How to use the Accessor

/* The UserData#set(Body, Object) method is not necessary. It's just meant to make it a bit easier because you don't have to check if a UserData instance already exists in the Body/Fixture/Joint's user data. The method sure needs some improvements, but I guess it's enough for you to get the idea. */

/** holds a {@link Box2DSprite} and {@link Breakable} for use with the according {@link Accessor Accessors}
 *  @author dermetfan */
public class UserData {

        public Box2DSprite sprite;
        public Breakable breakable;

        public UserData() {
        }

        public UserData(Box2DSprite sprite, Breakable breakable) {
            this.sprite = sprite;
            this.breakable = breakable;
        }

        /** creates a new {@link UserData} instance if the body's {@link Body#getUserData() user data} does not already hold one and puts the given object in (if it's a {@link Breakable} or {@link Box2DSprite})
         *  @param data the {@link Box2DSprite} or {@link Breakable} to put in the body's user data */
        public static void set(Body body, Object data) {
            UserData userData = body.getUserData() instanceof UserData ? (UserData) body.getUserData() : new UserData();
            if(data instanceof Box2DSprite)
                userData.sprite = (Box2DSprite) data;
            else if(data instanceof Breakable)
                userData.breakable = (Breakable) data;
        }

    }

///////////////////////////////////////////////////////////////////////////////////////////

Box2DSprite.setUserDataAccessor(new Accessor<Box2DSprite, Object>() {

    @Override
    public Box2DSprite access(Object object) {
        if(object instanceof Box2DSprite)
            return (Box2DSprite) object;
        if(object instanceof UserData)
            return ((UserData) object).sprite;
        return null;
    }

});

Breakable.Manager manager = new Breakable.Manager();
manager.setUserDataAccessor(new Accessor<Breakable, Object>() {

    @Override
    public Breakable access(Object object) {
        if(object instanceof Breakable)
            return (Breakable) object;
        if(object instanceof UserData)
            return ((UserData) object).breakable;
        return null;
    }

});

UserData.set(body, sprite);
UserData.set(body, breakable);