Java is a popular programming language written by Sun Microsystems. It's an object-oriented language that runs inside a "virtual machine" which, at least in theory, allows code to run on many different machines. Sun calls this "Write once, run anywhere", and in practice it actually works reasonably well, although it took years for this to happen.

Java is extremely popular in commercial software development - it's dominant in the area that I work in: operations support software for communications companies. It's not at all popular in the Free Software community because it's a proprietary product owned and controlled by Sun.

The Java home page is http://java.sun.com/; you can download a developer kit or run-time from there. Java per se is a proprietary product, but there are many free projects that aim to provide all (or part) of the Java environment as Free Software. http://www.dwheeler.com/java-imp.html lists many of those projects.

See also: JavaProgrammingBookmarks, JbossTips

Books

Bruce Eckels Thinking in Java

Nutshell

Development Tools

building: ant - "like make only without make's wrinkles"

modelling: argouml - a UML modelling tool that can generate Java skeletons.

debugging: jswat

database access: isql, squirrelsql

Profiling

There are many expensive commercial tools for profiling Java programs while they're running, but since Java has the JVMPI interface pretty much anyone can hook into the JVM at run time and find out what's going on. This is another category of programs where there seem to be far too many half-baked efforts, but here are my notes from a few hours of fooling around with various tools.

Mike's Profiler - http://mjp.sourceforge.net/ looks promising, has actual documentation and the maintainer seems to be maintaining it on an ongoing basis. The GUI is easy to understand. Unfortunately it seems to run very slowly and crash before JBoss can fully boot up. NOTE - I need to test 0.06 which has a statistical sampling mode

JTreeProfiler - http://sourceforge.net/projects/jcoverage/ this one streams to an XML file on a per method invocation basis so it's probably unsuitable for anything but trivial profiling (booting JBoss wrote a 2.3GB file). The analysis GUI is interesting and very graphical but hard to make sense of.

JPerfAnal - http://sourceforge.net/projects/jperfanal/ can you think of a more unfortunate name for a performance analysis tool? In any case this guy seems to have the right idea in that he reads the stock Sun profiler output so he doesn't need a platform-specific library to gather data. Unfortunately his GUI is lame and there's not one word of documentation.

Extensible Java Profiler - http://ejp.sourceforge.net/

JMP - http://www.khelekore.org/jmp/ GTK GUI, written in C.

JRat - http://jrat.sourceforge.net/ takes an interesting approach - you "instrument" your code after you build.

Infrastructure

tomcat - servlet engine, jboss - j2ee container, openejb - ejb container, avalon - server framework, maverick - presentation framework

Libraries

trace logging: log4j

Coding Conventions

Use Sun's http://java.sun.com/docs/codeconv/html/CodeConvTOC.doc.html and override what you feel is wrong. Personally, I feel that the 80 column constraint is too limiting; I prefer to allow people to use 132 column line widths.

When something is an identifier, and you feel that you must append "Id" in some form to the name, use ID not Id. It's easier to type and reflects that most people call it an "eye-dee" not an "id".

When a class has a member variable that holds multiple items, don't make a setFoo(Collection foos) method, make an addFoo(Foo foo) method instead. Maintain the collection (or set or whatever) internally. The difference is that there's some type checking in the second approach whereas the first approach lets the client pass a Collection full of anything in. It's also often more convenient for the client to call add() a few times rather than create their collection, fill it, and then set() it. If they want to look at it you can return either a collection/set/list (some interface) or you can return an array or iterator of some sort.

Misc

Here's a cute hack to enable token substitution in java property files: http://www.sys-con.com/java/source.cfm?id=1228

Using Java on Debian GNU/Linux

When I tried to run Sun's JDK 1.3.1 on Debian Sid (in March 2002) I got the following error: /home/tcabot/local/Linux/jdk/bin/i386/native_threads/java: error while loading shared libraries: libstdc++-libc6.1-1.so.2: cannot open shared object file: No such file or directory. This can be solved by linking to the existing libstdc++, i.e. ln -s /usr/lib/libstdc++-libc6.2-2.so.3 /usr/lib/libstdc++-libc6.1-1.so.2.

Java is a proprietary language (controlled by Sun) but there are a few Free implementations of the compiler, JVM, and class libraries. If you're running a new version of Debian you can apt-get gcj and kaffe.

$ gcj -C HelloWorldApp.java
$ kaffe -addclasspath . HelloWorldApp

Java is very resource-intensive: cpu, memory, and processes. So you might bump into the limits that Unix uses to limit individual user resource consumption. An important one is max user processes which you can see if you run ulimit -a. You probably want to bump this up to 1020 or so: ulimit -u 1020. Other limits that you might bump into are SHMMAX and SHMANY which you can set using files in /proc/sys/kernel/ or by setting values in /etc/sysctl.conf.

Arrays vs Collections

Why don't people use arrays in Java? It seems to me that people get so excited about Collection, Set, List etc that they forget about arrays. Arrays have some definite issues, but they also have some advantages. Arrays are fixed-length so they're not good if you're not sure how many things that you're going to put into them in advance. Their big advantage (which limits their use) is that they're strongly typed. This can be a big advantage in many cases, i.e. where you've got a bunch of the same thing. In this case an array is nice because it enforces type safety in a way that a Collection doesn't.

In summary, an array is the correct data structure to use where you have a known number of objects all of the same type.

Misc Notes

IBM's JVM 1.4.1 for GNU/Linux doesn't seem to recognize old-style timezone names. My /etc/localtime was a link to /usr/share/zoneinfo/US/Eastern and the JVM would never use daylight savings - it was always GMT+5. When I changed the link to /usr/share/zoneinfo/America/New_York it worked fine. See http://www.mainframeforum.com/t590240.html which seems to be relevant despite being a mainframe forum.

public class date { public static void main(String[] argv) throws java.io.IOException {System.out.println(new java.util.Date()); }}

-- TobyCabot - 28 Dec 2001 - 24 Jun 2003