Java Generics: On arraylist (unbounded wildcard type), add and addAll methods behaves differently

Question

To be direct here's an example:

    ArrayList <?> x = new ArrayList();
    ArrayList y = new ArrayList();
    x.add("abc"); // Clause 1. Compilation error - No problemo. Understood.
    x.addAll(y); // Clause 2. No compilation error

For unbounded wildcard, one cannot add. Why there is no compilation error on the last statement?

Answer 1

Because you are invoking the method with a raw type. The compiler can't perform type checks. If you make y user generics, addAll(..) will fail.