OpenJDK
OpenJDK is an free implementation of Java for Linux, Solaris and Windows.[1] There is also OpenJDK for BSD[2]. Several BSD and Linux distros carry packages of OpenJDK.
OpenJDK can compile and run Java programs. OpenJDK 6 implements Java 6, while OpenJDK 7 is a preview of future Java 7.
Usage
OpenJDK is a set of command-line tools. If OpenJDK is installed in /usr/local/jdk-1.7.0, then /usr/local/jdk-1.7.0/bin needs to be added to the PATH environment variable. The two most important tools are the javac compiler, and the java runner.
This example program would compute 12 - 4.
<lang java>/* TwelveMinusFour.java */ public class TwelveMinusFour { public static void main(String[] args) { System.out.println(12 - 4); } }</lang>
One can compile this program with javac, and run it with java.
<lang bash>$ javac TwelveMinusFour.java $ java TwelveMinusFour 8</lang>