Can Java methods return type Enum?

java generics enums method-signature
Dexygen picture Dexygen · Sep 15, 2009 · Viewed 36.1k times · Source

I could be wrong but I'm guessing from Why can't enums be declared locally in a method? that, since an enum in Java cannot be declared locally, that therefore it is problematic for a method to return type Enum? I can declare that a method should return an Enum (see below) but how would one then go about implementing such a method to return anything other than null, or a reference to an Enum declared outside the method? My first inclination would be to investigate using Generics for this but I'd like to avoid any deadends if the SO community can help me avoid them.

private Enum resources() {
    return null;
}

Answer

David Moles picture David Moles · Sep 17, 2009

I think you're correct, it's only going to be able to either return null or an Enum declared somewhere else. But you don't necessarily have to specify that "something else" at compile time.

  class EnumEnumerator<T extends Enum<T>> implements Iterable<T> {
    private final Class<T> enumClass;

    public EnumEnumerator(Class<T> enumClass) {
      this.enumClass = enumClass;
    }

    public Iterator<T> iterator() {
      T[] values = enumClass.getEnumConstants();
      return Arrays.asList(values).iterator();
    }
  }

Later, you invoke it by specializing the generic constructor and passing in the enum class you're interested in:

class EnumEnumeratorDemo {
    enum Foo {
        BAR, BAZ, QUX;
        @Override public String toString() {
            return name().toLowerCase();
        }
    }

    public static void main(String[] args) {
        for (Foo f : new EnumEnumerator<Foo>(Foo.class)) {
            System.out.println(f);
        }
    }
}

(Obviously this is a contrived example and in real life you should just call Foo.values(), but you get the idea.)

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