Can I have a class field that is the Class of a type that must extend another class and an interface?

Question

I'm sorry, I can't seem to form a better sentence to ask this question. I'll try to clarify:

My class (currently an enum, but porting it to a class isn't an issue if it resolves this) has a Class field that denotes the type that it will dynamically instantiate at runtime.

This type must extend the Fragment class (Android) and also implement MyInterface.

<F extends Fragment & MyInterface>

How do I define this in code?

What works currently is an incomplete solution, which accepts any Fragment regardless of whether it implements MyInterface:

public enum Something {
  Value1 (OneFragmentInterfaceImplementor.class),
  Value2 (AnotherFragmentInterfaceImplementor.class);

  public final Class<? extends Fragment> fragment;

  private Something (final Class<? extends Fragment> fragmentClass) {
    fragment = fragmentClass;
  }
}

What I really want, though is that this class not only extends Fragment, but also implements MyInterface.

I want the correct form of below:

public <F extends Fragment & MyInterface> enum Something {
  Value1 (OneFragmentInterfaceImplementor.class),
  Value2 (AnotherFragmentInterfaceImplementor.class);

  public final Class<F> fragment;

  private Something (final Class<F> fragmentClass) {
    fragment = fragmentClass;
  }
}

Am I asking for the impossible here?

Update A great suggestion was to create an abstract Fragment that implements the interface. Unfortunately, this won't work in my specific use-case because some of these fragments extend other already implemented fragments.

Create an abstract class that extends `Fragment` and implements `MyInterface`. Make your implementations extends this class. And use that with `Class`. — Sotirios Delimanolis, Mar 08 '14 at 23:49
@SotiriosDelimanolis post your comment as an answer. It's the correct way (and the only way) to do it. — Bohemian, Mar 08 '14 at 23:51
Just looked at the code and some of these fragments extends other already implemented fragments. — copolii, Mar 08 '14 at 23:51
@Bohemian I wonder why you cannot do `` when it is noted here: http://stackoverflow.com/q/745756/1065197 — Luiggi Mendoza, Mar 08 '14 at 23:53
You are absolutely allowed to do this with class, which you anyway want (no generics with enum). So you have practically no problem. Just put the <> diamond behind the class name. — ps-aux, Mar 09 '14 at 00:04
@lui you can to a type intersection, but not an unknown one ie not ` extends A & B>` — Bohemian, Mar 09 '14 at 00:16

Sotirios Delimanolis · Accepted Answer · 2014-03-09T00:02:39.693

For use within an enum, which cannot take generic type parameters, create an abstract class that extends Fragment and implements MyInterface. Make your implementations extend this class. And use that with Class.

private final Class<AbstractClassThatExtendsFragmentAndImplementsMyInterface> fragment = ...;

You stated

Unfortunately, this won't work in my specific use-case because some of these fragments extend other already implemented fragments.

An alternative (half-)solution is to have the field declared with just one bound, but the parameter in the constructor have two. I say half solution because you won't be able to get the type bounds when accessing the Class field.

public enum Something {
    Value1(OneFragmentInterfaceImplementor.class), Value2(
            AnotherFragmentInterfaceImplementor.class);

    public final Class<? extends Fragment> fragment;

    private <T extends Fragment & Interface> Something(final Class<T> fragmentClass) {
        fragment = fragmentClass;
    }
}

If you can use a class, then you can declare

public class Example<T extends Fragment & MyInterface> {
    private final Class<T> fragment = ...;
    // ...    
}

Beautiful! I've got many red lines in the IDE right now (refactoring, don't ask), but your suggestion to apply the restrictions to the private constructor shut it right up! — copolii, Mar 09 '14 at 00:06

score 0 · Answer 2 · answered Mar 08 '14 at 23:56

You cannot pass the 'implements' clause within the 'diamond' (<>) operator as it only accepts class types. To work around the requirement you need, you can create a class (or an abstract class if you do not want your users to create objects of this type) to extend Fragment and implement interface and then pass this within the diamond operator. Simply put:

class Holder extends Fragment implements MyInterface // Mark as abstract if user should not instantiate
public final Class<? extends Holder> fragment;

I've used the `` form before so I know for a fact that it works. Just not in this specific context. See here: http://stackoverflow.com/q/745756/1065197 — copolii, Mar 08 '14 at 23:58

Can I have a class field that is the Class of a type that must extend another class and an interface?

2 Answers2