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From: [e-mail](Peter van der Linden)
Newsgroups: comp.lang.java.programmer,comp.lang.java.help,comp.lang.java.gui,comp.answers,news.answers
Subject: Java Programmers FAQ
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Date: 17 Nov 1997 09:20:27 -0800
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Summary: This posting answers frequently-asked questions by Java programmers
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Archive-name: computer-lang/java/programmers-faq
Posting-Frequency: twice-weekly
Last-modified: 1997/11/17
URL: http://www.best.com/~pvdl/javafaq.html
Copyright: (c) 1997 Peter van der Linden
Maintainer: Peter van der Linden ([e-mail])

------------------------------------------------------------------------

Help stop Microsoft from trying to destroy Java Portability

As the Java Programmers FAQ maintainer, I ask you to drop all use of the
near-Java tools from Microsoft. Don't use any Microsoft near-Java tools
until Microsoft stops trying to undermine and divide Java.

   * don't use J++ or Visual J++ (use the tools from a score of other
     vendors)
   * don't use Internet Explorer (use Netscape)
   * don't use the Microsoft Java SDK. (Use Sun's JDK)

The biggest value of Java is its portability. If programmers accept
Microsoft tools that destroy Java portability by changing core Java
libraries and protocols, it will set the computer industry back years.

Support 100% Pure Java. Encourage Microsoft to make its near-Java
implementation pass the Java compatability test suite. Junk all non-standard
Microsoft not-quite-Java software. Use the free Java compilers downloadable
from http://java.sun.com.

------------------------------------------------------------------------

Frequently Asked Questions (with answers) for Java programmers

Most of the Java FAQs here are intended for experienced programmers. To
distinguish this from other Java FAQs, this is the "Programmer's FAQ" and
will be posted mostly in

   * comp.lang.java.programmer
   * comp.lang.java.help
   * comp.lang.java.gui
   * comp.answers.

Latest copy of this FAQ is available at: http://www.best.com/~pvdl

This FAQ reflects the latest and most current version of Java: JDK 1.1.4.

0. A WORD FROM OUR SPONSOR
1. LOOKING FOR INFORMATION
2. COMPILERS, ETC.
3. LANGUAGE ISSUES
4. AWT and Related Topics

     4.0 AWT
     4.1 APPLETS
     4.2 BROWSERS

5. CORE LIBRARIES
6. I/O
7. NETWORKING & DISTRIBUTED OBJECTS
8. MULTI-MEDIA
9. SECURITY
10. Java IDIOMS
11. FOR C and C++ AFFICIONADOS
12. FURTHER RESOURCES

-------------------------------------------------------------------------

0. A WORD FROM OUR SPONSOR

This Java FAQ is compiled and maintained as a service to the Java community.

Learning Java

Please consider these books when you are looking for a programming text. You
can get more info on these books at http://www.best.com/~pvdl

     0.1"Not Just Java" by Peter van der Linden, $35
     publ 1997 Sun Press/Prentice-Hall, ISBN 0-13-864638-4

     This should be your first book on Java -- and it doesn't teach Java
     programming. Instead, this book describes the trends and implications
     of Java and related technologies. Clear explanations with diagrams
     covering Java, CORBA, thin clients, network computers, RMI, multi-tier
     systems, Java threads, ActiveX, OLE, JavaBeans, JDBC, etc. For
     engineers, executives, and all the job titles in between.

     Look at reader reviews at:
     http://www.amazon.com/exec/obidos/ISBN=0138646384/4112-4025659-560808

     0.2 "Just Java Third Ed." by Peter van der Linden, $44.95
     publ Oct 1997 Sun Press/Prentice-Hall, ISBN 0-13-784174-4

     An introduction to Java Programming, for people who can already program
     in another language. Filled with common sense examples, and anecdotes
     from the computer industry. Learning Java is enjoyable, stimulating fun
     with this book. Comes with a CD including the compiler and sample code.
     Covers Java 1.0 and 1.1, with a preview of Java 1.2 Swing classes.

     Look at reviews at:
     http://www.amazon.com/exec/obidos/ISBN=0137841744/7892-5006833-225957

     0.3 "Expert C Programming" by Peter van der Linden, $35,
     publ 1994 Sun Press/Prentice-Hall, ISBN 0-13-177429-8.

     Everyone's second book on C. Unix Review gave it "5 stars". "Expert C
     Programming" teaches you the tips and techniques used by the very best
     C programmers in the world. Based on a decade in Sun's compiler and OS
     groups.

     Please look at reader reviews at:
     http://www.amazon.com/exec/obidos/ISBN=0131774298/4112-4025659-560808

     These books can be ordered online from http://www.cbooks.com Or by
     phone from Quantum Books 617 494-5042 (in USA) during business hours.

                             -------------------

1. Looking For Information

     1.0 Is Java "Open" or Proprietary?
          A.  Java is a freely-available open specification.  Sun
          Microsystems publishes the Java specifications and public
          interfaces as open specifications that can be independently
          implemented by anyone without charge (termed a "clean-room"
          implementation in industry jargon).  Specifically, Sun includes a
          royalty-free license in the front of the Addison-Wesley books
          containing the specification.

          Further, by submitting Java specifications to be ISO/IEC
          standards, Sun is giving up some control in exchange for the world
          being able to refer to a Java specification that is guaranteed to
          be open by ISO/IEC's charter and rules.

          Sun Microsystems has invested significant time and money to create
          a Java implementation. It has licensed, for a fee, the results of
          this work (the source code for the implementation) to 117
          companies and organizations who have asked for it, including
          competitor Microsoft.  Sun has also allowed free download and use
          of the Java implementation (binary executables) by anyone.  Over 1
          million computer users have downloaded the JDK binaries for free
          in the seven month period from February 1997.

     1.1 What is the best way to refer someone to the FAQ when they ask
     question I know is answered there?

          A. The Java Programmers FAQ gives the answer to your question in
          section N.n. ...

          This gives them the answer, and shows them where to go for future
          questions (it also demonstrates that the FAQ can answer their
          questions, providing an incentive to go there next time.)

     1.2 Where can I look at the definitive Java Language Specification?

          A. This is available online at:
               http://java.sun.com/docs/books/jls/html/index.html (Java 1.0)
          and the Java 1.1 update at:
               http://java.sun.com/docs/books/jls/html/1.1Update.html
          This is also available as a book in printed form (details at
          website).
     1.3 Where can I find a full list of Java books and book reviews?
          A. http://lightyear.ncsa.uiuc.edu/~srp/java/reviews.html
     1.4 How do I search Deja News for past postings on Java topics, e.g.
     the FAQ?
          A. Go to http://www.dejanews.com/home_sf.shtml
             + Under "Newsgroups" enter "comp.lang.java.programmer" (or
               whatever)
             + Under "Subject" enter "Frotteur" (or other topic you find
               pressing)
             + Click "Create Filter"
             + It will go to a new document, and you should click the link
               labeled

               nnn Documents (nnn is some number).
          This makes finding information very easy.

          Also look at http://sunsite.unc.edu/java/cgi-bin/query
          and look at http://asknpac.npac.syr.edu/ for Java newsgroup search

          http://www.javaworld.com/search.html can search the Javaworld
          newspaper

     1.5 How do I check on known bugs in JDK 1.1?
          A. Look at
          http://java.sun.com/products/jdk/1.1/knownbugs/index.html

          Note that only bugs that Sun deems "important" are listed there.
          The criteria for "important" are not published.

          After you have checked whether the bug is already listed, you can
          submit a bug report through:
          http://java.sun.com:80/cgi-bin/bugreport.cgi

          You can even send in an RFE or ease-of-use issue there!

          If you go to the JDC (that's the Java Developer's Connection) you
          can do a keyword search against the internal Javasoft bugtraq
          database, looking by keyword or bug number. JDC has free
          registration.
               http://developer.javasoft.com should do it.

               Also, there's a longer list of known bugs at:

               http://www2.vo.lu/homepages/gmid/java.htm
     1.6 Is there a Java port to Windows 3.1?
          A. Yes, several. IBM's ADK1.02 is available at the following
          locations:
               http://ncc.hursley.ibm.com/javainfo/latest/answers/faq0.html
               http://www.alphaworks.ibm.com/formula
          Netscape Navigator for Win3.1 has Java support [anyone know the
          version?]

          You could also consider JavaSoft's JavaPC $100 kit that converts a
          PC into a thin client Java system. Details are sparse as yet, but
          this is probably more for business users than personal PCs.

          Check http://java.sun.com/pr/1997/april/pr970402-07.html
     1.7 I see a lot of postings from magazines soliciting Java articles.
     Are these legitimate?
          A. Anyone considering writing for the "Java Developer's Journal",
          or any other Sys-Con publication, should be aware that there have
          been several instances of programmers complaining that JDJ has not
          paid them for commissioned articles.

          Another programmer reported that he was not paid by Sys-Con for an
          article for their Delphi magazine. There is a pattern of
          programmers complaining that SYS-CON has not paid them.

          The issue is that Sys-Con will only pay for articles that it
          actually prints, even when it explicitly commissioned the article
          from you. Some other publishers pay a cancellation or "kill" fee
          when they don't run a work they have commissioned. Programmer
          beware, and please don't support magazines that are in conflict
          over payment with their programmer contributors.
     1.8 What are the good folks at project Gnu doing with Java?
          A. Guava (the Gnu Java compiler) can be found at
               http://http.cs.berkeley.edu/~engberg/guavac/
          Kaffe (a JVM) can be found at
               http://www.kaffe.org
     1.9 Where can I find information about Java Certification?
          A. Sun is sponsoring an examination which programmers worldwide
          can take. Those passing can use the designation "Sun certified
          Java Programmer". There is also a second level test, involving
          writing a program to spec, and taking a further test. That results
          in the qualification "Sun Certified Java Developer". You can find
          out all about the exam at:
               http://www.sun.com/service/suned/cert/scjp_11.html
               and
               http://www.sun.com/service/suned/cert/scjd_11.html
          It costs $150 to sit the Java Programmer exam. It is not trivial
          to pass the Java certification exam. It requires understanding the
          objectives of the test, and the material that is tested for. These
          are given, along with sample questions, at the URL mentioned
          above.
     1.10 I'm looking for a Java style guide on naming conventions.
          A. Check out the section "Naming Conventions" in the language
          pecification
               http://java.sun.com/docs/books/jls/html/6.doc.html#11186
          Also take a look at Doug Lea's draft coding standard -
               http://gee.cs.oswego.edu/dl/html/javaCodingStd.html
     1.11  Where can I find information about future Java APIs?
          A.  JavaSoft has followed a policy of creating new APIs in
          consultation with leading industry participants, then posting the
          draft specification for public review and comments.  Check the
          JavaSoft roadmap of new APIs and products at
          http://Java.sun.com:80/products/api-overview/index.html
          Also, some APIs that are under consideration, possibly for JDK 1.2
          are at:
               http://java.sun.com/products/jdk/preview/docs/
     1.12 How can I find links to recent news about Java?
          A. http://lightyear.ncsa.uiuc.edu/~srp/java/javanews.html This
          site contains links to late-breaking online news stories about
          Java. Highly recommended. Also http://www.roaster.com/news/ is a
          good Java news source.

          This site is a source of independent news and commentary on the
          computer industry, including Java.
          http://www.pjprimer.com/media.html. (You have to subscribe
          ($10/year, 30 day free trial).
     1.13 What computers have Java ports?
          A. A list of JDK ports is available from
               http://java.sun.com/cgi-bin/java-ports.cgi



                             -------------------

2. COMPILERS, ETC.

     2.1 Why did I get an OutOfMemory error when porting working code from
     jdk 1.02 -> 1.1
          A. The preset memory limit has changed. It went down to 16Mb so as
          not to penalize low memory machines. You can adjust it with
               java -mx32m Frotz
          to get a 32Mb extent.

          Also see the Runtime methods freeMemory() and totalMemory().
     2.2 Why do I get a "Statement not reached" error from javac for no
     apparent reason?
          A. JDK1.0 has a limit of 63 words of storage for local variables
          in any method. longs and doubles require two words of storage, and
          all other primitive types and all reference types require one
          word. If you assign values to more than 63 words of local
          variables, you will get a "Statement not reached" error on the
          statement after you assign to the variable that contains the 64th
          word. In JDK 1.1, the low limit was removed.
     2.3 Is there a lex and yacc or preferably a flex and bison equivalent
     for Java?
          A. There is a lex equivalent called JavaLex and a yacc equivalent
          called CUP.

          LALR(1) parser
               JavaLex and JavaCup:
               http://www.cs.princeton.edu/~appel/modern/java/
          LL(k) parser
               JavaCC: http://www.suntest.com/Jack/
     2.4 Where can I find a byte code obfuscator?
          A. HashJava: http://www.blackdown.org/~kbs/hashjava.html
          http://www.primenet.com/~ej/
          http://www.math.gatech.edu/~mladue/HoseMocha.java
          There are also commercial products, such as SourceAgain from
          http://www.ahpah.com

          Some people have reported problems using these with JDK 1.1.

          This obfuscator has been updated to be fully compatible with JDK
          1.0 and 1.1:

          http://www.monmouth.com/~neil/Obfuscate.html
     2.5 Which program is used to create .zip files, compatible with the
     java* programs? (eg classes.zip, moz3_0.zip)
          A. Use the jar-tool from JDK1.1(.1):
               jar [ options ] [manifest] destination input-file
               [input-files]
          E.g.:
               jar cvf myJarFile.jar *.class
          creates a compressed archive
               jar cvfO myJarFile.zip *.class
          creates it fullsize (uncompressed) (note the 'O'-option used for
          JDK1.0.2)

          On Unix you can also use:
               zip -rn ".class" my_file.zip *
          Info-ZIP home page: http://www.cdrom.com/pub/infozip/
          Latest source code:
          ftp://ftp.uu.net/pub/archiving/zip/src/zip21.zip

          Netscape's command line version of its JAR packager and signing
          tool is called "zigbert". They also have a signing tool with GUI
          written in Java. More info at:
               http://developer.netscape.com/software/signedobj/jarpack.html
          If you zip your .class files for JDK 1.0.2 (for 1.1 you'll use a
          Jar)
               1. zip your files uncompressed (can use WinZip 6.2 up);
                    Unix command: zip -r0 classes.zip <directories>
               2. Make sure the main class has no directory beside it inside
               the archive, but your packages do;

               3. Put the archive in the same directory as the .html page;

               4. Put something like the following tag in the .html file:
                    <APPLET CODEBASE="."
                    ARCHIVE=my_zip_file.zip,myOtherZip.zip,thirdfile.zip
                    CODE="my_main_class.class"
                    WIDTH=600 HEIGHT=250>
                    </APPLET>
          From JDK 1.1 on, an example of the applet tag used with a jar file
          is
               <APPLET ARCHIVE=myfile.jar
               CODE=myapplet.class
               WIDTH=600 HEIGHT=250>
               </APPLET>
          These lines will use an applet called myapplet that can be found
          in the jarfile myfile.jar. You can supply several jar filenames in
          a comma-separated list. Jar files are in compressed PKZIP format.
     2.6 Can I compile a Java .class file to a binary executable, .exe on a
     PC?
          A. Some companies make products that do this, but it takes away
          one of the most significant benefits of Java: portability of
          executables. Consider carefully why you want to do this, and
          whether there is a Java way to accomplish your goal. There may be
          a good reason for compiling to native code, but it needs to be
          thought through.
     2.7 How can I performance profile my Java code?
          A. java -prof MyClass

          produces some basic output in a file called java.prof, showing the

          number of times methods were invoked. The output lines are of the
          form:


              # of calls     method called      called by        time spent
          On a Unix system, you can sort the file with something like
               sort -r +82 <java.prof > java.sort
          More and better Java tools are a third party opportunity.
     2.8 When I use javadoc and I click on any java class included in the
     jdk why do I get this message?
     Netscape is unable to find the file or directory named:
          /E|/Jwrkshop/JDK/bin/java.lang.Throwable.html

          A. References to the JDK classes assume that *all* generated html
          files are in the same directory and, in fact, that all files for
          all classes referenced are generated at the same time. There is no
          way to generate files incrementally and have them all reference
          each other, as you would like.

          As long as you have source for everything involved (including the
          JDK and all third-party classes), you can list all of your
          packages and all of the others on the javadoc command line and
          generate the whole set at once, but it is a real pain. Of course,
          if you receive any libraries as .class files, even this workaround
          will not suffice.

          Also javadoc will not generate the image files - you need to get
          them from the images directory under the JDK API documentation
          files. You can just copy the entire directory into your own doc
          directory. javadoc is a very nice concept, with a few
          implementation flaws.
     2.9 I'm working on a project with lots of classes and I use the JDK. A
     recompile from scratch takes forever when I do it a class at a time.
     How do I recompile everything?
          A. The first way is
               javac *.java
          Another way is
               javac -depend tip.java
          where "tip.java" is a class "at the tip of the iceberg", i.e. that
          depends on (uses) all the other classes. Typically, this may be
          your main class. However, "-depend" is known to be buggy and
          cannot be relied upon. It also doesn't issue compile commands in
          parallel to make use of multi-processor systems.

          Without the "-depend" option, the standard "javac files" doesn't
          look beyond the immediately adjacent dependencies to find classes
          lower down the hierarchy where the source has changed.

          The -depend options searches recursively for depending classes and
          recompiles it. This option doesn't help when you have dynamically
          loaded classes whose names cannot be determined by thecompiler
          from the dependency graph. E.g. you use something like

               Class.forName(argv[0]);

          The author of the code using those classes should make sure that
          those classes are mentioned in a Makefile.
     2.10 Why do I get the java.lang.UnsatisfiedLinkError when I run my Java
     program containing Native Method invocations?
          A. Your program is not able to find your shared library or DLL.

          On Windows 95/NT, make sure that the DLL exists in a path that is
          included within the PATH environment variable.

          On Solaris, make sure that the environment variable
          LD_LIBRARY_PATH includes the path of your shared library.

                             -------------------

3. LANGUAGE ISSUES

     3.1 Why doesn't my "hello world" program work?
          A. Common causes of failure are:
             + First, did you write an applet or an application? If an
               applet, you must make sure that you did extend the
               java.applet.Applet class. You must declare your class that is
               extended from Applet as "public". If you don't, unfortunately
               some browsers will still run the code, while others won't.
             + Your class name and the file name must match exactly, even
               letter case. If your class is HelloWorld, your source file
               must be HelloWorld.java
             + In JDK 1.0.2 the $CLASSPATH environment variable should
               include "." the current dir.
          In JDK 1.1, users are no longer required to set the CLASSPATH.

          From JDK 1.1.2 on, it is generally an ERROR if the user sets the
          CLASSPATH! (except that use of rmic apparently still requires it).
     3.2 How can I program linked lists if Java doesn't have pointers?
          A. Of all the misconceptions about Java, this is the most
          egregious. Far from not having pointers, in Java, object-oriented
          programming is conducted exclusively with pointers. In other
          words, objects are only ever accessed through pointers, never
          directly. The pointers are termed "references" and they are
          automatically dereferenced for you.

          Java does not have pointer arithmetic or untyped casting. By
          removing the ability for programmers to create and modify pointers
          in arbitrary ways, Java makes memorymanagement more reliable,
          while still allowing dynamic data structures. Also note that Java
          has NullPointerException, not NullReferenceException.

          A linked list class in Java might start like this:

               public class linkedlist {
               public linkedlist head;
               public linkedlist next;
               public Object data;
               public linkedlist advanceToNext(linkedlist current) { ...
               ... }

          Another choice for a linked list structure is to use the built-in
          class java.util.Vector which accepts and stores arbitrary amounts
          of Object data (as a linked list does), and retrieves it by index
          number on demand (as an array does). It grows automatically as
          needed to accommodate more elements. Insertion at the front of a
          Vector is a slow operation compared with insertion in a linked
          list, but retrieval is fast. Which is more important in the
          application you have?
     3.3 What is the true story about how parameters are passed in Java? Is
     is by value or by reference?
          A. All parameters (values of primitive types, & values that are
          references to objects) are passed by value [JLS sect 8.4.1].
          However this does not tell the whole story, as objects are always
          manipulated through reference variables in Java. Thus one can
          equally say that Objects are passed by reference (and the
          reference variable is passed by value). This is a consequence of
          the fact that variables do not take on the values of "objects" but
          values of "references to objects" as described in the previous
          answer.

          Bottom line: primitive type arguments (int, char, etc) _do not_
          change when the corresponding parameter is changed. The fields of
          object type arguments _do_ change when the corresponding parameter
          fields are changed.
     3.4 How, then, can I pass an object to a method, and have the method
     change the reference so it points to a different object back in the
     calling code?
          A. There are two ways. The obvious way is "just add another level
          of indirection" -- wrap the object in another class, whose purpose
          is simply to be passed as a parameter, allowing the nested object
          reference to be modified.

          The second alternative is a clearer variant of this. Pass in a
          single element array. Since arrays are objects, this works.

               void jfoo(Object ref[]){
                   ref[0] = new Object();
               }

               ...

               Object kludge[] = new Object[1];
               kludge[0]= myObj;
               jfoo(kludge);
               if (kludge[0] == myObj) ...
               else ...

     3.5 Why is String a final class? I often want to override it in some
     way.
          A. There are several reasons.

          The simplest is that being final guarantees that instances of
          String are immutable. (The String class implements immutable
          objects, but if it were not final it would be possible to write a
          subclass of String which permitted instances to be changed.) But
          that's an unsatisfying answer,
          because the real question is "Why must Strings be immutable?"

          One reason is efficiency. It's easier to optimize accesses to an
          object that is known to be immutable. Strings are very commonly
          used, even used behind the scenes by the Java compiler. Efficiency
          gains in the String class can yield big dividends. If no one can
          change a String, then
          you never have to worry about who else has a reference to your
          String.

          A more compelling reason is security. Before String was changed to
          be final (while Java 1.0 was still in beta) there was a race
          condition which could be used to subvert security restrictions. It
          had to do with having one thread change a pathname while another
          thread was about to open it.

          There are other ways to solve these problems, but making String
          final is the one that the designers chose.
     3.6 What happened to "private protected"?
          A. It first appeared in JDK 1.0 FCS (it had not been in the
          Betas). Then it was removed in JDK 1.1. It was an ugly hack
          syntax-wise, and it didn't fit consistently with the other access
          modifiers. It never worked properly: in the versions of the JDK
          before it was removed, calls to private protected methods were not
          dynamically bound, as they should have been. It added very little
          capability to the language. It's always a bad idea to reuse
          existing keywords with a different meaning. Using two of them
          together only compounds the sin.

          The official story is that it was a bug. That's not the full
          story. Private protected was put in because it was championed by a
          strong advocate. It was pulled out when he was overruled by
          popular acclamation.
     3.7 I extended the class called Frotz, and the compiler is giving me an
     error message "No constructor Frotz()" in the child class. Why?
          A. When you define a constructor for a class, unless you
          explicitly call the superclass' constructor at the start, a call
          to the superclass' parameterless constructor is implicitly
          inserted. The problem you're seeing is what happens when the
          superclass doesn't *have* a parameterless constructor. The
          solution is usually to call the correct version of the superclass'
          constructor with the right parameters.
     3.8 Why does <unexpected> happen in Java floating point?
          A. There are several unexpected things that seem to bite
          programmers with floating point. Invariably this is a result of
          the programmer not being fully conversant with FP, rather than
          some fault in Java.

          If you seem to be having problems with floating point, your
          problem probably stems from the fact that floating-point
          arithmetic is inherently imprecise. You can expect up to 7 digits
          of precision with floats and 16 digits with doubles. However, that
          does not mean that a number that can be exactly represented in 7
          digits decimal, can be exactly represented as a binary floating
          point number. On the contrary, that is usually not the case.

          Additionally, when Java converts floating point numbers to a
          String, as is done when they are output, enough digits are printed
          so the number can be read back in with no loss of precision. For
          this reason, you may see more "inaccuracies" in floating point
          output than you are used to. This policy actually gives you more
          consistent results than on a system where floating point output is
          deliberately rounded to make the output "pretty".

          There is a limitation of FP in JDK 1.0 (fixed in JDK 1.1). Namely,
          when you output a floating point number in Java 1.0, the result is
          system-dependent and contains no more than six digits after the
          decimal point. This bug is fixed in Java 1.1.

          For more information and detailed specifications on how Java deals
          with floating point, see the following URLs:
     http://java.sun.com/products/jdk/1.1/compatibility.html#incompatibilities

     http://java.sun.com/doc/language_specification/javalang.doc.html#1466
     http://java.sun.com/doc/language_specification/javalang.doc.html#5899


          If you want the rounded floating point output that most languages
          have, use the new java.text package of Java 1.1 to limit the
          number of digits that are output. If you need more precision than
          about 16 digits, use the BigInteger and BigDecimal classes of Java
          1.1.
     3.9 Why do I get the compiler error message
          urltest.java:8: Variable test may not have been initialized.

               URL test;
               try { test = new URL("http://osprey.avs.dec.com");
               } catch (MalformedURLException e) {}
               System.out.println("this is url " + test);
               }

          A. If an exception is raised in the try clause, test will not be
          given a value, yet you are using it in the catch clause. The
          solution is either to declare test with an explicit initial value
          of null, or (better) to print out the e.getMessage() string of the
          exception.
     3.10 Why do I get this compiler error message?

          public static void main(String[] args) {
          ^
          Statement expected.
          public static final float Conversion_Factor = 39.37;
          ^
          Type expected.

          A. A. Argument declarations and variable declarations inside
          methods are never given the "public", or "static" keywords. They
          are not public or static because they are local to a method. (They
          used not to be allowed the "final" keyword either, but there was
          not good reason for that restriction and it was dropped in JDK
          1.1). Move your public or static declarations outside the method.
          They are usually put at the beginning of the class.
     3.11 Why do I get this compiler error message?

          MyApplet.java:11: No constructor matching MyCheckbox(myApplet)
          found in class MyCheckbox.

          bp1 = new MyCheckbox(this);
          ^
          1 error

          A. The first thing to check is whether you gave a return value to
          the constructor, like this:

               public void MyCheckbox( Container parent )

          If you did, the compiler will think it is an ordinary method, not
          a constructor. This is a very common mistake, and quite hard to
          spot.
     3.12 Why do I get the compiler error message

T.java:96: Can't access protected method clone in class java.lang.Object.
OtherT is not a subclass of the current class.

          A. The method clone() is protected. It is protected because
          cloning is an operation which the object might want to restrict
          access to. If Object.clone() was declared as public, the object
          could never decide to make it more restrictive later. By declaring
          it protected, the object has the choice of making it *less*
          restrictive.

          This means that a method can clone its own objects, but a method
          cannot clone objects of another class, unless you do something
          like:

          class SomeObject implements Cloneable {
              public Object clone() throws CloneNotSupportedException {
                  return super.clone();
              }
          }

          [i.e. override clone to make it public, and call the superclass
          clone].

          class Foo {
              Bar bar;

              Foo (Bar b) {
              try {bar = (Bar) b.clone();}
              catch (Exception e) {}
          }
          ...

          class Bar implements Cloneable {
              public Object clone() throws java.lang.CloneNotSupportedException{
                  return super.clone();
              }
          }

     3.13 How do I transform a char into the corresponding int value, that
     represents the code value of the char?
          A. char c = 'A';
          int i = c;

          Going the other way is just

               c = (char) i;

          This question crops up so frequently because the BASIC language
          uses functions to map characters into ints, ASC( 'A' ) => 65
          causing BASIC programmers to seek the corresponding Java
          functions. Pascal and Ada have similar functions, and no doubt
          other languages too.
     3.14 If I extend a class with a subclass, are the constructors
     inherited?
          A. "Constructor declarations are not members. They are never
          inherited and therefore are not subject to hiding or overriding."
          The default constructor is not inherited, but provided (see
          Section 8.6.7, JLS).

          If you don't give your child class any constructors, a default
          no-arg constructor that invokes the superclass' constructor is
          provided for you. If the superclass doesn't have a no-arg
          constructor, see 3.7
     3.15 How do I allocate a multidimensional array?
          A. There are several ways. If you want a rectangular array, you
          can allocate the space for the array all at once. The following
          creates a 4x5 array:

               int arr[][] = new int[4][5];

          If you want each row to have a different number of columns, you
          can use the fact that a two-dimensional array is actually an array
          of arrays. The following code allocates a triangular array:

               int arr[][] = new int[4][]; // allocate the four row arrays
               for (int i = 0; i < 4; i++) // initialize each of the four rows
               arr[i] = new int[i + 1]; // row i has i + 1 columns

          Note that if you allocate an array of any kind of object (as
          opposed to primitive type), all the references will be null by
          default. These null references can result in NullPointerExceptions
          if you try to dereference them.

          In other words, after doing:

               int arr[] = new int[4];

          you can say "if (arr[2] == 0)"

          But after doing

               Integer Iarr[] = new Integer[4];

          you must fill in the object reference before using it, with e.g.

               Iarr[2] = myInt;

          or

               Iarr[2] = new Int(27);

          before you can say "if (Iarr[2].equals(myInt))"
     3.16 Does it make a difference to the class file in any way, if I
     import a package, versus use the full name, i.e.

          import java.rmi.server.*;
          RemoteObject ro;

     versus:

          java.rmi.server.RemoteObject ro;

          A. No, it makes no difference to the class files. Import is just a
          shorthand for quoting the full name package and class name (as in
          the examples in the question). Importing a class does not cause
          the class to be loaded at run time. There is no run time penalty
          for using the * form of import. The class file will contain the
          name of the packages it uses, and the loader will look for those
          classes as needed at runtime.

          The different forms of import may or may not make a difference to
          compile time. Such a difference is likely to be negligible, and
          should not be a factor in which form of import you use.

          Some people say that stating which classes you are importing can
          help program readability. In a program with many * import
          statements, it may take a programmer time to find which package an
          obscure class is imported from. If you explicitly list each class
          you import at the top of the
          program, you document which package each class you use comes from.
          These people suggest that you use

               import java.rmi.server.RemoteObject;

          in preference to:

               import java.rmi.server.*;

          Other people say that it is clearer still to use the full package
          and class name, at the point where you use classes in other
          packages. These people suggest that you use:

               java.rmi.server.RemoteObject ro;

          But that gets a little lengthy when you instantiate:

               java.rmi.server.RemoteObject ro = new java.rmi.server.RemoteObject();

          You always have the option of stating the full package and class
          name, whether you use import or not.

          Another good reason not to use the * form is when you are
          importing two packages that have classes of the same name and you
          want to use only one of those classes. E.g.

               import com.sun.*;
               import com.ms.*;

          where there is a class called SerialPort in both those packages.
          If you use the * form of import, you import both of the SerialPort
          classes and then must fully qualify the class each time you use
          it, to say which of the two you mean.

          In Java 1.0, if you import a class that has the same name as a
          class defined in that source file, you will get an error that the
          class names clash. In Java 1.1, the local class will be used when
          the package name is not given; use the package name of the
          imported class to use it.

          The best advice is to write the program so that it is as readable
          as possible. Where you have a group of well known classes, as in
          java.awt, there is no reason not to use "import java.awt.*;"
     3.17 What are "class literals"?
          A. A feature introduced in JDK 1.1. They are literals of type
          "Class" that hold a value representing any class. There is even a
          value to represent "void", like this:

          Class myCl1 = Character.class;
          Class myCl2 = Void.class;
          Class myCl3 = Object.class;

          You might use it like this:

          Class cl = thing.getClass();
          if (cl.equals(myCl1))
              System.out.println("It's a Character class");

          A class literal

               Component.class

          is the equivalent of

               Class.forName("java.awt.Component")

          The second can throw an exception, but the first cannot. If you
          don't know the name of the class when you write the code, you
          cannot use the first form.
     3.18 How do I copy the contents of an array (with primitive type
     contents) to another array?
          A. Use the method java.lang.System.arraycopy(Object src, int
          src_position, Object dst, int dst_position, int length);

          Note that there is no corresponding method to clear an array to
          0.0, 0, null, false, '\u0000' etc (except for allocating a new
          array, which will have the default initializations). This has lead
          several people to suggest the old COBOL trick of "initialize by
          rollup". In initialize by roll-up, the programmer sets the first
          element of the array to the desired value, and then moves elements
          array[0 to n-1] to array[1 to n]. Under some old IBM COBOL
          compilers, code was generated to move element 0 to 1, then 1 to 2,
          and so on, "rolling up" the desired value through the array. It
          was always a terrible idea, as it went outside the boundaries of
          defined behavior in COBOL, and was subject to being broken in any
          compiler release.

          In any event, intialization by roll-up is defined not to work in
          Java. If the src and dst arguments refer to the same array object,
          then the copying is performed as if the components at positions
          srcOffset through srcOffset+length-1 were first copied to a
          temporary array with length components and then the contents of
          the temporary array were copied into positions dstOffset through
          dstOffset+length-1 of the argument array.

          If you simply want to clear the same array to the same value many
          times, create two arrays. Fill one with the reset value, then use
          System.arraycopy to copy it into the work array each time you need
          to clear the work array.
     3.19 What is a fast way to set all elements of an array to a given
     value without duplicating the (possibly large) array?
          A. Using a loop that does it one by one is probably 20 to 40 times
          slower than good-old memset() in C.

          A fast way on many VM's is to set the first byte of the array,
          then use System.arraycopy repeatedly to fill the next byte, the
          next two bytes, the next four bytes, the next eight bytes, etc.
          (Note these are not overlapping slices, so the issue raised in
          Q3.18 does not
          arise).

          public static void bytefill(byte[] array, byte value) {
              int len = array.length;
              if (len > 0)
                  array[0] = value;
              for (int i = 1; i < len; i += i)
                 System.arraycopy( array, 0, array, i,
                          ((len - i) < i) ? (len - i) : i);
          }

          This is faster on Sun's VM than a simple loop, and probably faster
          even under JITCs because it only performs log2(array.length)
          bounds-checks at most. This is a clever code idiom applying the
          binary chop algorithm to arrays whose size is not a power of 2.
     3.20 Is there some declaration that I can use to make "acos", "cos",
     "sin", etc. (from java.lang.Math) recognizable in my own class, so I
     don't have to prefix "Math." to them?
          A. No. There is no good alternative. There are several bad
          alternatives:
               You could wrap the functions in your own class.

                    double sin(double x) { return Math.sin(x); } // etc. for each function

               If your class does not extend another class, you could make
               it extend Math, to bring the namespace in, but this is very
               poor OOP style.

               If the Math class provided a public constructor -- which it
               does not -- you could instantiate a local copy of Math with a
               shorter name, e.g. "M".

               If java.lang.Math were not final and your class did not
               extend another class, you could have your class extend Math,
               to bring the namespace in. However, the final prevents this
               and it is very poor OOP style to use inheritance to obtain a
               trivial name abbreviation (rather than to express a type
               hierarchy).
          The import stament only imports packages, subpackages, and
          classes. Members of classes are not imported, so

               import java.lang.Math.*;

          doesn't work.
     3.21 Why does b >>>= 1 give me the same result as b >>= 1?
          A. First of all, note that ">>" is a "signed" or "arithmetic"
          shift, namely, it replicates the sign bit on the left as it
          shifts.

          The ">>>" operator is an "unsigned" or "logical" shift; it does a
          shift right and zero fill. However, there are a couple of places
          where ">>>" looks like it does a signed shift!

          The issue occurs when you have a non-canonical type, byte, or
          short, with a negative value, e.g.

               byte b = -15; // 0xF1
               b = (byte) b >>> 4; // why isn't b 0x0f ?

          The initial expectation is that an unsigned shift right of 0xF1
          would successively be (in binary) 0111_1000 then 0011_1100 then
          0001_1110 then 0000_1111

          But that doesn't happen. The rules of arithmetic in Java say that
          all operands are converted at least to int before the operation
          (and possibly to a more capacious type). That means our byte is
          promoted to an int, so instead of shifting 0xf1, we are shifting
          0xfffffff1. If you shift right unsigned 4 places, you get
          0x0fffffff. When you cast that to a byte it becomes 0xff, or -1.

          The bottom line is that the final result is the same as if you'd
          performed the signed shift because the unsigned shift applied to
          the intermediate int, not to the original byte. This anomaly means
          that ">>>" is useless for negative bytes and shorts. It is
          probably safer and clearer not to use it at all, but to mask and
          shift like this:

               // not recommended
               byte b = -15;
               b = (byte) (b>>>4);
               System.out.println("b= "+Integer.toHexString(b) );

               // recommended

               b= -15;
               b = (byte) ( (b & 0xFF) >> 4 );
               System.out.println("b= "+Integer.toHexString(b) );

     3.22 How do I compare two Strings together?

          if (s1 == s2)

          is giving me funny results.

          A. The comparison using "==" on objects, like Strings, is asking
          the question "do these two objects have the same reference?". That
          is, do they have the same address, and hence are the same object?
          What you really want to do is ask "do these two Strings have the
          same *contents*?"

          You do that with any of (and there are others too)

               if (s1.equals(s2) )

               or if (s1.equalsIgnoreCase(s2) )
               or if (s1.startsWith(s2) )
               or if (s1.endsWith(s2) )
               or if (s1.regionMatches(s1_offset, s2, s2_offset, length) )
               or if (s1.compareTo(s2) < 0)

          Note that you can do this with literals:

               if ("apple".equals(s2) ) ...

          If you compare these the other way round, like this:

               if ( s2.equals("apple") ) ...

          and s2 is null, you will get a null pointer exception. Be alert to
          this.

                                   -----------------

4. AWT and Related Topics

     4.0 AWT

     4.0.1 Why do I get this when using JDK 1.1 under X Windows?

               java.lang.NullPointerException
               at sun.awt.motif.MFramePeer.<init>(MFramePeer.java:59)
               at sun.awt.motif.MToolkit.createFrame(MToolkit.java:153)
               at java.awt.Frame.addNotify(Frame.java)
               at java.awt.Window.pack(Window.java)

          A. There's a missing font on your system. Move font.properties
          from the "lib" subdirectory aside to font.properties.bak Then it
          won't look for the font and fail to find it.

          The problem occurs because the Motif AWT libraries use the Font
          "plain Dialog 12 point" as a fall-back default font.
          Unfortunately, when using a remote X server sometimes this font
          isn't
          available.

          On an X-terminal, the diagnostic may be slightly different, a segv

               % appletviewer HelloWorldApplet.html
               SIGSEGV 11* segmentation violation
               si_signo [11]: SIGSEGV 11* segmentation violation
               si_errno [0]: Error 0
               si_code [1]: SEGV_ACCERR [addr: 0x14]

          To determine which fonts you have, issue a command such as
               xlsfonts > ~/fonts.txt
          Then pick through the long list of fonts to determine which ones
          you want to use. The xfd program will let you look at a font:
               xfd -fn "your font name here" &
     4.0.2 Why is GridBagLayout so hard to use?
          A. GridBagLayout was contributed to Javasoft by a programmer who
          wanted to support the Java effort. It was intended as a proof that
          the AWT offered enough features for programmers to write their own
          layout managers. It wasn't designed with human factors and ease of
          use in mind. If it bothers you (it bothers me) then just don't use
          it. Create your GUI on several panels and use the other layout
          managers as appropriate to get the exact effect you want. The
          official story from the project leader of the AWT project, as
          explained to the Mountain View Java Users' Group on Dec 4 1996,
          is:
               "The case has been made and is now accepted that
               GridBagLayout is too hard to use for what it offers. GBL will
               continue to be supported, and something better and simpler
               will eventually be provided as well. This "better GBL" can be
               used instead of GBL."
          Bottom line: nobody has to waste any effort on GBL, there are
          better alternatives available now, and it will be replaced by the
          SwingSet "SpringLayout" Springs & Struts style layout manager to
          be introduced as part of the Java Foundation Classes with JDK 1.2.
     4.0.3 How do you change the font type and size of text in a TextArea?
          A. myTextArea.setFont(new Font("NAME", <STYLE>, <SIZE>));
               where:
                    NAME is the name of the font (eg Dialog or TimesRoman).
                    <STYLE> is Font.PLAIN, Font.ITALIC, Font.BOLD or any
                    additive combination (e.g. Font.ITALIC+Font.BOLD).
                    <SIZE> is the size of the font, e.g. 12. e.g. new
                    Font("TimesRoman", Font.PLAIN, 18);
     4.0.4 Can you have different fonts for individual words in a TextArea ?
          A. No. If you're trying to write a word processor, use the Canvas
          class to render on.
     4.0.5 How much text can be put in a TextArea?
          A. TextArea just uses the corresponding widget of the underlying
          window system. It will be bounded by the limit imposed in the
          native window system. In Windows95 TextAreas can hold about
          28Kbytes. The native widget allows 32Kb, but there is some
          overhead which reduces the amount available to the programmer.
     4.0.6 MyClass works fine except when I try to set a particular font. I
     just can't seem to get it to work in Win95, but I can get it to work on
     a MacOS and Unix.
          A. You probably specified a font name that isn't available under
          your Win95; this is one of those cross-platform differences that
          can bite you if you over-specify for one platform, like specifying
          "Arial" as a font and expecting it to work on something other than
          Windows.

          On both Windows 95 and Solaris 2.6, these fonts
               Dialog
               SansSerif
               Serif
               Monospaced
               Helvetica
               TimesRoman
               Courier
               DialogInput
               ZapfDingbats
          are revealed by this code:

               import java.awt.*;

               class foonly {
                  static public void main(String s[])
                  {
                      String n[]= new Frame().getToolkit().getFontList();
                      for (int i=0;i<n.length; i++)
                         System.out.println(n[i]);

                      System.exit(0);
                  }
               }

               In other words, You can get a String array of the names of
               the fonts by

                    String[] fonts = Toolkit.getDefaultToolkit().getFontList()

     4.0.7 Is it possible to draw a polygon or a line more than 1 pixel
     wide?
          A. JDK 1.1.1 doesn't have support for this. The standard
          workaround for drawing a thick line is to draw a filled polygon.
          The standard workaround for drawing a thick polygon is to draw
          several polygons.

          There is a useful class at
          http://www.apl.jhu.edu/~hall/java/GraphicsUtil.html It extends the
          drawxxx and fillxxx methods of java.awt.Graphics. It adds a Line
          Width argument to most of the drawxxx methods, a Color argument to
          most of the drawxxx and fillxxx methods, and a Font argument to
          draw String and drawChars.
     4.0.8 I use add(Component) to add Components to the Container. Is there
     any way to explicitly set the z-order of these Components?
          A. JDK 1.0 has no way to explicitly set the z-order of components.
          You can try it heuristically, based on the browser you're using,
          or you can use CardLayoutManager to ensure the panel you want is
          at the front.

          In JDK 1.1 the z-order of components ("z-order" means
          "front-to-back" order, i.e. which window is in front of which) can
          be controlled by using the the method add(Component comp, int
          index). By default, components are added 0 to N. The method paint
          of class Container paints its visible components from N to 0.
     4.0.9 When I call repaint() repeatedly, half my requests get lost and
     don't appear on the screen. Why is this?
          A. repaint() just tells the AWT that you'd like a paint to happen.
          AWT will fold several adjacent repaint requests into one, so that
          only the most current paint is done. One possible workaround might
          be to use a clip rectangle and only paint the different areas that
          have changed.
     4.0.10 What is the difference between
               Component's setForeground(Color c)
          and
               Graphics's setColor(Color c) ?
          A. First of all, these methods do the same thing: set the
          foreground color to the value of the parameter. The difference
          lies in where you use them. There is also a
          Component.setBackground that will set the background color.

          If you are in a constructor or an event handler (e.g. "click here
          to turn the canvas blue") you have a Component and should use the
          setForeground() method. If you are in a paint() method, that takes
          a Graphics context as its argument so you will typically use
          g.setColor(c).

          Unlike a Component, a Graphics object doesn't have a background
          color and a foreground color that you can change independently. A
          Graphics object arrives in the color(s) inherited from the drawing
          surface. From then on, any rendering (drawLine(), drawRect(),
          fillOval(), etc.) will be
          done in the setColor() color. Because they do different things,
          the Component and Graphics methods have different names.
     4.0.11 When I start a mouse drag inside a Component, and go outside the
     Component, still dragging, the mouse events still get sent to the
     Component, even though I am outside it. Is this a bug?
          A. No, it is the specified behavior. The java API documentation
          says:
               "... Mouse drag events continue to get sent to this component
               even when the mouse has left the bounds of the component. The
               drag events continue until a mouse up event occurs. ..."
          It is done for the convenience and ease of the application
          programmer. It allows you to handle all drags from the place of
          origin. If you don't want this, simply look at the coordinates of
          the mouseDrag Event, and if they are outside the Component, ignore
          them.
     4.0.12 What's all this about subclassing Canvas and overriding paint()
     ? Can't I just do a getGraphics() for a component, and draw directly on
     that?
          A. You can do that, and it might work up to a point (or it might
          not). A problem arises when the window system wants to refresh
          that component e.g. because it has been partially obscured and is
          now revealed. It calls paint(), and that has no knowledge of the
          other g.drawing() you have done.
     4.0.13 But couldn't the AWT just remember what has been drawn to a
     Graphics context, and replicate that instead of calling paint()?
          A. It could, but that is not how it works. In practice it is a lot
          simpler to be able to look at the paint method, and see explicitly
          all the things that will be done to draw that component. Bottom
          line: use paint(), not g=getGraphics(); g.drawString( ...
     4.0.14 How can I get the dimensions and resolution of the screen?
          A. java.awt.Toolkit.getDefaultToolkit().getScreenSize()
          java.awt.Toolkit.getDefaultToolkit().getScreenResolution()

          Screen resolution is in dots-per-inch.

          Take a look in the Toolkit class for other useful methods.

          Toolkit.getDefaultToolkit().getColorModel().getPixelSize() gets
          you the color model in terms of bits per pixel.

          Math.pow(2,Toolkit.getDefaultToolkit().getColorModel().getPixelSize())
          gets you the color model in terms of number of colors. Or use
          this:
               1 <<
               Toolkit.getDefaultToolkit().getColorModel().getPixelSize()
          That does a shift left to calculate the power of two.
     4.0.15 How do I allow for the size of the title bar and border when I
     draw a Frame?
          A. Use MyFrame.getInsets(). This returns a java.awt.Insets object
          which has four ints: top, left, bottom, right, giving the number
          of pixels each of those margins are inset from the top. You can
          use these value to adjust the Dimension object returned by
          component.getSize().
     4.0.16 When I run the Swing demo on Win95 I get an error "Out of
     environment space"
          A. That's because you don't have enough space for your DOS
          environment. You can fix this with:
             + Right click your MS-DOS Prompt icon or window and choose
               Properties.
             + Choose "Memory" and on "Initial Environment" choose 4096
               instead of "auto".
             + Run Swing again, you'll be OK.
     4.0.17 How do I resize a List? I had a List defined as

          List tlist = new List(10);

     but the Strings in the list were 80 characters long and only the first
     15 were being shown. I was not able to resize the List to display the
     contents without using the scroll bar.
          A. A List cannot be resized in a constructor, so add the following
          to the Applet (or wherever)

               public void paint (Graphics g) {
               tlist.setSize(200,200);
               }

          Then before showing panel/frame with the List:

               tlist.resize(400,400);

     4.0.18 How can my program tell when a window is resized?
          A. Override the setBounds() method of Component to do what you
          want. Of course, have it call super.setBounds() as well. Note that
          setBounds() replaces reshape() which is deprecated.

          Note the new APIs call the deprecated APIs instead of the other
          way round. For example, Component.setBounds calls
          Component.reshape, instead of reshape calling setBounds.
     4.0.19 Why doesn't my window  close when I click on the X in the title
     bar?
          A. Here's how to make your program do that.
          JDK 1.0.2: Handle Event.WINDOW_DESTROY to do a hide() and
          dispose() on the Frame.

          JDK 1.1:
          a) Listen for WindowEvent and do hide(); dispose(); in
          windowClosing() - this ought to be the "default" behaviour.
          b) enable AWTEvent.WINDOW_CLOSING and do the hide() and dispose()
          in processWindowEvent().
     4.0.20 How do I clear the contents of a TextArea?
          A. Set it to a null String with this:

               area.setText("");

     4.0.21 What are those preferredSize() and minimumSize() methods in
     Component?
          A. Those methods allow a LayoutManager to calculate the preferred
          and minimum sizes of the Components it is arranging. You can
          control the values that the LayoutManager gets by creating
          subclasses of the Components you are using and overriding these
          methods.
     4.0.22 How can I force a synchronization of the graphics state, e.g. of
     a cursor change, or an animation frame to be rendered?
          A. This is done by the sync() method in the toolkit. So just use:

               AnyComponent.getToolkit().sync();

     4.0.23 How do I plot a single pixel to the screen?
          A. Use g.drawLine(x1,y1,x1,y1) to draw a line one pixel in length.
          If you are drawing a very large number of individual pixels,
          consider using a java.awt.MemoryImageSource object and measure
          whether this offers better performance.
     4.0.24 How can I tab between components?
          A. In JDK 1.0, you have to read the key press, and program it
          explicitly. JDK 1.1 supports tab and shift-tab (previous field)
          automatically. The tab order is the order that the components were
          added to the container.
     4.0.25  What is the difference between "low level" and "semantic"
     events?
          A.It doesn't seem to be a distinction that is of use to the
          programmer. Semantic events are those generated when you frob a
          control (move a scrollbar, click on a button, select from a menu,
          etc). Low-level events are the others (resizing a window, dragging
          the mouse, striking a key, adding a Component to a Container,
          etc).

          Low level events: use the method enableEvents() and override
          performX() for those events in the object itself.
          Semantic events: Uses the method addXListener() to add objects
          which implement the XListener interface in order to get XEvent
          objects delivered (usually via the AWTEventMulticaster).
     4.0.26 Is it possible to have a Java window float above all other
     windows. for example a tool palette floats in a super-layer always
     above all the regular document windows on which you use the palette's
     tools?
          A. On MS Windows, a Window object floats above all other windows,
          unlike a Frame, which is layered in with ordinary windows. This
          behavior yields a "floating" effect. Whether a Window object is
          really supposed to float is another question entirely.

          On Mac, a Window object is either layered in with other windows,
          just like a Frame is, or else it is entirely modal -- depending on
          which VM you use. In Java -- there appears to be no easy way to
          get floating behavior. If anyone knows otherwise, please send in
          your comments.
     4.0.27 How do I center a dialog box?
          A. You cannot currently get the applet's absolute screen
          coordinates. Its location (0,0) is relative to the browser, not
          the screen itself. But you can center something that it pops up or
          displays centered on the screen with code like this:

          Dimension screen = Toolkit.getDefaultToolkit().getScreenSize();
          my_window.move(
                  ( screen.width -  window.size().width ) / 2,
                  ( screen.height - window.size().height ) / 2 );

          my_window.show().

          In a related fashion, you can center something on its parent like
          this. Note the intelligent use of APIs like translate() to do the
          work for you.

          void centre(Component parent) {
              pack();

              Point p = parent.getLocation();
              Dimension d = parent.getSize();
              Dimension s = getSize();

              p.translate((d.width - s.width) / 2, (d.height - s.height) / 2);
              setLocation(p);
          }

     4.0.28 How do I get back to a normal echo after I have used
     TextField.setEchoChar('*');
          A. TextField.setEchoChar('\0') works on most Windows based
          browsers... but for most other platforms (ie: Netscape under
          UNIX), it just locks up the textfield.

          There is only one good solution, and that is to make two
          TextFields on top of each other, one normal, and one with
          .setEchoChar('*'), and switch between them.

4.1 APPLETS

     4.1.1 What is the difference between an application, and applet and a
     servlet?
          A. An application is a standalone program. An applet is a
          downloadable program that runs in a web-browser. Typically an
          applet has restricted access to the client system for reasons of
          security.

          A Servlet is an application, but (like an applet) requires a
          context in which to run, namely web-server software. Servlets are
          used like CGI, but allow the server end to be written in Java as
          well as the client.

          The Web Server starts up a servlet when the URL is referenced, and
          now your applets have something that they can talk to (via
          sockets) on the server that can write files, open connections to
          other servers, or whatever.

          Don't confuse Sun's JWS "Java Web Server" with JWS "Java
          Workshop". Java Web Server supports servlets as does the
          lightweight and free server at Acme.com:
               http://www.acme.com/java/software/Acme.Serve.Serve.html
     4.1.2 My applet works on my machine, but fails when I put it on our web
     server. Why?
          A. It could be one of several reasons, and unfortunately the
          messages that you get in this situation aren't much help. In
          general, you can assume that either your applet's class files are
          corrupted somehow, or the web server can't find one or more of
          them when the browser needs them.

          Be careful of the following things:
             + Make sure you transfer the class files in binary mode, rather
               than text or ASCII mode.
             + Make sure you transfer *all* of the class files which are a
               part of your applet. Sometimes people are surprised by how
               many there are. There will be a class file for every class
               and interface you define, even if you define more than one in
               a single source file. If you use the Java 1.1 "inner classes"
               feature, there will be class files for each inner class as
               well.
             + Make sure you maintain the appropriate case distinctions in
               your filenames. If a class is called StUdLy, it must be found
               in a file called StUdLy.class.
             + Make sure you maintain the directory structure that matches
               your package structure. If you declare a class in package
               COM.foo.util, the class file needs to be in directory
               COM/foo/util under the applet's codebase directory. Again,
               case distinctions are important for package/directory names,
               just as they are for class/file names.
             + Make sure that the web server process will have read access
               to the class files, and search access to the directories that
               the files are in. For example, if the web server runs on a
               Unix machine, use the command "chmod o+r filename" for the
               files, and "chmod o+x dirname" for the directories.
     4.1.3 How do I use an image as the background to my applet? How do I
     set the background color of my applet the same as the browser?
          A. You can simply do a g.drawImage(yourImage, x,y, this) in the
          paint() routine of your applet. If the image isn't big enough to
          fill the entire background, tile it or scale it There is some code
          to tile at:
               http://www.geocities.com/SiliconValley/Vista/1337/
          (under AWT) Alternatively, the AWT can scale your background image
          to the size of
          the applet. The result quality will depend on the kind of image.
          Inside an applet class, you can use:

               drawImage(img, 0, 0, size().width, size().height, this);

          You can set the background color to match the background color of
          the browser by passing the value in as a parameter, like this:

          In the HTML applet tag:
               param name=BrowserColor value=F1F1F1
               (value should be the same hex as the HTML COLOR value).
               In the Applet init() method:

               String colparam = getParameter("BrowserColor");
               int col = Integer.valueOf(colparam,16).intValue();
               setBackground( new Color(col) );

          An applet cannot override the size imposed by the HTML. If you
          make the applet larger, the browser will still clip to the
          origional size. If you need more room, open up a new Frame, Window
          or Dialog to show your output.
     4.1.4 how do you make the applet's background transparent?
          A. there is no way to give an applet a transparent background that
          lets the web browser background show through. You can simulate it
          by giving the applet a background that matches the underlying
          browser background. It doesn't produce satisfactory results with a
          patterned background because of problems aligning the edges.

          Lightweight components (new in JDK 1.1) have a transparent
          background.
     4.1.5 How do you do file I/O from an applet?
          A. See answer to question 6.8.
     4.1.6 How do you get a MenuBar/Menu in an applet?
          A. In your applet's init() method, create a Frame instance and
          then attach the Menus, Menubar etc to that frame. You cannot
          attach the Menu or a Menubar to an applet directly.

          Or get the parent Frame like this:

               Container parent = getParent();
               while (! (parent instanceof Frame) )
                   parent = parent.getParent();

               Frame theFrame = (Frame) parent;

          This second suggestion probably won't work on Macs (where would
          the menubar go?) or in some browsers. In JDK 1.1, just use a popup
          menu, which isn't attached to a Frame.
     4.1.7 Can I get rid of the message "Warning:Applet Window" along the
     bottom of my popup windows in my Applet?
          A. This is a security feature that prevents the applet programmer
          from popping up a window that looks like it came from the native
          OS and asking for passwords or credit card info (etc). Users must
          always be aware of when they are talking to an unsigned applet.
          You can get rid of it by signing the applet, if the user accepts
          signed applets from you.

          In Netscape (only), using the Capabilities API to make the call

               PrivilegeManager.enablePrivilege("UniversalTopLevelWindow");

          before creation of the Frame eliminates the message, if the
          security manager passes it.
     4.1.8 When I subclass Applet, why should I put setup code in the init()
     method? Why not just a constructor for my class?
          A. The browser invokes your constructor, then setStub, then
          init(). Hence when your constructor is invoked, the AppletStub
          (and through it the AppletContext) is not yet available. Although
          in principle you can do things in the constructor that don't rely
          (even indirectly) on the AppletStub or AppletContext, it is less
          error-prone to simply defer all setup to the init method. That way
          you know that anything that needs the stub/context will have it
          available.
     4.1.9 How do I pull a non-class file, such as a .gif, out of a jar
     file?
          A. In your class, you should be able to do something like this:

               String imageFileName = "foo.jpg"
               URL imageURL = getClass().getResource(imageFileName);

               Toolkit tk = Toolkit.getDefaultToolkit();

               Image img = null;
               try {
                   img = tk.createImage((java.awt.image.ImageProducer)
                   imageURL.getContent());
               }
               catch (java.io.IOException ex) {
                   System.out.println(ex);
               }

          (Like anything involving Jar files, this is from JDK 1.1 on).
     4.1.10 I want to know about {applets,applications} but the lousy book I
     got just talks about {applications,applets}. What can I do?
          A. The truth is that 95% of the material is the same, whichever
          your book chooses to focus on. Some people write their apps to
          work completely in a Panel, then depending on whether they're
          running stand-alone or in a browser the Panel is either added to a
          Frame or an Applet. The trick is that you need to subclass the
          application's Frame in order to handle the WINDOW_CLOSING
          (previously WINDOW_DESTROY) event yourself. If you fail to do
          this, when running as an application, the window won't close.

          In this scenario the following code will tell you which
          environment you're running in:

               public boolean isRunningInBrowser() {
                  Component p = getParent();

                  while(p != null && !(p instanceof Frame)) {
                     p = p.getParent();
                  }

                  return (p == null);
               }

     4.1.11 How do I print a page with an applet?
          A. iBrowsers are starting to introduce support for this. Until
          they all have it, your best bet is to print a screendump.  Using
          the browser to print the page may leave a blank where the applet
          is.  Putting print support in the applet will print the applet
          only, not the rest of the browser page.

     4.1.12  How can I position my dialogs centered (not top left)?
          A.  Use some code like this:
               void centre(Component parent) {
                          pack();

                          Point p = parent.getLocation();
                          Dimension d = parent.getSize();
                          Dimension s = getSize();

                          p.translate((d.width - s.width) / 2, (d.height -
          s.height) / 2);
                    setLocation(p);
              }

     4.1.13  How can I get two applets on the same page to communicate with
     each other? centered
          A.(Easy way) Make them into one applet with two GUIs. (Harder) Use
          the static members of a common class to communicate information
          between the applets. This is something of a hack as it relies on
          class-loading behavior that may change in future. Try to avoid the
          need for applets to talk to each other. Also check the URL
          http://java.sun.com:81/products/hotjava/1.1/applet_environment.html
          which explains how it can be done in HotJava 1.1. Recommendation:
          stay far away from code which is browser-specific.

4.2 BROWSERS

     4.2.1 When will my favorite browser support Java 1.1?
          A: Netscape Communicator 4.0 with the feature patch supports most
          new features of Java 1.1, and Netscape has committed to the rest.
          Communicator is scheduled to fully support Java 1.1 in late 1997.
               URL:
               http://developer.netscape.com/support/faqs/champions/java.html#21
          Sun's HotJava browser fully supports the JDK 1.1 features.

          Internet Explorer 4.0, released in September 1997, does not fully
          support Java 1.1, omitting entirely RMI, JNI, signed JARs, and
          making around 50 non-standard modifications to the java.awt.* and
          other package. After weeks of negotiations, Sun has brought a
          breach of contract lawsuit against Microsoft because of the
          unauthorized changes. Programmers who support Java are recommended
          not to use Internet Explorer until Microsoft brings it back into
          compliance with the Java standard.

          Microsoft is trying to slow down the drive to Java, by spreading
          fear, uncertainty, doubt, and trying to fragment the Java
          environment. People say that sooner or later Microsoft will be
          forced to support portable Java, not try to undermine it. Until
          that day, developers should avoid Microsoft's near-Java knock-off
          when they choose their development tools.

     4.2.2 Is it possible to set and retrieve cookies from Java, in a manner
     that is compatible with all browsers supporting cookies?
          A. Short answer: no.
          Longer answer: probably no.
          Ultimate answer:
          A cookie is a morsel of tasty data that a server sends back to the
          client, and can retrieve on demand. It allows the server to retain
          some state information for each of its clients. The information is
          typically something like "what pages has the user seen?" or "is
          this a privileged user?".

          The DevEdge site on Netscape's home page has a javascript-java
          example on getting cookies. It's quite involved. Stick to just
          Java if you can.
     4.2.3 I am developing an applet and testing it in Netscape Navigator. I
     find that after I recompile, I press reload, clear the caches, retype
     the URL of the HTML wrapper, and I still have the old version. Why is
     this?
          A. It is because Netscape has completely failed to improve the
          defective code that does this monstrously wrong thing. It has been
          like this for many successive releases.

          Flushing the network cache will make no difference; that isn't
          where the caching is taking place. Although applets are sometimes
          "pruned" and their ClassLoaders garbage-collected, this doesn't
          happen predictably, so restarting Netscape is the only reliable
          work-around at the moment.

          A related question is "how do I make the browser reload from a
          URLConnection instead of just getting the content from the local
          cache?" The answer is to use
          java.net.URLConnection.setUseCaches(false)
          Browsers seem to vary in their conformance to this programmatic
          request. Netscape caching varies depending on whether a proxy
          server is in use, and which thread in the applet made the get
          request.
     4.2.4 So, why can't Netscape reload the applet when you press the
     Reload button?
          A. For the applet to be reloaded, the new version would have to be
          loaded in a different ClassLoader. Navigator/Communicator's policy
          for assigning ClassLoaders to applets doesn't take into account
          whether a reload has been done (although there is no technical
          reason why it couldn't).

          Some versions of Netscape reload the Applet if you hold down
          <Shift> while you click on reload. Until they fix it, use the
          appletviewer to test applets. And send them mail -- developers can
          only fix the bugs they know about.
     4.2.5 Should I use Microsoft CAB files or Java JAR files?
          A. The question contains its own answer.

          CAB format is a Microsoft-only format. So do not use it as it
          destroys software portability.

          JAR format is the Java standard format, based on PKZIP format
          including data compression. JARs were introduced with JDK 1.1

          See http://www.ibm.com/java/community/viewarchive4.html for more
          information.

          You should use the Java standard format JAR (Java Archive) files,
          not a vendor-specific format. JAR files are not just a Java
          standard, they are in industry-standard PKZIP format. One reader
          comments that both formats can be used with this tag:
               <APPLET NAME=myapplet
               ARCHIVE="myzip.zip"
               CODE="com/nnnnn/nnnn/cccccccc.class"
               WIDTH=n
               HEIGHT=n>
               <PARAM NAME="cabbase" VALUE="mycab.cab">
               </APPLET>
          IE3 does not support JAR
          IE4 supports compressed and uncompressed JAR, but not signed JAR
          IE3 and IE4 both support CAB (signed & unsigned)
                                   -----------------

5. CORE LIBRARIES

     5.1 I can't seem to change the value of an Integer object once created.
          A. Correct. Integer (Float, Double, etc) are intended as an object

          wrapper for a specific value of a number, not as a general purpose

          way of shipping a primitive variable around as an Object. If you
          need
          that it's easy enough to create: class General { public int i; }
     5.2 How do I print from a Java program?
          A. Use the Toolkit.getPrintJob() method

               Component c = this.getParent();
               while (c!=null && !(c instanceof Frame))
                  c=c.getParent();

               PrintJob pj = getToolkit().getPrintJob((Frame) c, "test", null);
               Graphics pg = pj.getGraphics();
               printAll(pg);
               pg.dispose();
               pj.end();

          This feature was introduced with JDK 1.1. A common place to put
          this is in the code that handles a button press. Printing from an
          untrusted applet is subject to a check from the SecurityManager.

          To print in JDK 1.0.2, some people have suggested using Jentec's
          JENI. (The Java Enterprise Network Interface). JENI is a set of
          classes that provides applets with file, print, email, and
          directory services to lots of different protocols and locally
          without the applet having to change or even be aware which is
          being used.

          JENI is free and available at http://www.jentec.com. For an
          example of how to print, see
          http://www.jentec.com/live/jeni/PrintingAFile.html. To save a
          file, see
          http://www.jentec.com/live/jeni/CreatingAFile.html.
     5.3 What are the properties that can be used in a PrintJob?
          A. The properties are
               awt.print.destination - can be "printer" or "file"
               awt.print.printer - print command
               awt.print.fileName - name of the file to print
               awt.print.numCopies - obvious
               awt.print.options - options to pass to the print command
               awt.print.orientation - can be "portrait" or "landscape"
               awt.print.paperSize - can be "letter","legal","executive" or
               "a4"

               The defaults are destination=printer, orientation=portrait,
               paperSize=letter, and numCopies=1

               You can search for info like this by joining the Java
               Developer Connection (it's free)

               http://developer.javasoft.com/developer/index.html

               Then do a search for "PrintJob".
     5.4 Is there any package in Java to handle HTML?
          A. See the answer to Question 8.14
     5.5 Why don't Dialogs work the way I want them to?
          A. Modal dialogs (dialog windows that stay up until you click on
          them) are buggy in many browsers and in the 1.0.2 JDK. One bug is
          that the dialog is not necessarily put on top when it is
          displayed. Most of the modal dialog bugs are fixed in JDK 1.1.
     5.6 Where can I find information about the sun.* classes in the JDK?
          A. You're not supposed to. Those classes are only to support
          functions in the java.* hierarchy. They are not part of the API,
          and won't be present in Java systems from non-Sun vendors. Some
          people have reverse engineered the code and published an API for
          these classes but you use it at your own risk, and it may change
          without warning.

          Worst of all, those programs will not have the portability of true
          Java but will only run on Sun JDKs. For the same reason you
          shouldn't use classes outside the java.* packages when using JDKs
          from other vendors.

          If you still insist on going ahead, check these URLs:
          http://java.sun.com/products/api-overview/index.html
          http://www.parmly.luc.edu/javaudio/
          http://www.users.interport.net/~mash/javamidi.html
     5.7 How do you read environment variables from with a Java program?

     A. Environment variables are not used in Java, as they are not platform
     portable. The Mac doesn't have environment variables for example. Use
     properties instead. Additionally, on some systems you can set a
     property from the command invocation line like this:
          java -Dfoo=$foo MyClass (Unix)
     or
          java -Dfoo=%foo% MyClass (MS-DOS)
     This sets the "foo" property to the value of the environment variable
     foo. Make sure you do not leave any spaces after the -D or around the =
     sign. Inside the program you get the value with:

          String env = System.getProperty("foo");

     More simply, just put the environment variable on the commandline
     and read it as arg[0]
          java MyClass %FOO% ; Win32
          java MyClass $FOO ; Unix
     5.8 How do you use the Date class to display the current time in my
     timezone? Date.toString() always uses PST. [jdk 1.1] (Pacific Standard
     Time -- the zone covering California where JavaSoft is).
          A. To make things easier for debugging Sun has decided that the
          toString() method should always use one format, if you want a
          different format you should use the new internationalization
          routines.

          As a Date is stored internally in GMT the obvious choice for a
          standard format is... PST time? Taligent (who wrote this junk) is
          in the PST zone. As an example of how the new method should work
          jdk1.1/src/java/util/Date.java contains the method:

               DateFormat formatter = new SimpleDateFormat (
                          "d MMM yyyy HH:mm:ss 'GMT'", Locale.US);

               formatter.setTimeZone(TimeZone.getTimeZone("GMT"));

          or even:

               formatter.setTimeZone(TimeZone.getDefault());

          It should be reasonably straight forward to adapt this code for
          your preferred format (e.g. change string to "hh:mm" or other) and
          timezone, e.g. change "GMT" to "ECT", "JST" or other timezone.

          Don't forget this code too, to prevent GMT being interpreted as
          GMT+1:00 hour.

               // + 1 is to work around bug in GregorianCalendar
               // XXX - need FIX
               // should probably be formatter.format( this );
               // or formatter.format(new Date());

               return formatter.format( new Date(getTime() + 1) );

     5.9 How are dates represented in Java?
          A. java.util.Date stores dates as long integers representing the
          number of milliseconds since 00:00:00 UTC Jan 1, 1970 (the birth
          of Unix, a date known as the "Epoch"). This is sufficient to
          represent dates from roughly 146,026,873 B.C. to 146,030,814 A.D.
          Dates earlier than the Epoch are represented as negative numbers,
          counting away from 1/1/1970.

          In JDK 1.1, Date was augmented by Calendar. Things didn't get any
          better. Instead of being ill-conceived and simple, it is now
          ill-conceived and complicated. The code was all licensed from
          Taligent. It didn't really improve matters. Dates are the lemon of
          Java, as Roedy Green truly notes.
     5.10 That may be, but how do I extract day, month, year from a Date?
          A. In JDK 1.1, Date represents an instant in time. Calendar
          translates between an instant in time, and individual fields like
          year, month, day, etc

          So get your Date -- the current Date can be had by

               Date now = new Date();

          Then construct a Calendar to do the translation:

               Calendar mycal = Calendar.getInstance();
               mycal.setTime(now);

          // Now read the Calendar's fields using get().

               System.out.println("Year = " + mycal.get(Calendar.YEAR));

          We hope all this junk will be fixed once and for all in a future
          release.

          The Calendar, DateFormat, and even TimeZone classes will give you
          the timezone of your system (wherever you are). The default time
          zone for SimpleDateFormats is PST (i.e. California).
     5.11 What kind of different date formats are allowed for the Date
     constructor with the string parameter.
          A. The Language Specification (which also covers the java.lang,
          java.util, and java.io packages) is the best reference for
          answering questions like this. It has much more detail than the
          API Documentation has, and it is available as a book or online at
               http://java.sun.com/docs/books/jls/html/javautil.doc.html
          The Date(String) constructor is deprecated in Java 1.1, by the
          way.
     5.12 How do I get Java talking to a Microsoft Access database?
          A. Use the JDBC-ODBC bridge. It is not especially challenging to
          set up, but it does require painstaking attention to detail. There
          is a step-by-step example in the van der Linden text "Just Java
          2nd Ed." mentioned in the sponsorship section of this document.

          Note that the Microsoft version of the Java kit does not support
          JDBC-ODBC access because it uses a non-standard native code
          interface. Also check the JDBC FAQ listed at the end of this
          document.
     5.13 I can't seem to change the current working directory.
          A. Correct. This missing functionality is an oversight that we
          hope will be corrected in future. Changing the user.dir property
          merely changes the text property, not the underlying reality that
          it is supposed to reflect.
     5.14 How do I create a Vector of ints?
          A. ints are primitive types and hence can't be stored by the
          Vector class which stores objects. You'll need to wrap the ints.
          Try this:

               int i =7;
               Vector holdsInts = new Vector(5,1);

               holdsInts.addElement(new Integer(i));
               int j = ((Integer)holdsInts.elementAt(0)).intValue();

     5.15 I have several worker threads. I want my main thread to wait for
     any of them to complete, and take action as soon as any of them
     completes. I don't know which will complete soonest, so I can't just
     call Thread.join on that one. How do I do it?
          A. You need to use the wait/notify mechanism to allow any of the
          worker threads to wake up your main thread when the worker has
          completed.
     5.16 How do I get random numbers?
          A. If you just need a quick random double between 0.0 and just
          less than 1.0

               double myrandom = Math.random(); // [0,1)

          The notation "[0,1)" is common math notation for "zero to .9999999
          etc" The Sun documents say this returns 0.0 to 1.0, but inspection
          of the source shows they are wrong. However, due to the inherent
          inaccuracies of floating point arithmetic, multiplying N by
          0.999999 etc can result in an answer of N, not N * .999999 . So
          watch out when N is big.

          Where things get even trickier is in the case where you want an
          int within a certain range, say 1 to 6 to simulate the throw of a
          die or 1 to 52 to represent a playing card. Class Random has a
          nextInt method that will return any integer:

               Random r = new Random();
               int i = r.nextInt();

          However, that has an (almost) 50% chance of being negative, and it
          doesn't come from the right range. So you just take the abs()
          value and then mod it into the right range:

               int dice_throw = 1 + Math.abs(i) % 6;

          Except, the abs() method fails gracefully in the presence of the
          Integer.MIN_VALUE (it returns the same, negative, result!). So it
          is better to AND to get the non-negative value: In general, to get
          a random int between high and low limits (incl.):

               Random r = new Random();
               int j = (Integer.MAX_VALUE & r.nextInt()) % (high-low+1) + low;

          The sentence states "(almost) 50% chance" because there is one
          more value in the negative integers than in the positive integers
          in two's complement arithmetic as used by Java. For most purposes,
          the bias introduced will be insignificant.

          A worse problem is that with the algorithm used, the low order
          bits are significantly less random than the higher order bits. And
          the low order bits are precisely the ones you get when you do the
          remainder operation.

     5.17 What does "deprecated" mean? I got this in a compiler error
     message.
          A. The compiler will flag a now-obsolete API as "deprecated". The
          word means "officially disapproved of". Compile again with the
          "-deprecation" option to see what is deprecated. In almost all
          cases, the old API has been replaced by a new one. Update your
          code to use the new one.

          An example of using a deprecated API is calling component.size().
          That has been replaced by component.getSize().
     5.18 Where/why should I use the Enumeration interface?
          A. It's a very convenient way to step through some of the library
          data structures, such as HashTable, Vector, and ZipFile. It is
          thread safe. If you're looking at an element in one thread while
          another thread is trying to delete it, it won't half vanish.

          Here's how you might look at every file in ZIP file:

               ZipFile z = new ZipFile("foo.zip");
               for (Enumeration e=z.entries(); e.hasMoreElements(); ) {
                   ZipEntry ze = (ZipEntry)e.nextElement();

               System.out.println("got " + ze.getName() );
               }

          You should look for opportunities in your own data structures to
          implement Enumeration, anywhere where the structure has repeated
          elements.
     5.19  Which version of WinZip is compatible with java.util.zip?
          A. You need WinZip version 6.2 or later. Version 6.1 or earlier is
          not good enough. WinZip can be downloaded from
          http://www.winzip.com/download.cgi. The pkzip software works fine.

                             -------------------

6. I/O

     6.1 How do I read a String/int/boolean/etc from the keyboard?
          A. The easiest way is to pick up the source for the 100% pure Java
          class EasyIn from http://www.best.com/~pvdl (same place as this
          FAQ) Compile it with your code and use it like this:

               EasyIn easy = new EasyIn();

               int i = easy.readInt(); // gets an int from System.in
               boolean b = easy.readBoolean(); // gets a boolean from System.in
               double d = easy.readDouble(); // gets a double from System.in

          ... etc.

          EasyIn is free, comes with source, and you can do what you like
          with it, including improve it, and send me back the results.

          If, instead, you want to "roll your own" code (why?!) In JDK 1.0.2

               java.io.DataInputStream in = new java.io.DataInputStream(System.in);
               String s = in.readLine();

          One way in JDK 1.1

               java.io.BufferedReader in =
                      new java.io.BufferedReader( new InputStreamReader(System.in));

               String s = in.readLine();

          Once you have the token in a String, it is easy to parse it into
          one of the other types, as shown earlier in the FAQ. Yes, it is
          bone-headed, as it makes the simplest case of keyboard I/O
          unnecessarily complicated.
     6.2 Why do I have trouble with System.out.println()?
          A. Check the spelling. The last two characters are the letters
          "ell enn" not "one enn".

          The name of the method stands for "print line", since it prints a
          String and goes to the next line, rather than staying on the same
          line as System.out.print() does. Yes, the name is yet another Java
          naming inconsistency, since the input equivalent is readLine(),
          not readln().
     6.3 How do I write to the serial port on my PC using Java?
          A. If the port exists as a pathname in the filesystem, you can
          open it as a file and read/write. You can also print text this way
          by writing to "prn" or "lpt1" on a pc, and "/dev/something" on
          Unix.
          Writing a formfeed at the end of the file is essential on
          Windows95.

//class that opens the printer as a file and writes "Hello World" to it
import java.io.*;
public class lpt
{
        public static void main (String[] argv) {
                try {
                        FileOutputStream os = new FileOutputStream("LPT1");
                                    //wrap stream in "friendly" PrintStream
                        PrintStream ps = new PrintStream(os);

                                    //print text here
                        ps.println("Hello world!");

                                    //form feed -- this is important
                                    //Without the form feed, the text
                                    //will simply sit in the print
                                    //buffer until something else
                                    //gets printed.
                        ps.print("\f");
                                    //flush buffer and close
                        ps.close();
                }
                catch (Exception e) {
                        System.out.println("Exception occurred: " + e);
                }
        }
}

          The bigger problem is if you wish to change the characteristics of
          the port (e.g. baud rate, parity, etc). Java currently offers no
          portable way to do this. You will need to use a native method, or
          execute a system command. At least two companies have written a
          library to drive the port. See
               http://www.sc-systems.com has a library for Windows95,
               WindowsNT, OS/2, Mac PPC, Solaris Sparc, Linux x86, FreeBSD
               x86, HP/UX PA-RISC, and possibly others too.
               http://www.cd.com/portio
          In addition, there is a Unix serial port utility available with
          source at:
     http://www.blackdown.org/java-linux/downloads/Private/jarvi/rxtx-1.1.1p2.tar.gz

     6.4 How do I append to a file?
          A. There are two ways. JDK 1.1 introduced new constructors for two
          of the output classes, that allowed you to set a boolean flag:

               public FileWriter(String fileName, boolean append) throws IOException
               public FileOutputStream(String name, boolean append) throws IOException

          Another way is to do this:

               RandomAccessFile fd = new RandomAccessFile(file,"rw");
               fd.seek(fd.length());

          Then write using fd. Note that the latter method does not take
          advantage of the "append" mode present in many operating systems
          (such as all unixes). Such a difference may make a difference with
          multiple processes or threads appending to the same output file.
          This can happen frequently, even if not intended by the
          programmer, e.g. with logfiles in multitasking environments. With
          the lack of file-locking mechanisms in Java the issue becomes even
          more significant.
     6.5 Is it possible to lock a file using Java ?
          A. Java does not feature an API to lock a file or regions within a
          file. Code that needs to do this must take one of three
          approaches:
               1. implement an advisory locking scheme using features that
               Java does have (synchronized methods). This allows you to
               lock files against use by other Java code running in the same
               JVM.


               2. Use an atomic operation like "file delete" and have all
               processes (Java and non-Java) follow the same protocol: if
               the file was deleted by you, you have the lock, and you
               create the file again to give up the lock.


               3. make calls to native code to issue the locking ioctls.
               This approach is not portable, but gives you a shot at having
               your locks respected by other programs using standard locking
               ioctls outside Java.
     6.6 How do I make the keyboard beep in Java?
          A. In JDK 1.1, java.awt.Toolkit has the method beep(). It does not
          work on NT 4.0 (bug).

               System.out.print("\07");
               System.out.flush();

          should work, and works in JDK 1.0.2, too. That's the ASCII BEL
          character (Java doesn't support the C abstraction of '\a' for an
          alert character).
     6.7 How do I execute a command from Java? How do I do I/O redirection
     in Java using exec() ?
          A. See answer to question 10.5
     6.8 How do you do file I/O from an applet?
          A. For security reasons, untrusted applets accessed across the
          network are restricted from doing certain operations, including
          I/O. This prevents rogue applets from sending out your private
          data, or deleting it. A trusted (signed) applet can perform these
          operations (JDK 1.1 on).

          The following suggestion is for server-side input.

          - You can read a file on the server if you can create a URL
          referencing the file. Then open a stream, then use any of the
          stream-based methods to read.

          This allows reading but not writing. It requires an http demon
          running on the server, which will usually be the case.

               try{
                  URL url = new URL("http://somewhere.com/test.txt");
                  // or URL url = new URL( getDocumentBase(), filename);
                  DataInputStream dis = new DataInputStream(url.openStream());
                  String s = dis.readLine(); //read till you get a null line.
               } catch(MalformedURLException e){System.out.println("URLException:"+e);}
               catch(IOException e){System.out.println("IOException:"+e);}

          You cannot write a file on the server this way.

          The following suggestions are for server-side output.

          It absolutely requires the cooperation of the server to allow an
          applet to write a file to the server. This cooperation may take
          any of several forms:
             + ftp server process
             + file server (custom written)
             + listening on a socket for data from applets
             + CGI script
             + Java RMI (remote method invocation)
          In particular:
               1. Or open a socket back to the server and read/write the
               data. Have a process on the server that listens for socket
               connections from applets and does the requisite I/O. This
               does I/O on the server.

               2. Or use a CGI script or servlet on the server to write when
               browsed. There is some source at
               ftp://ftp.oyster.co.uk/pub/java/fileIO/

               The following suggestions are for client-side I/O.

               3. Use a trusted applet (see section on security). This will
               permit local I/O without any of the restraints mentioned
               above. In this regard, the appletviewer and many other
               browsers regard applets loaded from a local filesystem
               (rather than across the net) as being more trustworthy, and
               perhaps even allowed to do I/O.

               4. Or use a browser that has a security policy that is
               configured to allow file I/O (such as Sun's appletviewer).

               Also see the answer to question 5.2 regarding the JENI
               library.
     6.9 I used a C program to write a binary file. When I instatiate a
     DataInputStream on the file in Java, and try to readInt, I do not get
     the correct numbers. Why is this?
          A. Java does everything in network byte order (big-endian order),
          as do many computers including Motorola, and SPARC. The Intel x86
          uses little endian order in which the 4 bytes of an int are stored
          least significant first. Rearranging the bytes on the way in will
          get you the results you need. This is only necessary when the file
          was written by a non-Java program on a little endian machine such
          as a PC.

          The following code will byte-swap little-endian integers into
          network standard order

               public int swap(int i) {
                   int byte0 = i & 0xff;
                   int byte1 = (i>>8) & 0xff;
                   int byte2 = (i>>16) & 0xff;
                   int byte3 = (i>>24) & 0xff;
                   // swap the byte order
                   return (byte0<<24) | (byte1<<16) | (byte2<<8) | byte3;
               }

     6.10 How do I make I/O faster? My file copy program is slow.
          A. This is the purpose of BufferedInputStream. It is a flaw in
          Java that buffered I/O is not the default, with a flag or
          different constructor to turn it off. I/O is the second worst
          designed package in Java, after the Date class.
     6.11 How do I do formatted I/O of floating point numbers?
          A. Look at http://www.apl.jhu.edu/~hall/java/CoreJava-Format.html
          for the html with the javadoc info. The actual file is called
          http://www.apl.jhu.edu/~hall/java/CoreJava-Format.java However,
          you must rename the file to Format.java for it to compile.

          Although many utilities claim to handle all varieties of C's
          printf, as far as has been found, this is the only one to
          correctly handle the equivalent of %e in printf.
     6.12 How do I read numbers in exponential format in Java?
          A: The program below (written by Steve Chapel) uses
          StreamTokenizer to read data from the standard input and
          recognizes doubles in exponential format (e.g. -1.23e-45).

               import java.io.*;

               public class ReadExponential {
                   public static void main(String argv[]) {
                       DataInputStream in = new DataInputStream(System.in);
                       StreamTokenizer st = new StreamTokenizer(in);
                       try {
                           while (st.nextToken() != StreamTokenizer.TT_EOF) {
                               switch (st.ttype) {
                               case StreamTokenizer.TT_NUMBER:
                                   double num = st.nval;
                                   int exp = 0;
                                   st.ordinaryChars('\0', ' ');
                                   st.nextToken();
                                   st.whitespaceChars('\0', ' ');
                                   if (st.ttype == StreamTokenizer.TT_WORD &&
                                       Character.toUpperCase(st.sval.charAt(0)) == 'E') {
                                       try {
                                           exp = Integer.parseInt(st.sval.substring(1));
                                       } catch (NumberFormatException e) {
                                           st.pushBack();
                                       }
                                   } else if (st.ttype < 0 || st.ttype > ' ')
                                       st.pushBack();
                                   System.out.println("Num  " + num * Math.pow(10, exp));
                                   break;
                               case StreamTokenizer.TT_WORD:
                                   System.out.println("Word " + st.sval);
                                   break;
                               default:
                                   System.out.println("Char '" + (char) st.ttype + "'");
                                   break;
                               }
                           }
                       } catch (IOException e) {
                           System.out.println("IOException: " + e);
                       }
                   }
               }

     6.13 I'm trying to read in a character from a text file using the
     DataInputStream's readChar() method. However, when I print it out, I
     get ?'s.
          A. Remember that Java characters are 16-bit Unicode characters,
          while many hosts systems store characters as 8-bit ASCII
          characters. Therefore, to read individual chacters from a text
          file, you need to ensure the proper conversion. The proper way to
          do this is to use an InputStreamReader, which converts from 8 to
          16 bit streams:

               FileInputStream fis = new FileInputStream("myfile.txt");
               InputStreamReader isr = new InputStreamReader(fis);
               char c3 = (char) isr.read();

          The less-favored way (because it is not so portable, as the
          encodings translation is not done) is just to read a byte and cast
          it into a character:

               FileInputStream fis = new FileInputStream("myfile.txt");
               DataInputStream dis = new DataInputStream(fis);
               char c1 = (char) dis.readByte();

     6.14 How do I delete a directory in Java?
          A. JDK 1.0 did not support directory removal. JDK 1.1 supports
          directory removal with the method:
               public boolean delete() in class java.io.File
          Make sure you don't have any open streams in the directory you're
          trying to remove. Do a close() on all streams, even if the
          underlying file is gone.
     6.15 How do I tell how much disk space is free in Java?
          A. There currently aren't any good Java APIs for system
          introspection. There is no Java way to control processes, or look
          at system resources. You have to Runtime.getRuntime.exec() to do
          "df" on unix or "dir" on Windows right now.
     6.16 What is the difference between the various ZIP formats: ZIP, GZIP,
     and PKZIP?
          A. GZIP files (the Gnu format) are not archives and compress only
          one file. GZIP is essentially a one file subset of the Zip format.
          Zip is an archive file format, popularized on PCs, that contains
          multiple compressed files. PKZIP is a set of
          commercially-available programs that create Zip files. All three
          PKZIP, GZIP and Zip use the deflate compression format, which is
          based on the LZ77 algorithm. This compression is also used by the
          zlib library and the PNG graphics file format. The format is
          specified in RFCs 1950, 1951, and 1952, and is unencumbered by
          licenses or patents.

          An alternative compression technology, LZW compression, is
          encumbered by Unisys's patent. LZW is used in GIF files and by the
          Unix compress command. Luckily, as well as being free from patent
          restrictions, LZ77 also gives better compression than LZW. LZW is
          the initial letters of the last names of the three computer
          scientists who developed the algorithm (Lempel, Ziv, Welch).

          The basic classes (all in java.util.zip) that read LZ77 Zip format
          are Deflater and Inflater. These are used by the stream classes
          DeflaterOutputStream and InflaterInputStream. The java.util.zip
          classes GZIPInputStream and ZipInputStream inherit from
          InflaterInputStream.

          PKZIP is a commercial program for DOS, Windows, and OS/2, sold by
          PKWARE Their FAQ (http://www.pkware.com/zipgfaq.html) specifically
          says
               "Because PKWARE has dedicated the .ZIP file format to the
               public domain, it is possible for other people to write
               programs which can read .ZIP files. NOTE THAT THE PKZIP,
               PKUNZIP, PKSFX PROGRAMS AND THEIR ASSOCIATED SOURCE CODE AND
               SUPPORT PROGRAMS ARE THE EXCLUSIVE PROPERTY OF PKWARE INC.
               AND ARE NOT PUBLIC DOMAIN SOFTWARE PROGRAMS.
          PKZIP's free (C language) source code was the basis for the JDK1.1
          Zip package AND the GNU GZIP program. The Zip classes' Javadocs
          acknowledge this.

          The "other people" PKZIP's FAQ refers to is the InfoZIP project, a
          bunch of hackers spread over the world producing free software
          that works on most ANSI C compilers and platforms. See
               http://www.cdrom.com/pub/infozip/
          Jar files are in PKZIP format, but are not as complete as a full
          filesystem archive format since file permissions are not saved.
          Some versions of WinZip are known to be inadequate for processing
          the full PKZIP format. Use infozip.

     6.17 How can I use characters other than ASCII in Java?
          A.Look for the article titled "Adding Fonts to the Java Runtime"
          at
          http://java.sun.com/products/jdk/1.1/docs/guide/intl/fontprop.html.

          This article explains how to add fonts to Sun's JDK, using the
          font.properties file. [If anyone has similar information for
          Netscape or IE, please send it in].
     6.18 I did a read from a Buffered stream, and I got fewer bytes than I
     specified, even though there are more bytes in the Stream.
          A. Correct. This is one of the "gotcha's" of buffered input. The
          system will try hard to buffer reads, even to the extent of
          sending you less data than you asked for. Nothing is lost of
          course, and you are always told how many bytes you actually got.
          You will get the remaining bytes on a subsequent read. This is
          also true with network operations (reads on sockets etc) and has
          always been true regardless of the language used. You really have
          to look at the "amount of data returned" field when using the
          read(byte[], int, int) method of BufferedInputStream.
     6.19 How do I get a directory listing of the root directory C:\ on a
     PC?
          A. The obvious approach of calling File.list("C:\"); does not
          work. There are two reasons why this fails. First, slash is an
          escape character in Java, so if you want a literal slash, you have
          to repeat it. Second, you need to give the name of the directory,
          i.e. dot. Putting this together, either of the following calls
          will work

          File.list("C:\\.");
          or
          File.list("C:/.");

                                ------------------

7. NETWORKING & DISTRIBUTED OBJECTS

     7.1 Should I use CORBA in preference to RMI? Or DCOM? Or what?
          A. If your distributed programs are all in Java, then RMI provides
          a simpler mechanism that allows the transfer of code,
          pass-by-value of real Java objects, and automatic garbage
          collection of remote objects.

          If you need to connect to C++ (or other language) systems or you
          need CORBA-specific services, then CORBA is your choice.
               http://www.javaworld.com/javaworld/jw-10-1997/jw-10-corbajava.html
          has a good intro to CORBA in the Java world.

          In July 1997, Sun announced that it was aligning RMI to work more
          closely with CORBA. Sun is simply adding an IIOP transport layer
          to RMI to support interoperability with CORBA. Java programs can
          then access CORBA-based objects through IIOP, the OMG's
          CORBA-based protocol. This is very good news for those building
          heterogenous Enterprise systems, although it will take some
          additions to IIOP to support the pieces that RMI uses.

          Microsoft spokespeople have tried to promote DCOM by spreading
          misinformation that RMI is changing or being dropped. That is
          totally wrong. The RMI API continues unchanged in its current
          form. Using DCOM would restrict your code to only ever run on
          Microsoft platforms using Intel hardware, and negates the "write
          once, run anywhere" Java philosophy. Non-portable, single vendor
          code should be avoided.
     7.2 Why does <my java debugger/IDE/other> hang for a couple of minutes
     if my Windows PC is not dialed up to the Internet?
          A. Java has networking support built in. When the Java program
          starts the Winsock dll automatically gets loaded. The first thing
          this does is to try to resolve the fully qualified domain name for
          your machine under the name "localhost". If your system doesn't
          have this name mapped, it will try to query a nameserver on the
          internet, which is typically (on a PC) your dialup ISP. So it
          either prompts you to connect to the ISP, or waits till the
          attempt times out.

          You can avoid the problem by giving your system another way to
          resolve DNS names. Edit the hosts file for your system (found in
          %windir%\hosts on Win95 and %windir%\system32\drivers\etc\hosts on
          NT) so that localhost and the full domain name are both mentioned.
          So if my system is called goober.best.com change the hosts file
          from
               127.0.0.1 localhost
          to
               127.0.0.1 goober.best.com localhost
          [If any networking gurus have improvements to this process,
          send'em in!]

          There is a longer, different procedure at
          http://www.best.com/~pvdl Please send me mail ([e-mail]) if
          either or both of these work or don't work for you, so I can
          update the FAQ.

     7.3 If I call the InetAddress.getByName() method with an
     IP-address-string argument, like "192.168.0.1", I get an
     UnknownHostException on some platforms, but not others. Code like

          Socket sock = new Socket("155.152.5.1", 23);

     triggers the exception. Why?
          A. This is a platform difference that arises out of different
          semantics in the underlying network libraries, and is [said to be,
          but subject to confirmation] fixed in JDK 1.1. On Solaris and
          Windows NT, the IP address string only works for IP addresses that
          have an associated hostname. On Linux and Windows 95, the IP
          address string works in all cases.
          http://www.cdt.luth.se/~peppar/java/InetAddress/ has a workaround.

          When InetAddress is instantiated with an IP address, a reverse DNS
          lookup is done. If the IP address is not associated with a valid
          hostname, the instantiation will fail. This is part of anti
          DNS-spoofing, and in JDK 1.1 works because the reverse lookup will
          not occur until the hostname is asked for. So in JDK 1.1,

               InetAddress in = InetAddress.getByName("155.152.5.1");

          should always work.

          [Note: this info is still to be confirmed. Net gurus?]
     7.4 I am using JDK 1.1.1 on Windows95, and when I start jdb I get
     "Uncaught exception: java.lang.UnsatisfiedLinkError no winawt in shared
     library path"

     The same program works OK using jdk1.1
          A. It *sounds* like your java\bin directory is not on your PATH
          and so the system can't find the winawt DLL.

          But actually the problem is the version of Microsoft's Visual C++
          that was used to build the product. VC++ 4.2 incorrectly generates
          code that depends on MSCVRT.DLL or in the case of java_g,
          MSVCRTD.DLL. These DLLs are not present in (some versions of)
          Win95. To make things even more interesting, some versions of
          Win95 (yes, there are at least four different ones...) ship with a
          broken MSVCRT.DLL (and MSVCRTD.DLL?) that seems to work, only it
          doesn't, and after a while it dies.

          Sun linked the winawt_g.dll with VC++ 4.2, which wrongly brought
          in MSVCRTD.DLL, the debug version of the VC++ runtime. You have to
          get that library from somewhere (like, say, VC++)
          in order to get jdb to run.

          You'll hit this problem any time you try to debug 1.1.1 code with
          jdb on a win95 system that doesn't have VC++ (or the MSVCRTD.DLL
          library from some other source) installed. At least this is a
          problem you can solve without waiting for the next release.

          Some people say that the missing library has been seen at
          http://cag-www.lcs.mit.edu/curl/Binaries/PC/ Others say you need
          to buy VC++ to get it.

     7.5 I want to pass a class file to willing recipients who are using my
     applet. Any ideas how?
          A. You could use a trick: put your .class file(s) in a .zip
          archive and use showDocument() on the URL. A person accessing this
          will get a dialog box put up asking them about saving the file to
          their local hard disk. You can see this in action and try it out
          yourself at:
               http://www.best.com/~rmlynch/saveit.html

     7.6 How do you succeed with new URL(someURL) from behind a proxy
     server?
          A. Tell the run time system what you are trying to do, like this:
               java -DproxySet=true -DproxyHost=proxy_host
               -DproxyPort=proxy_port \MyJavaProgram
          note proxyPort is optional and it defaults to 80.

     7.7 What is "swizzle", as in "Swizzle this object?"
          A. It means serialize. To swizzle an object is to recursively
          serialize or flatten composed objects.

     7.8 I have been using the Serializing capabilities in 1.1 to save some
     objects to disk. I added a new field to one of my objects that get
     serialized and now deserializing my old data no longer works. I get
     this exception:

          java.io.InvalidClassException: MacroData; Local class not compatible

          A. You need to add a declaration such as

               static final long serialVersionUID = 4021215565287364875L;

          in the modified class. The actual value of this long is supplied
          by the "serialver" utilitity suppied with the JDK. Any versions of
          a class other than the first version require this static to be
          defined
          in the class. This is how versioning is achieved.
     7.9 My socket code looks good, but is broken!
          A. When using sockets you typically open both inward and outward
          streams. If you close one of them, the other seems to 'break'
          instantly. Check whether this is happening for you, by adding the
          matched pair.

          [comments from net gurus welcome]
     7.10 How do I map between IP address and hostname?
          A. In Java 1.1 (earlier releases were buggy) use:

               String host = InetAddress.getByName("211.10.2.119").getHostName();

     7.11 How do I embed an anchor in a URL? Just putting it as part of the
     string in the constructor doesn't work.
          A. Like this:

               URL url = new URL("http://www.my_domain.com/my_page.html");
               URL anchor = new URL(url, "#section2");
               this.getAppletContext().showDocument(anchor);

     7.12 RMI seems to have stopped working for me in JDK 1.1. Why is this?
          A. The rules for where the client looks for a stub class seem to
          have changed making it necessary to reset your class path on the
          client after starting the rmi registry. In particular, it looks
          like rmic was not updated to the new "don't need $CLASSPATH"
          convention as the compiler was.

          There are several very good sources available from Sun which cover
          many simple and advanced RMI problems. They are:
             + The documentation, of course:
                    http://java.sun.com/products/jdk/1.1/docs/guide/rmi/index.html
             + Dedicated FAQs on RMI and Object Serialization
                    http://java.sun.com/products/jdk/rmi/faq.html
             + Mailing list [e-mail] with archive at
                    http://chatsubo.javasoft.com/email/rmi-users/
          Visit the archive!
     7.13 After a number of RMI client to server connections (55 on my
     system), subsequent RMI clients trying to connect fail. Why?
          A. You are hitting the default limit of 64 open file descriptors.
          Try increasing the limit in your OS.

          In addition there is currently a practical RMI connection limit
          imposed by the scalability of the VM and the performance of object
          serialization. In JDK 1.2 this is addressed. The actual number of
          active clients you will be able to support will depend on the
          workload mix you have (i.e. the number of clients, how often they
          talk to the server, and how much work must be done per call).
     7.14 How do I POST to a CGI script from an applet?
          A. For an untrusted applet, the CGI script can only be on the
          server that served the applet. Then use code like this:

               try { sock = new Socket(host, 80);
               dock = new DataOutputStream(sock.getOutputStream());
               dock.writeBytes("POST "+cgiloc+" HTTP/1.0\n");
               dock.writeBytes("Content-type: text/html\n");
               dock.writeBytes("Content-length: " +my_string.length() + "\n\n");
               dock.writeBytes(my_string+"\n");
               dock.close();
               sock.close();

               uresp = new URL(getDocumentBase(),"respond.html");
               getAppletContext().showDocument(uresp); }

          The my_string contains the data you want to POST to the CGI
          script. The string should be encoded in the special way CGI
          expects. The class method java.net.URLEncoder.encode(my_string)
          will do it.

          If you request a url via the URLConnection/HttpURLConnection, the
          server sets the content type, and your applet can use
          URLConnection.getContentType() to get the type. Alternatively, use
          setRequestProperty to set it, like this:

               url = new URL(cgiUrl);
               urlc = url.openConnection();
               urlc.setRequestProperty("Content-type", "application/x-www-form-urlencoded");

                                   -----------------

8. MULTI-MEDIA

     8.1 Why won't my audio file play?
          A. Java 1.1 and earlier releases use one audio format exclusively.
          The audio file must be in .au format, recorded at 8 KHz, mono, in
          mu-law encoding. If your audio clip is in a different format
          (e.g., .wav) or a different frequency it must be converted to the
          exact specifications above before Java can play it. Support for
          .wav and other formats is part of the Java Media Framework coming
          in JDK 1.2.

          Search at www.yahoo.com for GoldWave for Win 95, sox for Unix and
          similar conversion utilities for other systems. One conversion
          utility in Java is at http://saturn.math.uaa.alaska.edu/~hursha
          The source of a Java class to play linear PCM .WAV files is at:
          http://www.shef.ac.uk/~cs1mjp/Java/WhiteBoard/WavePlayer.html It
          can be used in any Java application or applet.
     8.2 Does Java support Animated GIFs?
          A. Java 1.0.2 and earlier releases use GIF and JPEG formats, and
          do not use the GIF89 animated GIF format. (An animated GIF is one
          that contains successive frames of an image, so when they are
          displayed in quick sequence the image appears to contain
          movement). When you display an animated GIF in Java 1.0.2, you
          will just get the first frame. There doesn't appear to be any easy
          way to get other frames from the image.

          The advantage of an animated GIF file is that there is only one
          file to download, and it is simple to do simple animations. The
          advantage of programmatic control over individual frames is that
          you control the rate and order of displaying them.

          Here's a surprise: JDK 1.1 supports the animated display of
          animated GIFs. For simple animations animated GIFs are a lot
          easier and lighter-weight than coding an animation explicitly.

          8.2.0  How do I create animated GIFs?
               A. Use GIFanimator from ULead (said to be the best)
               http://www.ulead.com, or GIF Construction Set from Alchemy
               Mindworks

          8.2.1 How do I prevent animated GIFs from flashing while
          displaying?
               A. The problem is most likely that in your paint method you
               have

                    g.drawImage(img, ix, iy, this);

               You should change this to

                    g.drawImage(img, ix, iy, getBackground(), this);

               This will change all the transparent regions of the image to
               the background color before painting to the screen. If you
               paint transparent images directly to the screen they flicker.

               If that does not solve it then check that:
                    1. ImageUpdate is

                    public boolean imageUpdate(Image img, int flags, int x,
                                               int y, int width, int height) {
                        if ((flags & (FRAMEBITS|ALLBITS))!= 0) {
                           repaint();
                        }

                        return (flags & (ALLBITS|ABORT)) == 0;
                    }

                    2. update is

                    public void update(Graphics g) {
                        paint(g);
                    }

               If you have a background Image behind the partly transparent
               animated GIF you will have to double buffer. You can crop the
               backgound image so you won't have to double buffer the full
               app and waste too much memory.
     8.3 Does Java support transparent GIFs?
          A. GIF89a images with a transparent background show up as
          transparent without further filtering. This has been supported
          from 1.0 on. Java correctly displays both animated GIFs and
          transparent GIFs.

          Even better, you can fill the transparent pixels with a color (so
          they appear non-transparent in Java). Just pass the fill color
          explicitly:

               drawImage(img, x, y, w, h, fillcolor, this);

          Further, you can filter the pixels of an Image to turn any bits
          you wish transparent. However, the most you can do is reveal what
          is underneath the image. You cannot reveal what is underneath the
          applet (i.e. on the browser itself). By default applets have a
          plain grey background.
     8.4 How do I play video in Java?
          A. Use the Java Media Framework Player API

          The spec can be found at
               http://java.sun.com/products/java-media/jmf/mediaplayer/
          Intel has released a SDK for the Java Media Framework Player API.
          The SDK is for Windows 95 and Windows NT For more information, see
               http://developer.intel.com/ial/jmedia
          SGI has released an implementation of JMF for IRIX:
               See http://www.sgi.com/Products/motion/
     8.5 How can I play *.au files from an application?
          A. You've got 2 options:
               a. Use the AudioClip or AudioPlayer class in sun.audio
                    http://www.javaworld.com/javaworld/javatips/jw-javatip24.html
               If you take this option, you can only play AU files. Worse,
               your code is no longer 100% pure Java, as it relies on a
               vendor library.

                    import sun.audio.*;

                    URL url; ...
                    AudioStream audiostream = new AudioStream(url.openStream());
                    AudioPlayer.player.start(audiostream);
                    ...
                    AudioPlayer.player.stop(audiostream);

               b. Use the new Java Media Framework API, allowing a wide
               range of video and audio formats to be played back. See
               previous question for implementations of this.
     8.6 How do I read in an image file, in an application?
          A. Use

               Image img = Toolkit.getDefaultToolkit().getImage(fname);

     8.7 When I initialize a component, I call MyComponent.getImage() to get
     its image. createImage() returns null! I know the image works
     elsewhere. What's wrong?
          A. A peer component needs to exist for your component before you
          can get its image. This is done by the method addNotify() (surely
          one of the most poorly named methods in all Java -- it doesn't
          mean "add a Notify". It means "Notify that the Component has been
          added to a Container". This tells the system, "whoops, I'd better
          create the peer for this Component right away). addNotify will be
          called for you when you add your component to a container.

          If you override addNotify(), don't forget to call
          super.addNotify() in your overriding version.
     8.8 How can I force a reload a fresh version of an image into my
     applet? My image file is changed periodically, and I want the applet to
     go and retrieve it, not cache it.
          A. You need to turn off caching for the URL.

               URL url = null;
               URLConnection con;
               try {
                   url = new URL(getDocumentBase(),"image.jpg");
                   con = url.openConnection();
                   con.setUseCaches(false);
               } catch (MalformedURLException e1) {System.err.println(e1.getMessage());}
               catch (IOException e2) {System.er.println(e2.getMessage());}

          Note: some programmers have reported that it caches anyway, even
          if that do this. That is a browser bug.

          One programmer reported that even after turning off caching and
          calling image.flush() before getImage(..), he was still seeing the
          same picture even though it had been changed on the server.

          He worked out a solution: access the image via a cgi script that
          returned a URL. This redirects the browser, and he put in an
          Expires: header as well to force the reload. Painful and
          laborious, but it got the result.
     8.9 How can I save an Image file to disk in jpg or gif format?
          A. A number of people have written utilities to do that. One of
          them is available at the same place as this FAQ:
               http://www.best.com/~pvdl
               Jef Poskanzer has written an abstract ImageEncoder class and
               implemented it for GIFs and PPMs. Those are at
               http://www.acme.com/java/software/
               Hong Shi wrote a PPM to JPEG convertor.
               http://www.ctr.columbia.edu/~hshi/report6880.htm
               Florian Raemy has written a program that encodes a JPEG then
               decodes it again. http://lcavwww.epfl.ch/demos/jpeg.html
     8.10 What causes this problem?

          $ appletviewer m.html
          Premature end of JPEG file
          sun.awt.image.ImageFormatException: JPEG datastream contains no image
          at sun.awt.image.JPEGImageDecoder.produceImage(JPEGImageDecoder.java:133)
          at sun.awt.image.InputStreamImageSource.doFetch(InputStreamImageSource.java:215)
          at sun.awt.image.ImageFetcher.run(ImageFetcher.java:98)

          A. There's a known bug in early releases of the JDK which can
          cause the above failure when reading a JPEG across a slow
          connection. The failure only occurs if the JPEG contains a large
          application data block (APPn marker) --- the problem is that the
          JPEG decoder is trying to skip over the APPn and failing if not
          all of the APPn has been received yet. The quoted error message is
          only one of several possible complaints, but they all stem from
          the same root.

          Photoshop is the most common source of JPEGs containing oversize
          APPn blocks. In particular, if you allow Photoshop 4 to save a
          thumbnail (preview) in a JPEG, the thumbnail plumps up Photoshop's
          private APPn marker to several K, which is usually enough to cause
          this problem.

          There are several possible workarounds:
               1. Get a newer JDK --- this problem is said to be fixed in
               1.1. (If you are putting images up on the Web, this isn't
               much of a solution, because you can't assume visitors to your
               site have an up-to-date Java installation.)

               2. When making JPEGs for Web use from Photoshop, make sure
               you have turned off the "save thumbnails" preference. (This
               is a good idea quite aside from bug workarounds, because the
               thumbnail is just a waste of download time as far as a Web
               browser is
               concerned.) You might still have a problem if you've got
               verbose comments or lots of paths being saved into the file,
               but 99% of the time, getting rid of the thumbnail will make
               Photoshop's APPn small enough to not trigger the Java bug.

               3. Use a tool such as 'jpegtran' (from the Independent JPEG
               Group) to strip out the Photoshop APPn entirely without any
               loss of image quality. Recommended answer for the compulsive
               byte-trimmer.

               4. (Last resort) Load and resave the image in a different
               image editor that won't insert any APPn or other overhead
               data. This implies a JPEG generational loss, so I don't
               recommend it if you are picky about image quality.

               Any large overhead marker will cause the same problem; 4K of
               comment text, say, in a COM marker. So Photoshop is not the
               only source of tickling this bug.
     8.11 How can I convert between GIF and JPEG formats?
          A. In a word: don't.

          There's hardly any overlap between the set of images that JPEG
          works well on and the set that GIF works well on. Sometimes, with
          enough care, you can get an acceptable conversion ... but most of
          the time gif<->jpeg conversion will just turn your image to mush.
          Far better to pick the right format in the first place.

          For more info see the JPEG FAQ at
          http://www.faqs.org/faqs/jpeg-faq/
     8.12 If you have an InputStream (rather than a file) that contains an
     Image, how can you display it?
          A: Using this method, and some adroit shuffling.
               Toolkit.getImage(URL url)
          Create a thread that pretends to be an http server. Make it listen
          to some port (8888 for example) for incoming requests. When the
          thread gets a request, it should simply whisk up the appropriate
          http headers and follow it by the InputStream. Thus the component
          that has the input stream and wants to do the getImage(url) can
          now invoke:

          Toolkit.getImage("localhost:8888/")

          The thread will act as a stream-to-url adapter, and send back the
          data It saves you from having to read 200K of JPEG data before you
          can begin drawing anything.
     8.13 How can I record sounds in Java?
          A. The Java Media Framework coming in JDK 1.2 will support this.
          In the meantime, there is a package for Win95/NT available at
          http://www.scrawl.com/store/

          It supports 8, 16-bit, stereo, mono, 11025, 22050, 44100 Hz
          record/play, load/save .WAV files.
     8.14 Does Java have any built-in support for displaying HTML?
          A. The JDK doesn't currently have any support for rendering HTML.
          The Swing software that will appear in JDK 1.2 has an elementary
          (graphics, tables, text) HTML bean that is good enough for simple
          rendering (help files, email, etc). Other alternatives include:
             + JavaBrowser http://www.ii.uib.no/~alexey/jb/index.html Free
               source, free for use under GNU LGPL licence, HTML 2.0 (sort
               of).
             + ICE Browser - Java Bean Component
               http://www.bgnett.no/datatech/ICEBrowser/ Free binaries for
               use in free applications. Commercial licensing available
               including source - flat fee licence. Thin HTML client!
               Lightweight! HTML 3.2
             + HotJava HTML Component - Java Bean Component
               http://www.javasoft.com/products/hotjava/bean/index.html $195
               for private use binary licence. HTML 3.2
             + HTML browser (free source)
               http://barium.tn.tudelft.nl/people/gool/java/html/Html.html
             + Web Window Browser http://www.opencube.com/example_wwb.htm
               $139 - no sources.
             + jHelp ($20-650)
               http://w3.nai.net/~rvdi/jhelp/jhelp2/jhelp.html jHelp is a
               HTML browser component written in Java, HTML 2.0

                                   -----------------

9. SECURITY

     9.1 What is a "trusted applet"?
          A. JDK 1.1 introduced the notion of a "trusted applet" which is
          one that has been cryptographically-signed to guarantee its origin
          and make it tamper resistant. Trusted applets can be granted more
          system access privileges than untrusted applets.

          You preconfigure your browser with a list of whose X.509
          certificate you trust, and then applets arrive with X.509's
          attesting to their keys. It's easier than it sounds.
     9.2 What is the story with Java and viruses? What is the blackwidow
     virus?
          A. Java was designed with security in mind. The security features
          make it very difficult, probably impossible, to attach a virus
          (self- copying code) to a Java applet. As far as is known, there
          has never been a Java virus.

          There has been mention of a "Java virus" called "BlackWidow" in
          the media (it was mentioned in Unigram in late 1996, and obliquely
          on the RISKS newsletter in February 1997). A request to the editor
          of Unigram for more information brought the answer that there was
          no more information, it was just a report of a rumor. As far as is
          known, this story exists *only* as rumors reported on by the
          press. There is no actual Java virus or blackwidow virus (there
          was a legitimate commercial product of that name, since renamed).
          If anyone has more concrete information about a virus that can
          attack a Java applet (again, this is thought to be impossible),
          please would they contact the FAQ maintainer with details.
     9.3 Why do I get the warning string at the bottom of popup
     windows "Unsigned Java Applet Window" in my applets?
          A. This is a security feature, to make certain that users can
          always tell that a window asking for their password and credit
          card details (or whatever) is from an applet. There should be no
          way for an untrusted applet to work around this message. See also
          the answer to 4.1.3.
     9.4 Where can I find crypto libraries for Java?
          A. Cryptographic libraries are not part of the Java release
          because US Government policy classifies strong cryptography under
          the same rules as munitions. Its export is regulated under the
          International Traffic in Arms Regulations. Many people regard this
          as a Kafka-esque (and futile) attempt to stem the use of
          cryptography inside the US.

          A comprehensive and free crypto library (called Cryptix) is at
               http://www.systemics.com/software
          Another crypto library for Java is at
               http://www.acme.com/java/software/Package-Acme.Crypto.html
          Blowfish, CRC16, CRC32, DES, DES3, IDEA, RC4, ROT13 (can they
          really call that "crypto"?), and more.

          One commercial Java encryption source (from Ireland) is
               http://www.baltimore.ie/jcrypto.htm
          A complete crypto API for Java (with HTML documentation) at:
               http://www.geocities.com/SiliconValley/Heights/8298
          The library provides comprehensive and complete range of crypto
          library and functions covering DES, 3DES, IDEA, Blowfish ...and
          RSA, DH, DSA and PGP access to Java programmers. The crypto
          functions are based on the C cryptlib, by Peter Gutmann. It would
          be illegal to export this under current US government rules, but
          the author of the code is outside the US, and not subject to US
          export regulations. Download it today before it becomes illegal.

          Also, early access to Sun's Java Cryptography Extension (JCE) is
          available for JDK 1.1 at:
               http://java.sun.com/products/jdk/1.1/jce
          (This may not be exported outside the USA and Canada).

          There's a Q&A archive at
               http://jeeves.javasoft.com/hypermail/java-security-archive/index.html

          An actual port of PGP v2.6.3i to Java is at
          http://tassun.math.nsc.ru
     9.5 How do I find out what these terms mean?
          A. Read Bruce Schneier's excellent book "Applied Cryptography 2nd
          Ed." for more info on what these terms mean. Read David Kahn's
          excellent (if exhaustive) book "The Codebreakers" for more info on
          the history and background of encryption.
     9.6 Where is Javasofts Security FAQ?
          A. Javasoft's security FAQs can be found at
          http://java.sun.com/docs/books/tutorial/applet/security/capabilities.html

          http://java.sun.com/sfaq/index.html
          See also
          http://java.sun.com/products/jdk/1.1/docs/guide/security/
                                ------------------

10. Java IDIOMS

     10.1  How do I convert a String to an int?

          A. There are several ways. The most straightforward is:

               int i = Integer.parseInt(<String>);

          or

               i = Integer.parseInt(<String>,<int radix>);

          Note: there are similar methods for Byte, Short, and Long.

               int i = Integer.valueOf(my_str).intValue();

          also works but involves the creation of an extra object. Note: use
          this for floating point values, as there are no parseDouble or
          parseFloat methods.

          float f = float.valueOf(my_str).floatValue();

     10.2 How do I convert an int to a string?
          A. String s = String.valueOf(i);
               or

                    String s = Integer.toString(i);

               or

                    String s = Integer.toString(i, radix);

               or

                    String s = "" + i; // briefer but may result in extra object allocation.

          Note: there are similar classes for Double, Float, Long, etc.
     10.3How do I print the hex value of an int?
          A. Another way is

               int i = 0xf1;
               System.out.println("i is hex " + Integer.toHexString(i) );

     10.4 How can you send a function pointer as an argument?
          A. Simple answer: use a "callback". Make the parameter an
          interface and pass an argument instance that implements that
          interface.

               public interface CallShow { public void Show( ); }

               public class ShowOff implements CallShow {
               public void Show( ) { .... }

               public class ShowMe implements CallShow {
               public void Show( ) { .... }

               public class UseShow { CallShow callthis;
               UseShow( CallShow withthis ) { callthis = withthis; }
               void ReadyToShow( ) { callthis.Show( ); }

               // in some other class that uses all this stuff:
               UseShow use_1 = new UseShow( new ShowOff() );
               UseShow use_2 = new UseShow( new ShowMe() );

          and then the ReadyToShow() method on use_1 or use_2 will call the
          appropriate method, as if you had stored a pointer to the method.
     10.5 How do I execute a command from Java?
          A. Use Runtime.getRuntime.exec( myCommandString ) Where
          myCommandString is something like "/full/pathname/command"
     10.6 How do I do I/O redirection in Java using exec() ?
          A. This solution works on Unix platforms using either JDK 1.0.2,
          or JDK 1.1. The trick is to use an array of Strings for the
          command line:

               String[] command = {"/bin/sh", "-c", "/bin/ls > out.dat"};

          If you don't do this, and simply use a single string, the shell
          will see the -c and /bin/ls and ignore everything else after that.
          It only expects a single argument after the -c.

          import java.io.*;
          import java.util.*;

          class IoRedirect {
              public static void main(String Argv[]) {
                  try {
                      String[] command = {"/bin/sh", "-c", "/bin/ls > out.dat"};
                      Process p = Runtime.getRuntime().exec(command);
                      p.waitFor();
                      System.out.println("return code: "+ p.exitValue());
                  }
                  catch (IOException e) {
                     System.err.println("IO error: " + e);
                  }
                  catch (InterruptedException e1) {
                     System.err.println("Exception: " + e1.getMessage());
                  }
              }
          }

     10.7 So why can't I exec common DOS commands this way (as in 10.6)?
          A. The reason is that many of the DOS commands are not individual
          programs, but merely "functions" of command.com. There is no
          DIR.EXE or COPY.EXE for example. Instead, one executes the command
          processor (shell) explicitly with a request to perform the builtin
          command, like so: Runtime.getRuntime().exec("command.com /c dir")
          for example. On NT, the command interpreter is "cmd.exe", so the
          statement would be
          Runtime.getRuntime().exec("cmd /c dir")

          This is a Windows, Windows95 and NT-specific problem.
     10.8 OK, how do I read the input from a command?
          A. As above (10.5, 10.6), adjusted like this:

               BufferedReader pOut=
                     new BufferedReader(
                     new InputStreamReader(p.getInputStream()));
               try {
                   String s = pOut.readLine();
                   while (s != null) {
                       System.out.println(s);
                       s = pOut.readLine();
                   }
               } catch (IOException e) { }

          Another possibility is to read chunks of whatever length as they
          come in:
          ...

               p = r.exec(cmd);
               InputStream is = p.getInputStream();
               int len;
               byte buf[] = new byte[1000];
               try {
                   while( (len = data.read(buf)) != -1 ) {
                   String str = new String(buf,0,0,len);
                   System.out.println( "Process out: " + str );
               }
               catch( java.io.EOFException eof ) {

               ...

               }

               catch( java.io.IOException ioe ) {

               ...

               }

     10.9 How do I compile code which has a cyclic dependency, i.e class
     pkg1.X contains a reference to class pkg2.Y ?
          A. You throw both classes at the compiler at the same time.
               javac pkg1/X.java pkg2/Y.java
     10.10 How can I store the errors from the javac compiler in a DOS file?
     javac foo.java > errorfile seems not to work
          A. javac write errors to stderr, so on NT use:
               javac myfile.java 2> errors.dat
          On Win95, this doesn't work (as command.com is very poor
          software), so you have to use the javac error redirection
          mechanism:
               javac -J-Djavac.pipe.output=true myfile.java > errors.txt
     10.11 How can I pretty-print Java source?
          A. Try http://www.CS.ORST.EDU/~speton/percolator/ (currently in
          Beta)
          Or http://www.parallax.co.uk/~rolf/download/jpp.pl
          Some Unix utilities work adequately:

          indent (fails with "//" comments though)
          cb (very few style choices though)

          alias printjava 'vgrind -lC++ -t -w \!* | lp'
          works pretty well too.
     10.12 What is the point of creating the temporary reference to
     this.layoutMgr? This code is from the 1.0 AWT, and the programmer was
     probably pretty skilled.

          public synchronized void layout() {
                 LayoutManager layoutMgr = this.layoutMgr;
                 if (layoutMgr != null) {
                    layoutMgr.layoutContainer(this);
                 }
          }

          A. The code makes a local copy of a global variable for one or
          both of two reasons.

          The first reason is that accessing local variables can be faster
          than accessing (non final) member variables It's good for loops or
          where there are many references in the source.

          The second reason is so that even if other threads update the
          global,

               this.layoutMgr = someOtherLayoutMgr;

          this method will still have a pointer to the original layoutMgr.

          If the local variable were omitted, and another thread used the
          setLayout() method to change layoutMgr to null between when the
          layout method checked for null and when it invoked layoutMgr's
          layoutContainer method, a NullPointerException would result.

          Note that the synchronized keyword on the layout method doesn't
          help any, since setLayout (which could make such a dire change)
          isn't synchronized. Synchronized methods only lock out other
          synchronized methods on this object. (The unhelpful synchronized
          keyword on the layout method is gone in JDK 1.1.)

          There are two alternative solutions. One would be to make
          setLayout synchronized and make layoutMgr private, so that it
          can't be set other ways. This provides a stronger form of thread
          serialization, in that you would never be able to see an old
          layout manager being used after it had been replaced. However, it
          is slower. Another option that provides no increase in thread
          serialization over the original would be to catch the
          NullPointerException.

          Threads programming is hard! This idiom was probably put in place
          by someone who got really bitten by this in the past.
     10.13 What is the difference between "a & b" and "a && b" ?
          A. "a & b" takes two boolean operands, or two integer operands. It
          always evaluates both operands. For booleans, it ANDs both
          operands together producing a boolean result. For integer types,
          it bitwise ANDs both operands together, producing a result that is
          the promoted type of the operands (i.e. long, or int). "|" is the
          corresponding bitwise OR operation. "^" is the corresponding
          bitwise XOR operation.

          "a && b" is a "conditional AND" which only takes boolean operands.
          It always avoids evaluating its second operand if possible. If a
          is evaluated to false, the AND result must be "false" and the b
          operand is not evaluated. This is sometimes called
          "short-circuited" evaluation. "||" is the corresponding
          short-circuited OR operation.

          Possible mnemonic: The longer operators "&&" or "||" try to
          shorten themselves by not evaluating the second operator if they
          can.
     10.14 If I create a thread, and then null out the reference to it, what
     happens to the thread? Does it get interrupted or what?
          A. The code looks like this:

               Thread t = new Thread( my_runnable_obj );
               t.start();
               ...
               t = null; // what happens to the thread?

          The answer is that *you* may no longer have a reference to the
          thread, but the JVM still does. Once a thread is started, and as
          long as it keeps running, it is a root object. Root objects are
          the starting points for "things in use" that the garbage collector
          uses.
     10.15 How do I calculate the number of days between two dates?
          A. There is no API for this (there should be), but you can
          calculate it by hand, like this:

               Calendar earlierDate = new GregorianCalendar();
               Calendar laterDate = new GregorianCalendar();

               earlierDate.set(1997, 1, 1, 0, 0, 0);    // Jan 01, 1997
               laterDate.set(1998, 1, 1, 0, 0, 0);      // Jan 01, 1998

               // the first getTime() returns a Date, the second takes
               // that Date object and returns millesecs since 1/1/70.
               // The API has misleading and horrible naming here, sorry.
               long duration =    laterDate.getTime().getTime()
                                - earlierDate.getTime().getTime();

               long nDays = duration / (24 * 60 * 60 * 1000);
               System.out.println("difference in days: " + nDays);

     10.16 How can a Java program determine the level of JDK support given
     by the underlying VM? I.e. is it running in a JDK 1.0.2 or 1.1 VM?
          A. Look at the java.version system property with:

               String ver = System.getProperty("java.version");

          There isn't a lot of standardization on the string contents
          however. Another possibility is to try { ... } a library method
          that didn't exist in JDK 1.0.2, and catch the exception. If you
          got an exception, it's a 1.0 implementation.
                                ------------------

11. FOR C and C++ AFFICIONADOS

     11.1 How do I translate C/C++ into Java or vice-versa?
          A. In general it is not simple to translate C/C++ into Java, as
          Java lacks the arbitrary pointer arithmetic of those languages. If
          your C code does not use pointer arithmetic, automatic translation
          gets a lot simpler. Try these URLs:
               http://www.ist.co.uk (search for X-Designer 4.6: Java
               edition).
               http://members.aol.com/laffra/c2j.html
               http://www.ilog.com/
               http://www.cs.wustl.edu/~schmidt/java_notes.html
          Going the other way there are currently three freely-available
          tools to translate Java into C. It seems that these have been done
          for hacking value, rather than practical purposes.

             + j2c from Japan,
                    http://www.webcity.co.jp/info/andoh/java/j2c.html
             + Toba from the Sumatra research project, translates 1.0.2
               .class files into .c source code
                    http://www.cs.arizona.edu/sumatra/toba
             + JCC from Nik Shaylor.
                    http://www.digiserve.com/nshaylor/jcc.html
          None of them support the AWT yet, and both j2c and JCC have
          additional restrictions.

          There's a product to convert Visual Basic to Java. Details at
               http://www.blackdirt.com and
                    http://www.javadelphi.com (also a Delphi-to-Java source
                    converter) and
                    http://www.tvobjects.com
               This program dumps info about the class file:
                    http://www.professionals.com/~cmcmanis/java/dump/index.html
               Chuck McManis was one of Sun's original Java whackers.

     11.2 How are finalizers different from C++ destructors?
          A. Java objects are not explicitly deleted and do not have
          destructors. Instead they are implicitly garbage collected when
          the JVM realises your program can no longer access them. Typically
          this technology is _not_ based on reference counting and _will_
          cope with circular references.

          Every object has a routine called finalize() which will be called
          before the object is collected. This is Java's nearest equivalent
          to C++'s destructor. However, it is not a good idea to rely on
          finalization for the timely freeing of resources.

          This is because garbage collection and hence finalization may be
          arbitrarily delayed, and may never happen at all if the program
          terminates before it runs out of memory. You should instead
          provide your objects with methods similar to Graphics.dispose() to
          free resources, and call the
          dispose() method explicitly when you have finished using them -
          typically within the "finally" clause of a "try/catch" block. You
          may then call your dispose() method from within your finalize()
          method as a last-ditch attempt to free the resource if someone
          forgets.

          Alas, all this means the C++ idiom of "object construction is
          resource aquisition" does not translate well to Java. However,
          note that 90% of destructors in C++ are there to free memory, and
          the GC means you don't need to do that in Java. As well as fixing
          an important source of bugs, the GC is essential to Java's
          security model; without it you could forge object references by
          preserving the reference after the object has been deleted.

          If your program appears to be crashing due to running out of some
          system resource (like File, Window or Graphics handles), it
          probably because the system is running out of handles before it
          has run out of memory. Check that you have called the dispose()
          method (or equivalent) on every object that uses system resources.
          You can help the GC a little bit more by explicitly
          NULLing out references that you've finished with.
     11.3 What's the Java equivalent of sizeof()?
          A: There isn't one. sizeof() in C and C++ is used in three main
          places:
               1) To check on the size of a primitive type. In Java, the
               sizes of primitive types are fixed in the language
               specification (a short is _always_ 16 bits; an int is
               _always_ 32 bits, etc), so this is no longer necessary.

               2) In memory allocation (i.e. malloc (32 * (sizeof(int));) In
               Java you always allocate a specific type of object, rather
               than a block of raw memory that you will fill as you like.
               The system always knows the size of the kind of objects you
               are allocating. So sizeof is not needed.

               3) in pointer arithmetic (i.e. p += sizeof (int)) Pointer
               arithmetic of this type is not allowed in Java, so this isn't
               necessary, either.

               For all these reasons, there is no need for a Java sizeof()
               operator.
     11.4 Does Java have the equivalent of "const" arguments in C and C++?
          A. Java 1.1 adds the ability to use the "final" keyword to make
          arguments constant. When used to qualify a reference type,
          however, this keyword indicates that the reference is constant,
          not that the object or array referred to is constant. For example,
          the following Java code:

               void foo(final MyClass c, final int a[]) {
                       c.field = 7; // allowed
                       a[0] = 7; // allowed
                       c = new MyClass(); // final means this is NOT allowed
                       a = new int[13]; // final means this is NOT allowed
               }

          is roughly equivalent to the following C/C++ code:

               void foo(MyClass * const c, int * const a) {
                       c->field = 7; // allowed
                       a[0] = 7; // allowed
                       c = new MyClass(); // const means this is NOT allowed
                       a = new int[13]; // const means this is NOT allowed
               }

          Java does not have any equivalent to the following C/C++ function
          declarations:

               void foo(const MyClass *c); // a pointer to a const class
               void foo(const int *a); // a pointer to a const int
               void foo(const int a[]); // a pointer to an array of const ints

     11.5 Are there any hacks around this?
          A. Certainly! There are always hacks around stuff. One way of
          enforcing constant values is to have two interfaces, a constant
          one and a non-constant one, e.g.

               public interface ConstFoo {
               int getValue();
               }

               public interface Foo extends ConstFoo {
               int getValue();
               void setValue(int i);
               }

          Then when you want to receive a parameter that cannot be modified
          you have:

               void noChange(ConstFoo foo);

          For a parameter that can be modified

               void change(Foo foo);

     11.6 How can I write C/C++ style assertions in Java?
          A. The two classes shown below provide an assertion facility in
          Java. Set Assert.enabled to true to enable the assertions, and to
          false to disable assertions in production code. The
          AssertionException is not meant to be caught--instead, let it
          print a trace. Since the exception is not meant to be caught, we
          just extend Error instead of RuntimeException. As with
          RuntimeException, a method does not need to declare that it throws
          Error. In addition programmers are less likely to write
          "catch(Error) ..." than "catch(RuntimeException)".

          With a good optimizing compiler there will be no run time overhead
          for many uses of these assertions when Assert.enabled is set to
          false. However, if the condition in the assertion may have side
          effects, the condition code cannot be optimized away. For example,
          in the assertion

               Assert.assert(size() <= maxSize, "Maximum size exceeded");

          the call to size() cannot be optimized away unless the compiler
          can see that the call has no side effects. C and C++ use the
          preprocessor to guarantee that assertions will never cause
          overhead in production code. Without a preprocessor, it seems the
          best we can do in Java is to write

               Assert.assert(Assert.enabled && size() <= maxSize, "Too big");

          In this case, when Assert.enabled is false, the method call can
          always be optimized away, even if it has side effects.

               public class AssertionException extends Error {
                    public AssertionException(String s) {
                       super(s);
                    }
               }

               public final class Assert {
                   public static final boolean enabled = true;
                   public static final void assert(boolean b, String s) {
                       if (enabled && !b)
                           throw new AssertionException(s);
                  }
               }

     11.7 How do I do stuff like scanf and sscanf in C/C++? And how do I do
     stuff like sprintf, e.g.

          float x = 12345.6789;
          printf("%6.3f/n", x);

          A. You can break a string like "5 loaves 2 fishes" into its parts
          by using java.util.StringTokenizer. This is the Java equivalent of
          sscanf().

          StreamTokenizer does a similar thing on a file or any stream (i.e,
          what scanf() and fscanf() do in C).

          To do formatted character output, create a format string, and then
          use that to format your binary value, e.g.

               import java.text.*;

               float fi = 1234.56789F;
               DecimalFormat df2 = new DecimalFormat( "0.000" );
               System.out.println( df2.format(fi) );

          gives:

               1234.567
          There are lots of different characters you can feed to the
          DecimalFormat constructor, not just "0". See
          $JAVAHOME/src/java/DecimalFormat.java source for details.

     11.8  What is the Java equivalent of C++ friend"?
          A: The keyword "friend" in C++ is a hack to allow a piece of code
          to access the private member declarations of another class. In
          Java, you would do this by labelling, not the friend, but the
          private members. Instead of making them private, make them
          protected. Classes in the same package have automatic "friendship"
          in that they can access each other's protected fields.

     11.9 Does anything like the C++ Standard Template Library exist for
     Java ?
          A: Yes, only it's better and simpler to use in Java. It's called
          the Java Generic Library. This library (JGL) is freely
          downloadable from http://www.objectspace.com/

          It includes about a dozen nice data structures (including sets and
          bags) and algorithms like unions, searching, and sorting.

          [Some Java vendors are bundling it with their next release]

                             -------------------

12. FURTHER RESOURCES

     12.1 Useful URLS
          Site with the latest copy of this FAQ: http://www.best.com/~pvdl/

          Other Java resources:
          The Java "Hall" of Fame: http://www.apl.jhu.edu/~hall/java/
          good glossary: http://oberon.ark.com/~roedy
          conversions: http://oberon.ark.com/~roedy/convert.html
          good JDBC FAQ: http://www.yoyoweb.com/Javanese/JDBC/FAQ.html
          general Java: http://java.miningco.com/

          Tutorial:
          http://java.sun.com:80/nav/read/Tutorial/index.html
          http://www.phrantic.com/scoop/onjava.html

          Java Book lists:
          http://lightyear.ncsa.uiuc.edu/~srp/java/reviews.html
          http://www.netcharts.com/majug/reviews.html
          http://www.avena.net/~dong/book.htm
          http://wwwiz.com/books

          Javasoft site: http://java.sun.com

          General sharing and exchange of Java info:
          http://www.gamelan.com
          http://www.JavaShareware.com
     12.2 Newsgroups
          These are the Java newsgroups since the reorganization that I
          arranged in April 1997

          comp.lang.java.help simple programming and setup questions
          comp.lang.java.announce (moderated) announcements
          comp.lang.java.advocacy for arguments: no it isn't, yes it is
          comp.lang.java.programmer programming in Java
          comp.lang.java.security security issues
          comp.lang.java.machine JVM and native interfaces
          comp.lang.java.databases JDBC,ODBC, java access to DBs.
          comp.lang.java.softwaretools IDES, editors, compilers, tools, etc
          comp.lang.java.gui AWT, IFC, JFC, AFC, Vibe, etc etc
          comp.lang.java.beans Software components in Java

          Please make an effort to post only to the single most appropriate
          group.

          As with the other language groups on Usenet (comp.lang.c,
          comp.lang.c++, etc) questions about products from specific vendors
          that only work on one specific platform are best posted to other
          newsgroups. For example, questions about ActiveX belong in
          comp.os.ms-windows.programmer.ole, not the Java groups. For
          questions about J/Direct try the group
          microsoft.public.java.visualj++ or microsoft.public.java.sdk.
          These are available from Microsoft's news server
          msnews.microsoft.com .
     12.3 Are there any Java libraries for sorting, etc?
          A. A sort class has not yet been made part of the standard
          library. It should be. If you have the demos that come with the
          JDK, take a look at the class SortDemo. Alternatively, use one of
          the several classic sorts available from Roedy Green. They are
          supplied free with heavily commented Java source code.

          See "QuickSort", "HeapSort" and "RadixSort" in the Java glossary
          at http://oberon.ark.com/~roedy/index.html .

          Also, try the Java Generic Library. This library (JGL) is freely
          downloadable from http://www.objectspace.com/

          Also Visual Engineering has JChart at: http://www.ve.com. No
          licensing fees.
     12.4 Why doesn't somebody write a shell in Java? Then they could use it
     on all platforms!
          A. Somebody has done just that. Look at http://www.jsh.net/
     12.5 Are there any URLs for other libraries?
          A. Indeed, there are. The Java3D Repository
          http://java3d.sdsc.edu/
     12.6 Are there any URLs for regular expression handlers in Java?
          A. http://www.oroinc.com/products/OROMatcher.html This comes from
          ORO Inc, is free, and handles Perl5 style exprs.

                                   -----------------

          Acknowledgements:
          Original FAQ copyright February 1997 Peter van der Linden

          Contributions from:

          Matt Kennel, Patric Jonsson, Brad Van Tighem, Tony Hursh , Glenn L
          Vanderburg, Peter Jones, John McDowall, Jim Driscoll, Uday, Dave
          Harris, Bill Wilkinson, Tom Valesky, Dan Drake, Giles Thomas,
          Mitch Baltuch, Guy Ruth Hammond, Gordon Keith, Jason Brome, Shani
          Kerr, Steve Chapel, Timothy Wolters, Robert Lynch, Jake Cormier,
          Sean C Sullivan, Joseph A. Millar, Jim Frost, Jim Balter, Jeff
          Bauer, John Kochmar, Carl Burke, William Stubbs, Mark Smith,
          Volker Turau, Real Gagnon, Russell Gold, Max Hailperin, Bill
          Tschumy, Marco Nijdam, Marc Pawlowsky, Laurence Vanhelsuwe,Ian
          Macgregor, Mike Faulkner, Rich Koch, Will Clark, Govind Seshadri,
          Rich Simkin, Ian Stiles, Kieren, Darren Christie, Tom Lane,
          Michael Jungmann, Rob Mayoff, George Ruban, Tom McCann, David
          Hopwood, Thomas Phan, Kai Stuke, Rolf Howarth, Derek Snider, David
          Boydston, Andy Godwin, John F. Dumas, Doug Bell, David J. Biesack,
          Tiger Quimpo, Martin Hugh Rogers, Brian Krahmer, Ian Burrell,
          Nikki Locke, Bin Li, Jackson Thompson, Steve Odendahl, Greg Smith,
          Jeffrey C. Ollie, Mark Halvin, Jeremy Cook, Lak Ming Lam, Peter S.
          Morris, Mark Halvin, Juergen Keil, Alex Stewart, Mike Abney,
          Rodney Stephenson

          [<your name here>: send in a suggested FAQ *with the answer*]

          ----

          I am maintaining a FAQ list to address specifically programming
          issues
          (not a general tutorial on Java). Please feel free to mail me
          entries for it.
          Question with answer gets you a credit in the FAQ.
          I can be emailed at: [e-mail]
          Sun Certified Java Programmer.

          -- end --






--
Peter van der Linden,                     Java Programmer's FAQ
Certified Java Programmer.                --->    http://www.best.com/~pvdl