IdStack implements Iterable<String> { public E peek() { if (isEmpty()) { return null; } return map.get(deque.peek()); }  void push(String id, E item); E peek(); E pop(); boolean isEmpty(); int size(); String peekId(); void clear(); E get(final String id); boolean containsId(final String id); E remove(final String id); boolean isTop(final String id); @NonNull @Override Iterator<String> iterator(); Collection<E> values();  }
@Test public void peek() throws Exception { assertThat(uut.peek()).isNull(); uut.push("123", 123); uut.push("456", 456); assertThat(uut.peek()).isEqualTo(456); }