### Question:
ValueOptionMap extends AbstractMap<K, V> { @Override public boolean containsKey(Object key) { return get(key) != null; }  @SafeVarargs static Set<T> keys(T... keys); @Override abstract V get(Object key); @Override V put(K key, V value); @Override abstract Set<K> keySet(); @Override boolean containsKey(Object key); @Override Set<Map.Entry<K, V>> entrySet(); @Override int size(); @Override void forEach(BiConsumer<? super K, ? super V> action);  }### Answer:
@Test public void containsKey() { Mock<String> map = new Mock<>("a", "b", "c"); assertThat(map.containsKey("a"), is(true)); assertThat(map.containsKey("A"), is(false)); }