While it's always possible to begin our discussion of Java bytecode by simply asserting some facts ("Java bytecode is stored in a tree format"), given that most of the audience is already familiar ...

Next, open com.javaworld.sample.helloworld.Activator.java in the Eclipse Java editor. You will notice that you are now able to access the HelloService interface but not the HelloServiceImpl class.

For traditional Java, such as the HelloWorld.java class shown next, javac is used to compile the class into a .class file and then java is used to run that class file.