Java泛型-将一个泛型转换为另一个

Question

Here's my code:

public class AnimationManager<STATE> {


private Map<STATE, Animation> animationMap = new HashMap<STATE, Animation>();


public void addAnimation(STATE state, BufferedImage[] frames, int frameDuration){
        animationMap.put(state, new Animation(frames, frameDuration));
}

public <E extends Enum<E>> void initAnimations(Player player, Constants.Heros chosen_hero, Class<E> heroStates){
        LinkedList<BufferedImage[]> frames = AssetsManager.getHeroGraphics(chosen_hero);

        assert frames != null;

        player.width = frames.get(0)[0].getWidth();
        player.height = frames.get(0)[0].getHeight();

        for(int i=0;i<frames.size();i++)
            addAnimation(heroStates.getEnumConstants()[i],frames.get(0),15);

}

}

At this line:

addAnimation(heroStates.getEnumConstants()[i],frames.get(0),15);

compiler shouts: Wrong 1st argument type, found 'E', required 'STATE'.

When initAnimations() method is in a different class (without STATE) it compiles and works like a charm. How is that possible that he doesn't want to get the first parameter and convert it to a STATE type?

Answer 1

The type parameter STATE are unbounded and can be invoked with any class. The type parameter E is bounded to be a subclass of Enum. The variable heroStates are a Class which means the getEnumConstants() method returns an array of E (which means an array of some subclass of Enum). That can not safely be cast to the type of STATE.

There are several ways to proceed from here, depending on your specific requirements. If casting can be avoided, that would be preferable.

If you know that STATE type parameter always is going to be an enum, you could put a bound on STATE and skip the type parameter E altogether:

class AnimationManager<STATE extends Enum<STATE>>
...
void initAnimations(Player player, Heros chosen_hero, Class<STATE> heroStates)

Otherwise, if you do not know that STATE is an enum, then you can avoid the class reflection exercise of getting the enum types by letting the caller of your initAnimations() method provide them:

void initAnimations(Player player, Heros chosen_hero, List<STATE> heroStates)
...
addAnimation(heroStates.get(i), frames.get(0), 15);

and the invocation would be something like

AnimationManager<Foo> animationManager = ...;
animationManager.initAnimations(null, null, Arrays.asList(Foo.values()));

The third option, which I really don't like, would be to cast. In that case you just need to change the problematic line to

addAnimation((STATE)heroStates.getEnumConstants()[i], frames.get(0), 15);

That will generate a warnings for an unchecked cast, which means the generics type system cannot guarantee that you're not ending up with ClassCastException s when you run your code.

Good luck with your game!