Why can't a class extend an enum?

Question

I am wondering why in the Java language a class cannot extend an enum.

I'm not talking about an enum extending an enum (which can't be done, since java doesn't have multiple inheritance, and that enums implicitly extend java.lang.Enum), but a class that extends an enum in order to only add extra methods, not extra enumeration values.

Something like:

enum MyEnum
{
    ASD(5),
    QWE(3),
    ZXC(7);
    private int number;
    private asd(int number)
    {
        this.number=number;
    }
    public int myMethod()
    {
        return this.number;
    }
}

class MyClass extends MyEnum
{
    public int anotherMethod()
    {
        return this.myMethod()+1;
    }
}

To be used like this:

System.out.println(MyClass.ASD.anotherMethod());

So, can anyone provide a rationale (or point me to the right JLS section) for this limitation?

Answer 1

I think an answer to why they did it this way comes from this question:

In your example, how would you instantiate a MyClass? Enums are never explicitly instantiated (via a new MyEnum()) by the user. You'd have to do something like MyClass.ASD but not sure how that would work.

Basically, I don't know what syntax would work for your proposed addition. Which is probably why they made them final etc...

EDIT ADDED

If the author of the original Enum planned ahead (unlikely), and you are not worried too much abut thread safety, you could do something like this: (BTW, I'd probably scream at anybody who actually did this in production code, YMMV)

public enum ExtendibleEnum {

   FOO, BAR, ZXC;

   private Runnable anotherMethodRunme;  // exact Interface will vary, I picked an easy one
                           // this is what gets "injected" by your other class

   public void setAnotherMethodRunMe(Runnable r) {  // inject here
      anotherMethodRunme= r;
   }

   public void anotherMethod() {  // and this behavior gets changed
      anotherMethodRunme.run();
   }
}