You must be careful, however, because not every Java
    platform is free. Sun continues distributing an executable 
    Java platform which is nonfree, and other companies do so 
    too.
    
    The free environment for Java is called IcedTea; the 
    source code Sun freed is included in that. So that is the
    one you should use. Many GNU/Linux distributions come with
    IcedTea, but some include nonfree Java platforms.
    
    To reliably ensure your Java programs run fine in a free
    environment, you need to develop them using IcedTea.
    Theoretically the Java platforms should be compatible, but
    they are not compatible 100 percent.