Learning Java, 4th Edition

Case Study: The Enum Class

If you take a look at the definition of the java.lang.Enum class in Java 5 or later, you’ll see a rather bizarre-looking generic type declaration:

    Enum< E extends Enum<E> > { ... }

In trying to parse this, you may be hampered by two thoughts, which we’ll try to dispel right away. First, upon quick inspection this may appear to be recursive. The type variable E seems to be defined as something that’s not yet finished being defined. But it’s not really. We often have mathematical equations of the form x = function( x ) and they are not recursive. What they really call for is a special value of x that satisfies the condition. Next, although it’s pretty clear that E is a subtype of some formulation of the generic Enum type, you may jump to the conclusion that E itself must be a generic type. Remember that concrete types can extend generics just as well as generics can.

With these thoughts in mind, let’s hunt for some arrangement that satisfies these bounds. Let’s focus only on the bound for a moment:

    E extends Enum<E>

E is a subclass of some parameterization of Enum and, in particular, the parameterization of Enum on the subclass type itself. To say this again, what it does is to require that any invocations of the Enum type are by subclasses of some parameterization of the Enum type. And specifically, the parameterizations of the Enum type supply their own type as the type parameter to their parent, Enum. What kind of class satisfies this condition?

    class