The JDK comes with the source code of the core java api packages. For testing and debugging purposes, I want to modify the behaviour of some of the standard java packages. Let's say, I want to modify the java.util.ArrayList. I tried to recompile from the supplied src the java.util.ArrayList class. But it gives me "error: Class names, 'java.util.ArrayList', are only accepted if annotation processing is explicitly requested". What's wrong with this?