Generic arrays

public class Array<E> {

    public E[] elements;

    private int size;
    private int capacity;

    @SuppressWarnings("unchecked")
    public Array() {
        size = 0;
        elements = (E[]) new Object[(capacity = 2)];
    }

    public void append(E element) {
        if (size == capacity) {
            @SuppressWarnings("unchecked")
            E[] temp = (E[]) new Object[(capacity *= 2)];

            for (int i = 0; i < size; ++i)
                temp[i] = elements[i];

            elements = temp;
        }

        elements[size++] = element;
    }

    public E get(int idx) {
        if (idx >= 0 && idx < size)
            return elements[idx];
        else
            throw new IndexOutOfBoundsException();
    }

    public static void doSomething(Array<Integer> arr) {
        Integer good = arr.get(0);

        /*
            Integer bad1 = arr.get(1);
            Integer bad2 = arr.get(2);
            Integer bad3 = arr.get(3);

            Replace `bad1', `bad2', and `bad3' with
            these and they will work without any problems.
        */

        // `bad1', `bad2', and `bad3' all produce a
        // runtime exception.
        Integer bad1 = arr.elements[1];
        Integer bad2 = ((Integer[]) arr.elements)[2];
        Integer bad3 = (Integer) arr.elements[3];

        System.out.println(good + bad1 + bad2 + bad3);
    }

    public static void main(String[] args) {
        Array<Integer> arr = new Array<>();
        arr.append(1);
        arr.append(2);
        arr.append(3);
        arr.append(4);
        Array.doSomething(arr);
    }
}