Question

Surfing on the source code of Java, I found the following declaration:

public abstract class Enum<E extends Enum<E>>

How should it be interpreted? I’m stuck with it…

Thank you.

Answer 1

It’s like quining! @LES2 is on the right track.

public abstract class Foo <E extends Foo<E>> 
{

   public static void use(Foo<E> foo) {
      // use foo
   }
}

If you have the following class:

public class FooImpl extends Foo<FooImpl> {
  // ...
}

then the magic that these recursive templates gives you, is that:

• the Foo template requires that its parameter extends itself (Foo).
• If the parameter class E, in turn, extends Foo<E> (it has to because of the previous point) then you have ensured that the Foo template has an “awareness” of its subclass, since its subclass is passed into it as a template parameter
• which in turn means that Foo‘s methods can safely downcast the this pointer to its derived subclass E.