### Question:
CacheMap implements Map<K, V> { public V remove(Object key) { V v = mainDataMap.remove(key); Set<K> keys = new HashSet<>(extraKeyMap.keySet()); V v2 = extraKeyMap.remove(key); for (K nk : keys) { if ((key != null && key.equals(nk)) || (key == null && nk == null)) { V v3 = extraKeyMap.remove(nk); if (v2 == null) { v2 = v3; } } } return v == null ? v2 : v; } CacheMap(); void clear(); boolean containsKey(Object key); boolean containsValue(Object value); Set<Map.Entry<K, V>> entrySet(); @SuppressWarnings("unchecked") V get(Object key); boolean isEmpty(); Set<K> keySet(); V put(K key, V value); void putAll(Map<? extends K, ? extends V> t); V remove(Object key); int size(); Collection<V> values(); String toString();  }### Answer:
@Test public void testRemove() { Map<Object, Object> definitions = new CacheMap<>(); Object putValue = new Object(); String putKey = "test"; definitions.put(putKey, putValue); Assert.assertNull(definitions.remove(null)); String removeKey = new String("test"); Object removeValue = definitions.remove(removeKey); Assert.assertEquals(putKey, removeKey); Assert.assertEquals(putValue, removeValue); Assert.assertTrue(definitions.isEmpty()); Assert.assertNull(definitions.remove(null)); }