@Nullable
    public abstract E pop();
@Test
    public void pushPop() {
        final PriorityQueue<Priority, String> queue = populateQueue();
        assertEquals("7", queue.pop());
        assertEquals("2", queue.pop());
        assertEquals("4", queue.pop());
        assertEquals("1", queue.pop());
        assertEquals("3", queue.pop());
        assertEquals("5", queue.pop());
        assertEquals("6", queue.pop());
        assertEquals("8", queue.pop());
    }