### Question:
CacheKeySet implements Set<E> { public boolean contains(final Object o) { for (Collection keySet : keySets) { if (keySet.contains(o)) { return true; } } return false; } CacheKeySet(final Collection<E>... keySets); int size(); boolean isEmpty(); boolean contains(final Object o); Iterator<E> iterator(); Object[] toArray(); T[] toArray(final T[] a); boolean add(final Object o); boolean remove(final Object o); boolean containsAll(final Collection<?> c); boolean addAll(final Collection c); boolean removeAll(final Collection<?> c); boolean retainAll(final Collection<?> c); void clear();  }### Answer:
@Test public void testContainsIsSupported() { Set<Integer> keys = new HashSet<Integer>(keySet); for (Integer key : keys) { assertThat(keySet.contains(key), is(true)); } }