### Question:
AbstractTwoLevelMap { public Set<K> keySet() { return this.impl.keySet(); }  V getItem(final K key1, final K key2); Map<K, V> getItems(final K key); void remove(final K key); Set<K> keySet();  }### Answer:
@Test public void keySetReturnsEmptyCollectionIfMapIsEmpty() { final AbstractTwoLevelMap<String, MockValue> map = new MockTwoLevelMap(); MatcherAssert.assertThat(map.keySet(), IsEquivalent.equivalentTo(Collections.emptyList())); }