java - How can I require a generic parameter to be an enum that implements an interface?

Question

I'm not 100% convinced that this is a good idea, but I bumped into some code today that's currently implemented as:

class MyWidget <T extends Enum<T> > {
  MyWidget(Map<T, Integer> valueMap) {
    mValueMap = valueMap;
  }

  Map<T, Integer> mValueMap;
}

where MyWidget then offers methods that use mValueMap to convert the passed-in Enum to/from an Integer.

What I was considering doing was trying to refactor this, so that I'd declare my enumeration:

interface MyInterface {
  public Integer getValue();
}

enum MyEnum implements MyInterface {
  foo, bar;
  public Integer getValue() {
    return ordinal();
  }
}

And I'd then be able to rewrite MyWidget into something that looked vaguely like this:

public class MyWidget<T extends Enum<T> extends MyInterface> {
  ...
}

and would then be able to call the getValue() method from MyInterface on T-type objects within MyWidget. The problem, of course, is that "<T extends Enum<T> extends MyInterface>" isn't valid syntax. Is there any way to pull this off?

I don't want to just have MyWidget<T extends MyInterface>, because it's also important that T be an enumeration.

Thanks in advance!

Answer 1

Use an '&' instead:

public class MyWidget<T extends Enum<T> & MyInterface> {
    ...
}

The JLS calls this an "intersection type", but I can find no mention of it in the Java tutorials. I'll just say that it does exactly what you were wishing that "extends" would do.

Also, I should mention that you can have as many types as you want in the intersection type. So if you wanted, you could do:

public class MyWidget<T extends Enum<T> & MyInterface & Serializable & Cloneable> {
    ...
}

[Note: this code sample should not be construed as an endorsement of the Cloneable interface; it was merely handy at the time.]