Two methods for creating generic arrays with the same result, but only one works

Question

I recently tried to create an array of generics and found out it is not allowed:
Cannot create generic array of OptionSet<T>

I decided to make a test class and found out that there is a different method with exactly the same result that does work:

public class Test {

    @SuppressWarnings("unchecked")
    public static final void main(String[] args) {
        A<String> a = null;
        A<String> b = null;

        A<?>[] array1 = array(a, b); // fine, only a warning
        A<?>[] array2 = new A<String>[] {a, b}; // Error: Cannot create a generic array of Test.A<String>
    }

    @SuppressWarnings("unchecked")
    private static final <T> A<T>[] array(A<T>... a) {
        return a; // fine, only a warning
    }

    private static final class A<T> {}

}

Why is that? Both methods have exactly the same result, but for some reason one throws an error and the other works fine, although it gives a warning.

Answer 1

It's because of casting is happening in case of method call via return type private static final <T> A<T>[] array(A<T>... a) which is not in case of A<?>[] array2 = new A<String>[] {a, b}; where you're directly creating the instance.

to make it work You need to cast explicitly like below:

A<?>[] array2 = (A<String>[])(new A<?>[] { a, b });

What's happening is the elements are A<String> but not the array itself; it's still generic A<?> until you cast it explicitly.