2.4.5 Java

Java was developed by Sun and at present (mid-2002) eclipsed only by XML as the current Big Thing. Source code is compiled to machine-independent bytecode, which is then either interpreted, or compiled to machine code on the fly, by a Java Virtual Machine (JVM) on a particular platform. The main interest to astronomy might be in developing machine- and architecture-independent interfaces either to codes or archives. It's also, I think, of use as a stepping-stone to C++, since its object-oriented features are less obscured by syntax than they are in C++. A very good textbook, written (you won't be surprised to guess) by two of the language's authors, is The Java Programming Language [arnold98]; once you've mastered the basics, there's a good deal of helpful advice in Effective Java [bloch01]. There are numerous resources at Sun's Java site at <http://java.sun.com>.

Java is a mixture of an interpreted and a compiled language. Java source code is compiled by the Java compiler into bytecodes, which are then interpreted by a Java Virtual Machine. These bytecodes are low-level, but architecture-independent so that, in principle, only the virtual machine and its runtime need to be ported to a new machine, whereas the code should be completely portable. This does not work quite as well in practice as it does in principle, but it does mean that Java is not too far off the ideal of run-anywhere code.

Since Java bytecodes are ultimately interpreted, Java programs have the potential to be rather inefficient. However, just-in-time compilers (JIT), which analyse running code and optimize the most heavily-used parts, and similar developments, should help the situation improve in the future. This is possible because JITs have access to both the program source code and the running program, and so can combine the features of both an optimizer and a profiler, and thus optimize more aggressively than a traditional compiler could. This, combined with Java's good built-in support for different platforms and for resource-discovery, means that, possibly somewhat surprisingly, Java has been suggested as a suitable language for high-end codes. The Java Grande Forum is concerned with investigating and promoting developments in this area.

Note that it is relatively easy to write slow Java (for example, adding Strings is a lot slower than using a StringBuffer; input and output streams are not (extensively) buffered, so if you have a lot of I/O, you would be well advised to use Buffered{Input,Output}Stream objects wrapping your I/O objects; small-object creation is extensively and increasingly optimised, but it's still not dirt-cheap, so look out for that in a small loop). That means that using a profiler (see Section 2.5.1) is particularly important. However, with Java, that's easy, because there's one built in to the JVM. Give the option -Xrunhprof:help to find out how to invoke it. For example:

java
            -Xrunhprof:cpu=samples,file=myprog.hprof
            myclass

Java is still developing. At present (mid-2002) Java 1.3.1 is long in the tooth but stable. Version 1.4 is in beta, and on the point of being released properly; it includes a few changes to the language, most noticeably the inclusion of an assert construct.