### Question:
CacheKeySet implements Set<E> { public int size() { int size = 0; for (Collection keySet : keySets) { size += keySet.size(); } return size; } 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 testSizeSumsAllCollections() { assertThat(keySet.size(), is(9)); }