code: public String getString(final String _key)
      {
          String ret = null;
          if (this.attributes.containsKey(_key)) {
              ret = this.attributes.get(_key).getValue();
          }
          return ret;
      }
docs: Returns the value for a key as a String.
  
  @param _key key for the searched value
  @return string of the value if exist; otherwise <code>null</code>