Why in java enum is declared as Enum<E extends Enum<E>> [duplicate]

This is a common question, and understandably so. Have a look at this part of the generics FAQ for the answer (and actually, read as much of the whole document as you feel comfortable with, it's rather well done and informative).

The short answer is that it forces the class to be parameterized on itself; this is required for superclasses to define methods, using the generic parameter, that work transparently ("natively", if you will) with their subclasses.

Edit: As a (non-)example for instance, consider the clone() method on Object. Currently, it's defined to return a value of type Object. Thanks to covariant return types, specific subclasses can (and often do) define that they return a more specific class, but this cannot be enforced and hence cannot be inferred for an arbitrary class.

Now, if Object were defined like Enum, i.e. Object<T extends Object<T>> then you'd have to define all classes as something like public class MyFoo<MyFoo>. Consequently, clone() could be declared to return a type of T and you can ensure at compile time that the returned value is always exactly the same class as the object itself (not even subclasses would match the parameters).

Now in this case, Object isn't parameterized like this because it would be extremely annoying to have this baggage on all classes when 99% of them aren't going to utilise it at all. But for some class hierarchies it can be very useful - I've used a similar technique myself before with types of abstract, recursive expression parsers with several implementations. This construct made it possible to write code that was "obvious" without having to cast everywhere, or copy-and-paste just to change concrete class definitions.

Edit 2 (To actually answer your question!):

If Enum was defined as Enum<E extends Enum>, then as you rightly say, someone could define a class as A extends Enum<B>. This defeats the point of the generic construct, which is to ensure that the generic parameter is always the exact type of the class in question. Giving a concrete example, Enum declares its compareTo method as

public final int compareTo(E o)

In this case, since you defined A to extend Enum<B>, instances of A could only be compared against instances of B (whatever B is), which is almost certainly not very useful. With the additional construct, you know that any class that extends Enum is comparable only against itself. And hence you can provide method implementations in the superclass that remain useful, and specific, in all subclasses.

(Without this recursive generics trick, the only other option would be to define compareTo as public final int compareTo(Enum o). This is not really the same thing, as then one could compare a java.math.RoundingMode against a java.lang.Thread.State without the compiler complaining, which again isn't very useful.)

OK, let's get away from Enum itself as we appear to be getting hung up on it. Instead, here is an abstract class:

public abstract class Manipulator<T extends Manipulator<T>>
{
    /**
     * This method actually does the work, whatever that is
     */
    public abstract void manipulate(DomainObject o);

    /**
     * This creates a child that can be used for divide and conquer-y stuff
     */
    public T createChild()
    {
        // Some really useful implementation here based on
        // state contained in this class
    }
}

We are going to have several concrete implementations of this - SaveToDatabaseManipulator, SpellCheckingManipulator, whatever. Additionally we also want to let people define their own, as this is a super-useful class. ;-)

Now - you will notice that we're using the recursive generic definition, and then returning T from the createChild method. This means that:

1) We know and the compiler knows that if I call:

SpellCheckingManipulator obj = ...; // We have a reference somehow
return obj.createChild();

then the returned value is definitely a SpellCheckingManipulator, even though it's using the definition from the superclass. The recursive generics here allow the compiler to know what is obvious to us, so you don't have to keep casting the return values (like you often have to do with clone(), for example).

2) Notice that I didn't declare the method final, since perhaps some specific subclasses will want to override it with a more suitable version for themselves. The generics definition means that regardless of who create a new class or how it is defined, we can still assert that the return from e.g. BrandNewSloppilyCodedManipulator.createChild() will still be an instance of BrandNewSloppilyCodedManipulator. If a careless developer tries to define it to return just Manipulator, the compiler won't let them. And if they try to define the class as BrandNewSloppilyCodedManipulator<SpellCheckingManipulator>, it won't let them either.

Basically, the conclusion is that this trick is useful when you want to provide some functionality in a superclass that somehow gets more specific in subclasses. By declaring the superclass like this, you are locking down the generic parameter for any subclasses to be the subclass itself. This is why you can write a generic compareTo or createChild method in the superclass and prevent it from becoming overly vague when you're dealing with specific subclasses.