### Question:
ConcurrentLinkedDistinctQueue extends ConcurrentLinkedQueue<E> { public synchronized boolean add(E e) { remove(e); return super.add(e); }  synchronized boolean add(E e); @Override synchronized boolean addAll(Collection<? extends E> c);  }### Answer:
@Test public void testAddElement() { testee.add("one"); testee.add("one"); assertListContainsOnly("one"); testee.add("two"); assertListContainsOnly("one", "two"); }