code: public Element insertByKey(Object key, Object object)
      {
          Element element = new Element(this, key, object);
          add(element);
          return element;
      }
docs: Insert the specified object into the bucket and associate it with
  the specified key. Returns the Element which holds the newly-
  inserted object.
  
  @param key key of the object element to be inserted.
  @param object the object to be inserted for the specified key.
  @return the Element holding the target object just inserted.