public void clear() {
        mSize = 0;
        mElements.clear();
    }
@Test
    void testClear() throws Exception {
        Stack<String> stack = new Stack<String>();
        stack.push("one");
        stack.push("two");
        assertThat(stack.isEmpty(), is(false));
        stack.clear();
        assertThat(stack.isEmpty(), is(true));
    }