避免在未检查的类强制转换异常方面发出警告？

Question

Code first: )

   /**
     *
     * @param arrayList the ArrayList
     * @return a random object from the ArrayList
     */
    public static Object randomArrayListObject(ArrayList<?> arrayList) {

        Random random = new Random();
        int i = random.nextInt(arrayList.size());
        assert i >= 0;
        assert i < arrayList.size();
        return arrayList.get(i);

    }

    public static Class<? extends Juice>  randomArrayListJuiceClass(
            ArrayList<Class<? extends Juice>> arrayList) {

        final Object o = randomArrayListObject(arrayList);
        assert Juice.class.isAssignableFrom((Class) o);
        if (Juice.class.isAssignableFrom((Class) o)) {
            @SuppressWarnings("unchecked")
            Class c = (Class<? extends Juice>) o;
            return c;
        } else throw new RuntimeException("wtf? not a Juice Class??"); 
    }

I'm getting this warning:

Unchecked assignment: 'java.lang.Class' to 'java.lang.Class<? extends drinks.Juice>'

I put all that monster code in randomArrayListJuiceClass() to try to avoid such warnings but still I can't figure it out. What can I do?

My goal is just to return the (some kind of) Juice Class without warnings... I know I'm only feeding randomArrayListJuiceClass() some Juice , I just have it throwing an Exception just in case I was sloppy and fed it something else.

What should I do?

Answer 1

Your method for getting a random element should be generic and bind its return type to the type of the input ArrayList

public static <T> T randomArrayListObject(ArrayList<T> arrayList) {

Your other code then becomes

final Class<? extends Juice> o = randomArrayListObject(arrayList);
assert Juice.class.isAssignableFrom(o); // even this is kind of useless
return o;

Assuming your assertions are on, you won't need another exception thrown.