Generics and static methods

Question

I have this static method

public static List<? extends A> myMethod(List<? extends A> a) {
  // …
}

which I'm calling using

List<A> oldAList;
List<A> newAList = (List<A>) MyClass.myMethod(oldAList);

This gives a warning because of the unchecked cast to List<A>. Is there any way of avoiding the cast?

Answer 1

You need to define the type returned matches the argument (and extends A)

public static <T extends A> List<T> myMethod(List<T> a) {
    // …
}

Then you can write

List<E> list1 = .... some list ....
List<E> list2 = myMethod(list1); // assuming you have an import static or it's in the same class.

or

List<E> list2 = SomeClass.myMethod(list1);