code: public boolean containsKey(Object key) {
  	if (key == null) {
  	    throw new NullPointerException("key cannot be null");
  	}
  	else if (key instanceof String) {
  	    Node<V> n = lookup((String)key);
  	    return n != null && n.isTerminal();
  	}
  	else {
  	    throw new ClassCastException("The provided key does not implement" +
  					 " String: " + key);
  	}
      }
docs: Returns {@code true} if this map contains a mapping for the specified key.
  
  @throws NullPointerException if key is {@code null}
  @throws ClassCastException if key is not an instance of {@link
  String}