Compile JAVA with gcj

to compile the common helloworld example

public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, World!");
}
}

with the gnu gcj compiler to JAVA bytecode
gcj -C HelloWorld.java

and interprete with the VM
gij HelloWorld

... or compile it to native code
gcj --main=HelloWorld -o HelloWorld HelloWorld.java
./HelloWorld

Guide to GNU gcj
yolinux.com - LinuxTutorialJava
Linux Journal - Developing Gnome Applications with JAVA