tee@uusi:~$ apt-cache search sun java jdk --names-only # multiverse
tee@uusi:~$ sudo apt-get install sun-java6-jdk 
tee@uusi:~$ cat HelloWorld.java 
public class HelloWorld
{
     public static void main(String[] args) 
     {
          System.out.println("Hello, world!");
     }
}
tee@uusi:~$ javac HelloWorld.java 
tee@uusi:~$ java HelloWorld 

See also: http://en.wikibooks.org/wiki/List_of_hello_world_programs#Java