Problem with JavaFX ChangeListener?

Knute Johnson <>
Thu, 11 Dec 2014 15:08:08 -0800
I'm trying to teach myself JavaFX and have hit a snag. In the program
below I add a ChangeListener to a TextField. The Java8 method works
fine, adding an anonymous ChangeListener class works fine but I can't
create a class that implements ChangeListener and get it to compile.

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>?



import javafx.application.*;
import javafx.beans.*;
import javafx.beans.value.*;
import javafx.event.*;
import javafx.scene.*;
import javafx.scene.control.*;
import javafx.scene.layout.*;
import javafx.stage.*;

import java.time.*;

public class test99 extends Application {
     public static void main(String... args) {

     public void start(Stage primaryStage) {
         VBox root = new VBox(10);
         Scene scene = new Scene(root,320,240);

         TextField textField = new TextField();
         textField.setOnAction(ae ->
textField.textProperty().addListener((observable,oldValue,newValue) -> {

         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)
         textField.textProperty().addListener(new LengthListener(4));

textField.textProperty().addListener((observable,oldValue,newValue) -> {
          ((StringProperty)observable).setValue(newValue.length() > 4 ?
          newValue.substring(0,4) : newValue);

         textField.textProperty().addListener(new ChangeListener<String>() {
             public void changed(ObservableValue<? extends String>
              String oldValue,String newValue) {
 > 4 ?
                  newValue.substring(0,4) : newValue);

         textField.setText("Hello World!");



Knute Johnson

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