code: public Object propertyGet(String propertyName) {
          if (propertyExists(propertyName) == false) {
              throw new NoSuchElementException("Unknown property: " + propertyName);
          }
          return data.get(propertyName);
      }
docs: Gets the value of the property.
  
  @param propertyName  the property name, not empty
  @return the value of the property, may be null