Take a look at com.netbeans.developer.impl.{Java,}Help classes. The structure is like this:

public class Help {
 // ...

 public interface Impl { ... }
 private static class XMLImpl implements Impl {...}
}

public class JavaHelp implements Help.Impl {...}

Now if you have both files open, and make a change within Help.java to the Impl interface, you are prompted to update XMLImpl, but not JavaHelp.

[phamernik] seems to be duplicated with 4471.