@Test
  public void dequeueTest() {
    // When the stack empty...
    try {
      queue.dequeue();
      fail("A IllegalStateException should be thrown.");
    } catch (IllegalStateException e) {
    }

    // Adding some elements...
    queue.enqueue(1);
    queue.enqueue(2);
    queue.enqueue(3);
    assertEquals(1, queue.dequeue());

    // Checking that this method removes something.
    assertEquals(2, queue.dequeue());
  }
  @Test
  public void sizeTest() {
    // When the stack empty...
    assertEquals(0, queue.size());

    // Adding some elements...
    queue.enqueue(1);
    assertEquals(1, queue.size());

    // Testing it backwards...
    queue.dequeue();
    assertEquals(0, queue.size());
  }
  @Test
  public void isEmptyTest() {
    // When the stack empty...
    assertEquals(true, queue.isEmpty());

    // Adding some elements...
    queue.enqueue(1);
    assertEquals(false, queue.isEmpty());

    // Testing it backwards...
    queue.dequeue();
    assertEquals(true, queue.isEmpty());
  }