code: public boolean containsKey(Object key) {
      EntryImpl<K, V> entry = _entries[keyHash(key) & _mask];
      while (entry != null) {
        if (key.equals(entry._key)) { return true; }
        entry = entry._next;
      }
      return false;
    }
docs: Indicates if this {@link FastMap} contains a mapping for the specified
  key.
  
  @param key the key whose presence in this map is to be tested.
  @return <code>true</code> if this map contains a mapping for the
  specified key; <code>false</code> otherwise.
  @throws NullPointerException if the key is <code>null</code>.