### Question:
AbstractTwoLevelMap { public Map<K, V> getItems(final K key) { return this.impl.computeIfAbsent(key, k -> new ConcurrentHashMap<>()); }  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 previouslyUnknownValueMapCanBeRetrieved() { final AbstractTwoLevelMap<String, MockValue> map = new MockTwoLevelMap(); final Map<String, MockValue> values = map.getItems("foo"); MatcherAssert.assertThat(values, IsNull.notNullValue()); }