Re: Problem with JavaFX ChangeListener?

The other question I have is why does the ObservableValue require a cast
to StringProperty in both of the working examples and why do I declare
it as <? extends String>?
         class LengthListener<String> implements ChangeListener<String> {
             private final int length;

             public LengthListener(int length) {
                 this.length = length;
             }

             @Override public void changed(
              ObservableValue<? extends String> observable,
              String oldValue,String newValue) {
                 if (newValue.length() > length)
                     ((StringProperty)observable).setValue(
                      newValue.substring(0,length));
             }
         }
         textField.textProperty().addListener(new LengthListener(4));