Java 中的通配符，用于从相同 class 继承的类

Question

Having the next classes in Java:

class A {
}
class B extends A {
}
class C extends A {
}

class ClassMethods {
    A method1(ArrayList<? extends A> list) {
        return list.get(1);
    }
    void method2(ArrayList<?> list, A el) {
        list.add(el);
    }

    void method3(A elem) {
        ArrayList<A> listA = new ArrayList<A>();
        listA.add(new A()); listA.add(new A());
        ArrayList<B> listB = new ArrayList<B>();
        listA.add(new B()); listB.add(new B());
        ArrayList<C> listC = new ArrayList<C>();
        listC.add(new C()); listC.add(new C());

        this.method1(listA);
        this.method1(listB);
        this.method1(listC);

        this.method2(listA, elem);
        this.method2(listB, elem);
        this.method2(listC, elem);
    }
}

Is there any way to make method2 working? Without overloading it. Because we write in the list, we may need a "? super x" specifier. But "super B" will not work for a C class for example. So I think the only possibility to make it work is to have one with "? super B" and an overload with "? super C". I can change only the "<...>" part of it, this is how the problem was proposed, and to explain if it can not be done.

Answer 1

Suppose A was Object and B was String , would you expect to be able to take a List<String> strs , Object item and call strs.add(item) ?

The closest thing you could do is dd a type parameter.

<T> void method2(ArrayList<? super T> list, T el) {
    list.add(el);
}

but that is the wrong way around.

If you try to inline method2 you will see the error more directly.

    listA.add(elem);
    listB.add(elem); // No chance.
    listC.add(elem); // No chance.