Java FXML : Nullpointerexception

Question

The code should open a to-do window in which tasks can be specified in a text field. These are stored in a VBox as a checkbox. If the CheckBox are marked as "checked", they will be removed from the list. However, these should be available in a new window using the "Done" button. The Done window can be opened, but the program cannot access the VBox doneBox. NPE is caused by: doneBox.getChildren().add(l); in Controller1

Are there any solutions for this problem?

Error message:

Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
    at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1774)
    at javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1657)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
    at javafx.event.Event.fireEvent(Event.java:198)
    at javafx.scene.Node.fireEvent(Node.java:8411)
    at javafx.scene.control.Button.fire(Button.java:185)
    at com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:182)
    at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:96)
    at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:89)
    at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
    at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
    at javafx.event.Event.fireEvent(Event.java:198)
    at javafx.scene.Scene$MouseHandler.process(Scene.java:3757)
    at javafx.scene.Scene$MouseHandler.access$1500(Scene.java:3485)
    at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1762)
    at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2494)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:394)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:295)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$2(GlassViewEventHandler.java:432)
    at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:410)
    at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:431)
    at com.sun.glass.ui.View.handleMouseEvent(View.java:555)
    at com.sun.glass.ui.View.notifyMouse(View.java:937)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.lambda$null$4(WinApplication.java:186)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.reflect.misc.Trampoline.invoke(Unknown Source)
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.reflect.misc.MethodUtil.invoke(Unknown Source)
    at javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1769)
    ... 48 more
Caused by: java.lang.NullPointerException
    at Controller1.initialize(Controller1.java:93)
    at Controller1.handleDoneButton(Controller1.java:79)
    ... 58 more

Main:

import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;

public class ToDo extends Application {

    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage primaryStage) throws Exception {

        Parent root = FXMLLoader.load(getClass().getResource("Layout1.fxml"));
        
        Scene scene = new Scene(root);
        primaryStage.setTitle("ToDo List");
        primaryStage.setScene(scene);
        primaryStage.setResizable(false);
        primaryStage.show();
        
    }

}

Controller1:

import java.io.IOException;
import java.util.ArrayList;

import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.CheckBox;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class Controller1 {
    
    ArrayList<CheckBox> list = new ArrayList();
    ArrayList<CheckBox> donelist = new ArrayList();
    String[] part;
    
    @FXML
    VBox taskBox;
    @FXML
    TextField task;
    @FXML
    CheckBox c;
    @FXML
    Button done;
    @FXML 
    VBox doneBox;
    
    @FXML
    private void handleAddButton(ActionEvent event) {
        String text = task.getText();
        c = new CheckBox(text);
        list.add(c);
        task.clear();

        c.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            @FXML
            public void handle(ActionEvent e) {
                for (int i = 0; i < list.size(); i++) {
                    if (list.get(i).isSelected() == true) {
                        taskBox.getChildren().remove(list.get(i));
                        donelist.add(list.get(i));
                        list.remove(i);
                    }
                }

            }
        });
        taskBox.getChildren().add(c);
    }

    @FXML
    private void handleDoneButton(ActionEvent event) {
        Parent root;
        try {
            root = FXMLLoader.load(getClass().getClassLoader().getResource("Layout2.fxml"));
            
            // new window
            Stage stage1 = new Stage();
            stage1.setTitle("Done Tasks");
            stage1.setScene(new Scene(root, 190, 190));
            stage1.setResizable(false);
            stage1.show();
            
            initialize();
            
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    // Error in this method:
    @FXML
    private void initialize() {
         
        for (int i = 0; i < donelist.size(); i++) {
            part = donelist.get(i).toString().split("'");
            String text = part[1];
            
            Label l = new Label(text);
// Caused by: java.lang.NullPointerException
   // at Controller1.initialize(Controller1.java:93):
            doneBox.getChildren().add(l);
        }
    }
}

Layout1:

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.ScrollPane?>
<?import javafx.scene.control.TextField?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.BorderPane?>
<?import javafx.scene.layout.HBox?>
<?import javafx.scene.layout.VBox?>
<?import javafx.scene.text.Font?>

<BorderPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="303.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="Controller1">
   <center>
      <ScrollPane prefHeight="328.0" prefWidth="587.0" BorderPane.alignment="CENTER">
         <content>
            <VBox fx:id="taskBox" prefHeight="309.0" prefWidth="303.0" />
         </content>
      </ScrollPane>
   </center>
   <bottom>
      <HBox prefHeight="27.0" prefWidth="600.0" BorderPane.alignment="CENTER">
         <children>
            <TextField fx:id="task" prefHeight="25.0" prefWidth="213.0" promptText="Aufgabe hier eingeben" HBox.hgrow="ALWAYS" />
            <Button mnemonicParsing="false" onAction="#handleAddButton" prefHeight="25.0" prefWidth="81.0" text="Hinzufügen" />
         </children>
      </HBox>
   </bottom>
   <top>
      <AnchorPane prefHeight="45.0" prefWidth="600.0" style="-fx-background-color: #2A2C2E;" BorderPane.alignment="CENTER">
         <children>
            <Label alignment="CENTER" layoutX="118.0" layoutY="4.0" text="ToDo" textFill="#e5e7e9" textOverrun="CLIP">
               <font>
                  <Font name="System Bold" size="26.0" />
               </font>
            </Label>
            <Button fx:id="done" layoutX="252.0" layoutY="11.0" mnemonicParsing="false" onAction="#handleDoneButton" text="Done" />
         </children>
      </AnchorPane>
   </top>
</BorderPane>

Layout2:

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.control.Label?>
<?import javafx.scene.control.ScrollPane?>
<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.BorderPane?>
<?import javafx.scene.layout.VBox?>
<?import javafx.scene.text.Font?>

<BorderPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="292.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="Controller1">
   <top>
      <AnchorPane prefHeight="44.0" prefWidth="292.0" style="-fx-background-color: #2A2C2E;" BorderPane.alignment="CENTER">
         <children>
            <Label layoutX="14.0" layoutY="5.0" text="Done Tasks" textFill="WHITE">
               <font>
                  <Font size="23.0" />
               </font>
            </Label>
         </children>
      </AnchorPane>
   </top>
   <center>
      <ScrollPane prefHeight="362.0" prefWidth="292.0" BorderPane.alignment="CENTER">
         <content>
            <VBox fx:id="doneBox" prefHeight="349.0" prefWidth="283.0" />
         </content>
      </ScrollPane>
   </center>
</BorderPane>

You shouldn't be calling initialize from other methods, which line specifically is 93, the one causing the NPE? — matt, Jul 03 '20 at 11:51
Does this answer your question? [What is a NullPointerException, and how do I fix it?](https://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) — matt, Jul 03 '20 at 11:55
You're using the same controller for both layouts, but it isn't going to use the same *instance* There will be two instances of Controller1, the one you call 'initialize' on is the controller for layout1. — matt, Jul 03 '20 at 12:07
The NPE is caused by doneBox.getChildren().add(l); in the Controller1 class — Bloom, Jul 03 '20 at 12:07

score 0 · Accepted Answer · answered Jul 03 '20 at 12:14

You have two different instances of Controller1. I strongly suggest making two controllers to make this clearer.

If you must this could be a workaround, you can create an FXMLLoader object to get the reference to the created Controller1.

    Parent root;
    try {
        FXMLLoader loader = new FXMLLoader(getClass().getClassLoader().getResource("Layout2.fxml") );
        root = loader.load();
        Controller1 controller2 = loader.getController();
        // new window
        Stage stage1 = new Stage();
        stage1.setTitle("Done Tasks");
        stage1.setScene(new Scene(root, 190, 190));
        stage1.setResizable(false);
        stage1.show();
        
        //don't call initialize.
        //initialize();  
        for (int i = 0; i < donelist.size(); i++) {
            part = donelist.get(i).toString().split("'");
            String text = part[1];
        
            Label l = new Label(text);
            controller2.doneBox.getChildren().add(l);
        }
    } catch (IOException e) {
        e.printStackTrace();
    }

Java FXML : Nullpointerexception

1 Answers1