code: public boolean containsKey(String key) {
  
          if (key == null) {
              throw new NullPointerException("Missing resource key.");
          }
  
          PropertyBundle p = this;
  
          do {
              String value = p.key2values.get(key);
              if (value != null) {
                  return true;
              }
          } while ((p = p.parent) != null);
  
          return false;
  
      }
docs: /*[deutsch]
  <p>Ermittelt, ob zum angegebenen Schl&uuml;ssel ein Eigenschaftenwert vorhanden ist. </p>
  
  @param   key     the key of property resource
  @return  {@code true} if the property for given key exists else {@code false}