From pvdl@best.com Wed May 06 19:19:21 1998 Path: news.nic.utwente.nl!news.cs.utwente.nl!Cabal.CESspool!bofh.vszbr.cz!newsfeed.wli.net!ptdnetP!newsgate.ptd.net!newshub.northeast.verio.net!news1.best.com!nntp1.ba.best.com!not-for-mail From: pvdl@best.com (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 Supersedes: <6id7e6$gu$1@shell15.ba.best.com> Followup-To: poster Date: 6 May 1998 10:19:21 -0700 Lines: 6027 Approved: news-answers-request@MIT.EDU Message-ID: <6iq62p$ehl$1@shell15.ba.best.com> NNTP-Posting-Host: shell15.ba.best.com X-Trace: 894475163 22452 pvdl 206.184.139.132 Summary: This posting answers frequently-asked questions by Java programmers Xref: news.nic.utwente.nl comp.lang.java.programmer:156830 comp.lang.java.help:26461 comp.lang.java.gui:15571 comp.answers:27127 news.answers:45148 Archive-name: computer-lang/java/programmers/faq Posting-Frequency: weekly Last-modified: 1998/5/6 URL: http://www.afu.com/javafaq.html Copyright: (c) 1997,1998 Peter van der Linden Maintainer: Peter van der Linden Frequently Asked Questions (with answers) for Java programmers _____________________________________________________ ________| |________ \ | Java Programmers FAQ http://www.afu.com | / \ | Copyright (c) 1997-98 Peter van der Linden | / / |_____________________________________________________| \ /___________) (__________\ The Java FAQs here are intended for people who already have some programming experience, maybe not in Java though. Go to the FAQ home page at http://www.afu.com for other Java information and downloads, and the most up-to-date copy of the FAQ. Report FAQ updates to faqme AT afu.com. ------------------------------------------------------------------------ Please support Java Portability. The biggest value of Java is its portability. * Portability makes it easier for companies to change/upgrade operating systems and platforms. * Portability makes it easier for Java programmers to transfer their skills to new employers. Microsoft does not want users to easily move their software to different platforms. So they made unauthorized changes to Java to try to undermine portability. The changes are described by an independent Java users organization at http://www.javalobby.org/action/msjava-diffs.htm. A Federal judge issued a preliminary injunction against Microsoft in March 1998, prohibiting them from selling their incompatible J++ product as Java. Here is a link to http://www.interhack.net/people/cmcurtin/rants/ write-once-run-anywhere/write-once-run-anywhere.html explaining why Java portability is important. The best way for programmers to support portable Java is to reject non-standard tools from the only company pushing them: Microsoft. As a Java programmer please join the Java Lobby, an independent organization dedicated to representing non-vendor interests in Java. It's free, and you can sign up by visiting http://www.javalobby.org for details. Other ways to encourage portable java: * Use development environments from other vendors, or convert Microsoft Visual J++ to use Sun's JDK, following the instructions at http://www.orbiter.demon.co.uk/ * Use Netscape Communicator (not Internet Explorer) * If forced to use Internet Explorer, use the Java Plug-In to get a standard Java system inside it. * Use a JVM from GNU, Kaffe or Sun (not Microsoft's J++ SDK) Free standard Java compilers and Java Plug-In can be downloaded from http://java.sun.com. Free standard Java Virtual Machines can be downloaded from http://www.kaffe.org. and http://www.oryxsoft.com/projects/gvm and http://www.redhat.com/linux-info/jolt Free Java AWT software can be downloaded from http://www.biss-net.com/biss-awt.html and the files are all at ftp.java-linux.org (the linux site) too. Free Java software can be downloaded from http://www.gnu.org/software/java/java.html ------------------------------------------------------------------------ 0. A WORD FROM OUR SPONSOR * 1. LOOKING FOR INFORMATION * Is Java "open" or "proprietary? How do you refer someone to the FAQ? Where can I look at the definitive Java Language Spec? Where can I find a full list of Java books and book reviews? What if my question is not answered in this FAQ? How do I check on known bugs in JDK 1.1? Is there a Java port to Windows 3.1? Where can I find information on the Java 3D API? What are the good folks at project Gnu doing with java? Where can I find information about Java Certification? Where can I find a Java style guide on naming conventions? Where can I find information about future Java APIs? How can I find links to recent news about Java? What computers have Java ports? 2. COMPILERS, ETC. * Why did I get an OutOfMemory error when porting? Why do I get a Statement not reached error? Is there a lex and yacc? Where can I find a byte code Obfuscator? Which program is used to create .zip files? Can I compile a Java .class file to a binary? How can I performance profile my Java code? I get a javadoc when looking at JDK classes How do I recompile everything? What is java.lang.UnsatisfiedLinkError? Why do I get an error in accessing a private outer method? 3. LANGUAGE ISSUES * Why doesn't my hello world program work? How can I program linked lists if Java doesn't have pointers? What is the true story about how parameters are passed in Java? How, then, can I pass an object to a method, and have the method change the reference? Why is String a final class? What happened to private protected" I extended the class called Frotz, and...? About Java floating point may not have been initialized"> Variable may not have been initialized Why do I get this compiler error message (Statement expected, Type expected)? Why do I get this compiler error: No constructor matching? Why do I get the compiler warning "Can't access protected method clone..."? How do I transform a char into the corresponding int value? Are constructors inherited? How do I allocate a multidimensional array? What are the tradeoffs between importing packages versus using Fully Qualified Name? What are class literals? How do I copy the contents of an array to another array? Is there a fast way to set all elements of an array? Is there a shortcut for Math acos, cos, sin, etc? Why does b>>>=1 give the same result as b >>=1? How do I compare two Strings? How can I be type-safe without creating a multitude of subclasses? Why is there a standard JNI? What are the naming conventions in Java? What are the differences between an interface and an abstract class? How do static methods interact with inheritance? How do I get unsigned ints? 4. AWT and Related Topics * 4.0 AWT o Why do I get this when using JDK 1.1 under X: (Font problem)? (links to be continued...) 4.1 APPLETS 4.2 BROWSERS 4.3 SWING 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 13. MOST COMMON JAVA NOVICE PROBLEMS 14. FOR WINDOWS USERS 15. Java GOTCHA'S ------------------------------------------------------------------------- 0. A WORD FROM OUR SPONSOR Learning Java This Java FAQ is maintained as a service to the Java community. Please consider these books from the FAQ author when you are looking for a programming text. * "Just Java 1.1" Programming in Java, for people who can already program in another language. More info at http://www.amazon.com/exec/obidos/ISBN%3D0137841744 "Not Just Java" Java explained for managers. More info at http://www.amazon.com/exec/obidos/ISBN=0138646384 Sample chapters are on Sun Microsystems webpage at http://www.sun.com/971124/cover-linden/. ------------------- 1. Looking For Information * 1.0 Is Java "Open" or "Proprietary"? o 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 perpetual, irrevocable, free and 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 promised ISO that it will abide by ISO's rules for developing a standard, in particular it will take account of industry input. Just as with other ISO standards, "open" does not mean that anyone who suggests any new feature will get it into the Java standard; Sun has the casting vote in the future evolution of Java, but it does not own the specification nor the sole right to implement to it. On the spectrum of "single vendor proprietary technology" to "open to all to implement", Java lies much more towards the open end than the single vendor end. Sun Microsystems has invested significant time and money to create its Java implementation. Sun has licensed its source code to more than 140 companies and organizations who want to build it into a commercial product of their own, including competitor Microsoft. Sun has also allowed free download and use of the Java implementation (binary executables) by anyone. Over 2 million computer users have downloaded the JDKbinaries for free in the twelve month period to December 1997. The relative openness of Java contrasts with systems that are only available from one vendor, whose interfaces are developed in secret, without an open process for others to participate, and whose owners do not allow competing implementations of the same API. Typically, such systems also feature "private" APIs that are published late or not published at all, to allow the single vendor to gain a competitive advantage for their other products. Typically such proprietary systems do not make the source code available for inspection by all. Some programmers want to see the Java source for free. They say that nothing is truly open without the source code. Sun allows this. If you aren't planning on incorporating Java into a commercial product, or doing any sort of external redistribution, there is an Internal Noncommercial Use Source License available at no charge. Please see URL http://java.sun.com/nav/business/source_form.html 1.1 What is the best way to refer someone to the FAQ when they ask question I know is answered there? o A. The Java Programmers FAQ (at http://www.afu.com) answers 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, supplying an incentive to go there next time. It's regarded as elementary politeness to look for the FAQ of a newsgroup and read it, before posting any questions. In general, FAQs for any newsgroup are available by looking at past postings in a group, or by searching Deja News (see Q 1.4), or via anonymous FTP at directories under ftp://rtfm.mit.edu The pathnames are called things like /pub/usenet-by-group/comp.lang.java.programmer/Java_Programmers_FAQ which may help you get to the right one directly, as it takes some time to get a directory listing there. Alternatively, you can look for newsgroup names on the same ftp site by going to the same site and looking under /pub/usenet-by-hierarchy/. That has subdirectories such as alt/, ba/, ca/, comp/, and subdirectories under them such as /pub/usenet-by-hierarchy/comp/lang/ and so on. This helps you explore the world of newsgroups with FAQs. If you do not have anonymous FTP access, you can access the rtfm.mit.edu archives by mail server as well. Send an E-mail message to mail-server@rtfm.mit.edu with "help" in the body for more information. "RTFM" stands for "Read The effing Manual" -- you must expect to put in some time and effort to master a new area of study. 1.2 Where can I look at the definitive Java Language Specification? o A. This is available online at: + http://java.sun.com/docs/books/jls/html/index.html (Java 1.0) and the Java 1.1 inner classes spec at: + http://java.sun.com/products/jdk/1.1/docs/guide /innerclasses/spec/innerclasses.doc.html and the other Java 1.1 update at: + http://java.sun.com/docs/books/jls/html/1.1Update.html It is also available as a book in printed form (details at website). Also see the "Clarifications and Amendments" http://java.sun.com/docs/books/jls/clarify.html. You can also see the virtual machine (execution environment) specification at http://docs.sun.com:80/ab2/java1/@Ab2CollToc?subject=java 1.3 Where can I find a full list of Java books and book reviews? o A. http://www.flathill.com/languages/java/ and http://www.javaworld.com/javaworld/books/jw-books-index.html 1.4 What if my question is not answered in this FAQ? o A. Go to http://www.dejanews.com/home_ps.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). The chances are that you will find several answers to your question. Some may not be complete or completely accurate however. That is the nature of Usenet, and free information. If you still don't have an answer, then post your question on the most appropriate of the newsgroups. Don't spam the newsgroups by posting to multiple groups. Knowledgeable posters tend to ignore questions like that. 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? o A. Look at the Java Developer Connection at http://java.sun.com/jdc. 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 also send in an RFE (Request For Enhancement) or ease-of-use issue there. You can even vote on the priority you would assign to a particular bug fix! Join the Java developer connection (it's free) by going to http://java.sun.com/jdc Then browse webpage http://developer.javasoft.com/developer/bugParade/#votes 1.6 Is there a Java port to Windows 3.1? o 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. Java will never be well-supported under Win3.1 because Win3.1 lacks the basic features expected of a modern OS (primarily lengthy filenames and multithreading support). See also Q1.13. You could also consider JavaSoft's JavaPC kit that converts a PC into a thin client Java system. Details are at http://java.sun.com/products/javapc/index.html. It is probably more for business users than personal PCs. Unlike the 16-bit versions of Netscape Navigator and Microsoft Internet Explorer, which provide a Java Virtual Machine that is only compliant with the JDK 1.02 API, the JavaPC software allows IS managers to deploy JDK 1.1 compatible Java applications on PCs running DOS and Windows 3.x. 1.7 Where can I find information on Java 3D? o A. The Java 3D FAQ at http://tintoy.ncsa.uiuc.edu/~srp/java3d/faq.html may have the answers you're looking for. It contains general information about Java 3D, as well as programming tips. 1.8 What are the folks at project Gnu doing with Java? o 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 See also http://www.oryxsoft.com/projects/gvm and http://www.hungry.com/products/japhar (Now one merged project). 1.9 Where can I find information about Java Certification? o 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/ and then search for "sun certified java". 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. o A. Check out the section "Naming Conventions" in the language specification + 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 See also Q3.25 for some basic rules of thumb. 1.11 Where can I find information about future Java APIs? o 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? o 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? o A. A list of JDK ports is available from + http://java.sun.com/cgi-bin/java-ports.cgi 1.14 What IDEs (Integrated Development Environments) are there? o A. Some popular IDEs include: Apptivity 2.0 (Progress) http://www.apptivity.com CodeWarrior Professional http://www.metrowerks.com GRASP (product is free) http://www.eng.auburn.edu/grasp Java WorkShop 2.0 (Sun) http://www.sun.com/workshop/java Javelin, Visual Object Development for Java http://www.stepahead.com.au JBuilder (Borland) http://www.borland.com/jbuilder JDE for emacs http://sunsite.auc.dk/jde/ Kawa 2.52.01 (Webcetera) http://www.tek-tools.com/kawa PARTS alpha (ObjectShare)http://www.objectshare.com PowerJ ? (Sybase) http://www.sybase.com/products/powerj SilverStream http://www.silverstream.com Super Mojo (Penumbra) http://www.penumbrasoftware.com SuperCede 2.0 (Asymetrix)http://www.supercede.com teikade 1.8R2 (PFU Ltd) http://www.pfu.co.jp/teikade Together Java ? (?) http://www.oi.com Visaj 1.0.1 (Imperial SW Tech) http://www.imperial-software-tech.co.uk VisualAge 1.0 (IBM) http://www.software.ibm.com/ad/vajava Visual Cafe 2.1 (Symantec) http://cafe.symantec.com Visual J++ (Microsoft) (non-standard; not recommended) Xelfi 0.94 http://www.xelfi.com 1.15 What is "San Francisco"? o A. San Francisco is the code name for a very large Java project led by IBM, and involving other companies. The project is to provide a Java framework for data processing applications. The basic classes are provided for general ledger, sales order, inventory management, etc etc, and these classes can be extended and customized for particular industries (vertical markets). IBM's SF project competes with products from companies like SAP and Baan. Of course, the SF project is multi-platform and uses Java beans and GUI interfaces. More information on SF is available at http://www.ibm.com/Java/Sanfrancisco 1.16 What large Office-style applications have been written in Java? o A. Well, the first one to consider is IBM's San Francisco project, mentioned in the answer to Q1.15. There is also Lotus's e-suite - a suite of Java applets and beans including a spreadsheet and a word processor. These became available in March 1998. Another office suite in Java is Applix Anyware at http://www.applix.com/anyware/index.htm. Applix became available in downloadable demo form in April 1998. However the niche for single user office productivity applications is pretty well already dominated by Microsoft products, and it is unrealistic to think that Java software will unseat shrink-wrapped software simply because it is written in Java. This is why Corel replanned its Java rewrite of Corel Office before taking it to FCS. When Corel did that, it also increased its investment in Java from 33% of R&D budget to 50%, at the expense of Windows. Most of Java development is taking place for custom applications internal to a company. Most programmers of any kind have never worked on MS Office, but work on internal applications, and so it is with Java. These projects don't have the high profile of major vendor's products, but they are the mainstay of the industry. ------------------- 2. COMPILERS, ETC. * 2.1 Why did I get an OutOfMemory error when porting working code from jdk 1.02 to 1.1 o 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? o 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? o 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/JavaCC/ 2.4 Where can I find a byte code obfuscator? o A. http://www.primenet.com/~ej/ http://www.math.gatech.edu/~mladue/HoseMocha.java Some people have reported problems using these with JDK 1.1. This obfuscator has been updated to be fully compatible with JDK 1.1: http://www.monmouth.com/~neil/Obfuscate.html Obfuscators are intended to foil decompilers. Decompilers translate byte code back into Java source code. Mocha was the first and most well known of the decompilers; it's no longer supported. There is a decompiler (written in C++) at http://web.unicom.com.cy/~kpd/jad.html Because it is in C++, there are different versions for every architecture (hah!) There are also commercial products, such as SourceAgain from http://www.ahpah.com There's a very good Java Code Engineering and Reverse Engineering FAQ page at http://Meurrens.ML.org/ip-Links/Java/codeEngineering/. 2.5 Which program is used to create .zip files compatible with the java* programs? (eg classes.zip, moz3_0.zip) o 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 2. Make sure the main class has no parent directory inside the archive, (in other words, don't build an archive with foo/bar/myMain.class, unless your myMain is in a package called foo.bar. Instead start it at myMain.class). Your packages must be placed in the archive using their corresponding filesystem pathnames. 3. Put the archive in the same directory as the .html page; 4. Put something like the following tag in the .html file: + From JDK 1.1 on, an example of the applet tag used with a jar file is + These lines will use an applet called myapplet that can be found in the jarfile myfile.jar. An example applet tag of a jar file used to hold classes in packages is 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 program to a binary executable, .exe on a PC? o A. Some companies make products that do this. See the webpages for Symantec http://www.symantec.com, Supercede www.supercede.com, and Tower Technology http://www.twr.com. The first two are targetted to Windows. Tower Technology supports several flavors of Unix. Also, there is a native Java compiler from IBM, known as the HPJ (High Performance Java) compiler. One user has reported that it created a 2Mb executable from a 12K java file, and did not run any faster. If you want to create a native executable to make executable use and distribution easy, consider a Jar file instead. Compiling to native code takes away the most significant benefit of Java: portability of executables. Further, if you want your Java DLL (or .exe) to interact with C++, you'll have to specify which specific C++ compiler and/or actually compile some sort of linkage via the appropriate C++ compiler. Because C++ does not have a standard ABI there is a big problem with interoperability. Every C++ compiler uses a different object model, a different way of laying out class members, and a different way of "mangling" names for the linker. C is much simpler. The only question here is how structures are "packed" (i.e., are integers aligned on four-byte bounds?). All the C++ compilers can interact with C code, thanks to 'extern "C"' declarations. Consider carefully why you want to compile to a native executable, 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? o 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.sort More and better Java tools are a third party opportunity. One Java profiler is JProbe Profiler, available from http://www.klg.com/jprobe. JProbe is said to be easy to use. Another profiler is OptimizeIt, available from http://www.optimizeit.com. Each of these profilers has performance tuning, which shows which methods took how much time, and also memory tuning, which shows what objects are in memory and how they were allocated. Both are important things to know. The latest version of the CodeWarrior IDE http://www.metrowerks.com has a time-based profiler for Java code. Java Workshop from Sun also has a time-based profiler. 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: o /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 burdensome. 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? o 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? o 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. (This need is true for both standard (untrusted) applications and trusted applets. At least, if you use the Java Plug-in to give yourself standard Java inside a browser). On Solaris, make sure that the environment variable LD_LIBRARY_PATH includes the path of your shared library. 2.11 An anonymous class can't seem to access a private outer method. Why is that? o A. It's a known bug in the JDK 1.1.4. The code is: public class MyDialog { void Setup() { addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { myCloseWindow(); } } ); // anon inner class } void CloseWindow() { // outer method dispose(); } } This code sends javac into an infinite loop. The workaround is to make the private method non-private, or to make the inner class a named class. 2.12 What are the major Java releases and their contents? o A. There have been two Java releases from Sun so far, plus a number of bugfix (dot-dot) releases. The releases are: + JDK 1.0.2 This was the release FCS in May 1996. It had some security fixes over JDK 1.0. + JDK 1.1 This release introduced a new event model in the window system. It also made JDBC support and beans support a standard feature. It changed and standardized the native code interface to JNI. It also introduced inner classes. + JDK 1.2 This release (due Fall 1998) made the Swing library a standard feature. Swing is a set of rich platform-independent graphical components. ------------------- 3.LANGUAGE ISSUES * 3.1 Why doesn't my "hello world" program work? o 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 to point to the classes.zip file, and in fact the README advises users not to set it. From JDK 1.1.2 on, it is generally an ERROR if the user sets the CLASSPATH to include classes.zip. CLASSPATH is only needed to + point to the roots of the programmers own packages, + use rmic + use unbundled packages like Swing in JDK 1.1 + point to native code libraries. If you're not doing any of these, do not set CLASSPATH. If you have set it, unset it. 3.2How can I program linked lists if Java doesn't have pointers? o 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? 3.3What is the true story about how parameters are passed in Java? Is is by value or by reference? o 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? o 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 ... Note that changing a global variable/object inside a method is an egregious programming practice -- it usually violates basic OOP constructs. 3.5Why is String a final class? I often want to override it in some way. o 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. 3.6What happened to "private protected"? o A. It first appeared in JDK 1.0 FCS (it had not been in the Betas). Then it was removed in JDK 1.0.1. 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? o 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 happen inJava floating point? o 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 * o 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 may not have been initialized">Why do I get the compiler error message o 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("bad URL:" + e.getMessage()); } 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 to declare test with an explicit initial value of null. 3.10 Why do I get this compiler error message? o 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? o 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. * o 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 you transform a char into the corresponding int value, that represents the code value of the char? o 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? o 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? o 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. o import java.rmi.server.*; RemoteObject ro; versus: o 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"? o A. A feature introduced in JDK 1.1. They are literals of type "Class" that hold a value representing any class. There are even values to represent "void" and an array, like this: Class myCl1 = Character.class; Class myCl2 = Void.class; Class myCl3 = Object.class; Class myCl4 = String[].class; Class myCl5 = int[][].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? o 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. Although it went outside the boundaries of defined behavior in COBOL, on S/360 computers and their successors. this could compile into a single machine instruction. The roll-up behavior was documented by IBM, and the technique was common enough that it was worth optimizing the trick in microcode. In any event, initialization 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? o 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? o 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". There is a cheating way to do this which works because all the methods are static. Declare java.lang.Math M = null; angle = M.cos(i); Please don't do this. 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? o 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? o 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. 3.23 How can I be type-safe without having to create a multitide of subclasses (IntegerLinkedList, StringLinkedList, etc.)? o A. Generic programming in java (the rough equivalent of C++'s templates) works reasonably well since all java classes are subclasses of Object. There is, however one potential problem - there is always a possibility that a generic container may contain different classes of objects. This naturally leads to the question of how to do this in a type-safe way. If you've created a generic LinkedList class, how can you be type safe without having to create a multitide of subclasses (IntegerLinkedList, StringLinkedList, etc.)? One way to handle this would be to offer up an additional constructor in your generic class that takes a parameter of type "Class" and uses that parameter along with Class's "isInstance" method to guarantee that Objects added to the container are the expected type. + public class LinkedList { protected Class type = Object.class; public LinkedList(Class type) { this.type = type; } public void addElement(Object element) throws Exception { if(!type.isInstance( element )) throw new Exception( "Expected element of type (" + type + ") got element of type (" + element + ")"); ... } } Note that the comments in the source for isInstance() refer to a "specified Class parameter", suggesting that you are supposed to write something like: + public void addElement(Object element) throws Exception { Class c = element.getClass(); if(!type.isInstance(c)) This works, but the documentation for isInstance is clear that the parameter should be an Object rather than a Class. Also, be aware of "Collections" that are coming in JDK 1.2. More information about this is available at the Java Developer Connection at the Java website http://java.sun.com. 3.24 Why is there a standard JNI? o A. JNI is the Java Native Interface. It defines the way that a Java program can call C programs. The industry has agreed on, and Sun has codified, JNI as the standard. Microsoft shuns the standard and uses a protocol of its own called Raw Native Interface, RNI. You might think that once a Java program uses JNI, portability is lost, and hence it doesn't matter if vendors diverge from the JNI standard. However, it does affect programmers and especially end-users. When you use native code, you lock your program into one platform (and other platforms if you port the native code). But programs that use native code should still be able to use a JVM, and other Java tools and libraries from any vendor. This is possible when everyone uses the same native code interface. Microsoft's non-standard RNI is the reason that programs using the Microsoft JVM cannot use the standard Java jdbc-odbc library. That library has a piece written in C. It works for all JVM's except Microsoft's. The standard JNI thus has two purposes: + Source code compatibility between different platforms. + Binary code compatibility between different JVMs on the same platform. 3.25 What are the naming conventions in Java? o A. The naming conventions are straightforward: + 1. package names are guaranteed uniqueness by using the Internet domain name in reverse order: com.javasoft.jag - the "com" or "edu" (etc) part used to be in upper case, but now lower case is the recommendation. + 2. class and interface names are descriptive nouns, with the first letter of each word capitalized: PolarCoords. Interfaces are often called "something-able", e.g. "Observable", "Runnable", "Sortable". + 3. object and data (field) names are nouns/noun phrases, with the first letter lowercase, and the first letter of subsequent words capitalized: currentLimit + 4. method names are verbs/verb phrases, with the first letter lowercase, and the first letter of subsequent words capitalized: calculateCurrentLimit + 5. constant (final) names are in caps: UPPER_LIMIT Also see Q1.10. [Reference: Java Language Specification, section 6.8 Naming Conventions.] 3.26 What are the differences between an interface and an abstract class? o A. The principal difference is that an abstract class is used when you have a type/subtype relationship. An interface does not imply a type/subtype hierarchy; it merely forces a class to have some methods with a given signature. That allows some other code to rely on being able to call those methods. Interfaces allow you to treat an object as an abstraction without having to know anything about its particulars. If, for example, you wanted to have a bunch of persistent objects on which you need to provide persistence services, these objects of many varying kinds would all in their individual ways implement a Persistent interface that prescribes a save() method. The persistence service simply treats each different object as a Persistent and calls save() on it. Each object of all the unrelated classes knows how to save() itself when told to do so. The service has no need to worry about the actual class of each object. You want to use abstract classes when you plan to have related subtypes that define the abstract behavior. The subclass should have a "kind of" relationship to the superclass. A user record is a kind of record, a dog is a kind of mammal, etc. Inheritance provides a "kind of" relationship. By way of contrast, an interface provides a "plays the role of" relationship. An image loader could play the role of an Observable thing. Just about any class could play the role of a "sortable thing". Interfaces therefore often have a name that is a "something-able thing". A class may only extend one other class, but it can implement an arbitrary number of interfaces. Abstract classes can have abstract methods and they can also have fully defined members. Interfaces can't have any code in them at all, though they can have final data. Another minor difference is that calls to a subclass method are faster than calls to an interface method (because the interface method involves a bit more lookup at runtime about what is actually being called). 3.27 How do static methods interact with inheritance? o A. Static (per-class, rather than per-object) methods do not participate in overriding (choosing the right method at runtime based on the class of the object). Probably the best and simplest way to think about this (and to write your code) is to write every invocation of a static method using the fully qualified class name so: class A { public static method1() { A.method2(); } public static method2() { } } class B extends A { public static method3() { A.method1(); } public static method2() { } } Now it is perfectly clear that the static method2() that is called is A.method2(), not B.method2(). This is true regardless of whether you use the fully-qualified class name or not, but the use makes it obvious to all. 3.28 How do I get unsigned ints in Java? o A. Java doesn't have unsigned ints. The reason is that this is a poorly designed area of C -- the rules for what type you end up with when you mix signed and unsigned in expressions are complicated -- and they changed between K&R and ANSI C (you might have heard this under the name "unsigned preserving vs. value preserving"). Worse, they depended on the underlying hardware, so they varied from platform to platform, causing bugs in all kinds of unexpected places. The book "Expert C Programming" goes into this in more depth (p25). So, to avoid bringing over the hidden complexities, Java does not bring over unsigned types from C. Use type char if you are ok with 16-bit unsigned quantities. Otherwise go to the next larger type and use masking. Specifically, to convert an int to its unsigned representation, use: ((long)i) & 0xFFFFFFFF This promotes the signed int to long and chops off the sign-extension, leaving it as an unsigned 32-bit quantity. Also worth noting is that if you're going to work with unsigned bytes, int is a more efficient larger type to use than short or char, since smaller values have to be promoted to int to do any arithmetic or testing on them. 3.29 How do I send a variable number of arguments to a method, or get multiple return values back? o A. (Easy) Use method overloading to support different parameters. This makes things easy on the caller but can get out of hand if you want to support a wide number and variety of parameter types. You should ask yourself if your code design is well-organized if you need to do this. (More complicated) Use arrays. It's possible to declare arrays inline as shown below: foo("A param", new Object[] {"param3", "param4", new Integer(5)} ); // ... void foo(String param1, Object param2[]) { System.out.println(param1); for (int i = 0; i < param2.length; i++) { System.out.println(param2[i].toString()); } } You can even pass arrays of arrays using this method. Of course, inside the method you need to be able to decode what the arguments are and how you use them. Alternatively you can invent a class that just contains all the possible fields you might want to pass into a method (plus booleans to say if each field is set or not), and make an object of that class be a parameter to the method. You can return multiple values from a method the same ways; either have the method return an array or a wrapper object. Remember the wise words of Prof Alan Perlis however, "if your procedure has more than about half a dozen parameters, you probably forgot a few." Perlis meant that passing large numbers of arguments into a function suggests that your function is badly-organized. ------------------- 4.AWT and Related Topics * 4.0 AWT * 4.0.1Why do I get this when using JDK 1.1 under X Windows? o + java.lang.NullPointerException at sun.awt.motif.MFramePeer.(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" & 4.0.2Why is GridBagLayout so hard to use? o A. There are two reasons. First, detailed GUI layout is difficult. Simple layouts are easy. Second, 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 spend any effort on GBL, there are better alternatives available now, and it will be replaced by the SwingSet "SpringLayout" Springs & Struts style layout manager. SpringLayout was to be introduced as part of the Java Foundation Classes with JDK 1.2, but it was dropped from the beta release as the code was not ready in time. Javasoft generously made the preliminary code available and invited programmers to hack on it and submit the results. 4.0.3How do you change the font type and size of text in a TextArea? o A. myTextArea.setFont(new Font("NAME",