Java List toArray(T[] a) implementation

Question

I was just looking at the method defined in the List interface:

Returns an array containing all of the elements in this list in the correct order; the

Answer 1

My guess is that if you already know the concrete type of T at the point you're calling toArray(T[]), it's more performant to just declare an array of whatever it is than make the List implementation call Arrays.newInstance() for you -- plus in many cases you can re-use the array.

But if it annoys you, it's easy enough to write a utility method:

public static  E[] ToArray(Collection c, Class componentType) {
    E[] array = (E[]) Array.newInstance(componentType, c.size());
    return c.toArray(array);
}

(Note that there's no way to write E[] ToArray(Collection c), because there's no way to create an array of E at runtime without a Class object, and no way to get a Class object for E at runtime, because the generics have been erased.)