public E pop() {
        if (mSize == 0) {
            throw new EmptyStackException();
        }
        return mElements.set(--mSize, null);
    }
@Test
    void testIllegalPop() throws Exception {
        Assertions.assertThrows(EmptyStackException.class, () -> {
            Stack<String> stack = new Stack<String>();
            stack.pop();
        });
    }