Java Enum definition

Question

I thought I understood Java generics pretty well, but then I came across the following in java.lang.Enum:

class Enum>
         


        

Answer 1

The following is a modified version of the explanation from the book Java Generics and Collections: We have an Enum declared

enum Season { WINTER, SPRING, SUMMER, FALL }

which will be expanded to a class

final class Season extends ...

where ... is to be the somehow-parameterised base class for Enums. Let's work out what that has to be. Well, one of the requirements for Season is that it should implement Comparable. So we're going to need

Season extends ... implements Comparable

What could you use for ... that would allow this to work? Given that it has to be a parameterisation of Enum, the only choice is Enum, so that you can have:

Season extends Enum
Enum implements Comparable

So Enum is parameterised on types like Season. Abstract from Season and you get that the parameter of Enum is any type that satisfies

 E extends Enum

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Maurice Naftalin (co-author, Java Generics and Collections)