Getting Null Pointer Exception with setCenter() method in JavaFX with Scene Builder 8.0

Question

I created a small program to reproduce the error I am getting in my actual project. I have a controller class called MainWindow.java that is responsible for two .fxml files: MainWindow.fxml and AnchorTest.fxml.

Code for the controller class:

package projecterror.controller;

import java.io.IOException;

import javafx.application.Application;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.geometry.Rectangle2D;
import javafx.scene.Scene;
import javafx.scene.control.Menu;
import javafx.scene.control.MenuItem;
import javafx.scene.layout.AnchorPane;
import javafx.scene.layout.BorderPane;
import javafx.stage.Screen;
import javafx.stage.Stage;

public class MainWindow extends Application {

    private Stage primaryStage;
    private BorderPane mainWindow;

    @FXML
    Menu menuFile, menuAnalysis;
    @FXML
    MenuItem menuNew;


    @FXML
    private void initialize() {
        menuNew.setOnAction((event) -> {
            try {
                FXMLLoader loader = new FXMLLoader();
                loader.setLocation(getClass().getResource("../view/AnchorTest.fxml"));
                AnchorPane anchorTest = (AnchorPane) loader.load();
                mainWindow.setCenter(anchorTest);
            } catch (IOException e) {
                e.printStackTrace();
            }
        });
    }

    @Override
    public void start(Stage primaryStage) throws Exception {
        this.primaryStage = primaryStage;
        this.primaryStage.setTitle("Test Project");

        initMainWindow();

    }

    public void initMainWindow() {
        try {
            FXMLLoader loader = new FXMLLoader();
            loader.setLocation(getClass().getResource("../view/MainWindow.fxml"));
            mainWindow = (BorderPane) loader.load();

            Scene scene = new Scene(mainWindow);
            primaryStage.setScene(scene);

            //Fullscreen
            Screen screen = Screen.getPrimary();
            Rectangle2D bounds = screen.getVisualBounds();
            primaryStage.setX(bounds.getMinX());
            primaryStage.setY(bounds.getMinY());
            primaryStage.setWidth(bounds.getWidth());
            primaryStage.setHeight(bounds.getHeight());

            primaryStage.setResizable(false);
            primaryStage.show();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

}

MainWindow.fxml is a BorderPane that has a MenuBar on top.

Code:

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

<?import javafx.scene.control.Menu?>
<?import javafx.scene.control.MenuBar?>
<?import javafx.scene.control.MenuItem?>
<?import javafx.scene.control.SeparatorMenuItem?>
<?import javafx.scene.layout.BorderPane?>

<BorderPane style="-fx-background-color: #DCDCDC;" xmlns="http://javafx.com/javafx/9.0.1" xmlns:fx="http://javafx.com/fxml/1" fx:controller="projecterror.controller.MainWindow">
    <top>
      <MenuBar BorderPane.alignment="CENTER">
        <menus>
          <Menu fx:id="menuFile" mnemonicParsing="false" text="File">
            <items>
                  <Menu fx:id="menuAnalysis" mnemonicParsing="false" text="Analysis">
                    <items>
                      <MenuItem fx:id="menuNew" mnemonicParsing="false" text="New" />
                    </items>
                  </Menu>
            </items>
          </Menu>
        </menus>
      </MenuBar>
   </top>
</BorderPane>

AnchorTest.fxml is an AnchorPane that has as children two AnchorPane.

Code:

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

<?import java.lang.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.layout.AnchorPane?>

<AnchorPane xmlns:fx="http://javafx.com/fxml/1">
    <children>
            <AnchorPane layoutX="6.0" layoutY="450.0" />
            <AnchorPane layoutX="6.0" layoutY="569.0" prefHeight="116.0" prefWidth="730.0">
                  <Button layoutX="6.0"   layoutY="26.0" mnemonicParsing="false" prefHeight="25.0" prefWidth="102.0" text="Button 1" />
                  <Button layoutX="115.0" layoutY="26.0" mnemonicParsing="false" prefWidth="102.0" text="Button 2" />
                  <Button layoutX="226.0" layoutY="26.0" mnemonicParsing="false" prefWidth="102.0" text="Button 3" />
                  <Button layoutX="115.0" layoutY="75.0" mnemonicParsing="false" prefWidth="102.0" text="Button 4" />
                  <Button layoutX="383.0" layoutY="26.0" mnemonicParsing="false" prefWidth="102.0" text="Button 5" />
                  <Button layoutX="491.0" layoutY="26.0" mnemonicParsing="false" prefWidth="102.0" text="Button 6" />
                  <Button layoutX="600.0" layoutY="26.0" mnemonicParsing="false" prefWidth="102.0" text="Button 7" />
                  <Button layoutX="491.0" layoutY="75.0" mnemonicParsing="false" prefWidth="102.0" text="Button 8" />
                  <Button layoutX="600.0" layoutY="75.0" mnemonicParsing="false" prefWidth="102.0" text="Button 9" />
                  <TextField alignment="TOP_CENTER" layoutX="341.0" layoutY="26.0" prefHeight="25.0" prefWidth="28.0" text="10" />
            </AnchorPane>
         </children>
</AnchorPane>

When on MainWindow.fxml, after I click on the menu File > Analysis > New, it is supposed to place AnchorTest.fxml content into MainWindow.fxml center. However, I get the following stack trace error:

 Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
        at projecterror.controller.MainWindow.lambda$0(MainWindow.java:35)
        at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
        at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
        at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
        at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
        at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
        at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
        at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
        at javafx.event.Event.fireEvent(Event.java:198)
        at javafx.scene.control.MenuItem.fire(MenuItem.java:462)
        at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.doSelect(ContextMenuContent.java:1405)
        at com.sun.javafx.scene.control.skin.ContextMenuContent$MenuItemContainer.lambda$createChildren$343(ContextMenuContent.java:1358)
        at com.sun.javafx.event.CompositeEventHandler$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.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$MouseEventNotification.run(GlassViewEventHandler.java:380)
        at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:294)
        at java.security.AccessController.doPrivileged(Native Method)
        at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$354(GlassViewEventHandler.java:416)
        at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
        at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:415)
        at com.sun.glass.ui.View.handleMouseEvent(View.java:555)
        at com.sun.glass.ui.View.notifyMouse(View.java:937)
        at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
        at com.sun.glass.ui.win.WinApplication.lambda$null$148(WinApplication.java:191)
        at java.lang.Thread.run(Thread.java:745)

The line that is flagging the NPE is:

mainWindow.setCenter(anchorTest);

I have read other questions similar to this but mostly all of their problems was regarding the path of the fxml files. I firmly believe that is not the problem since in my actual project I use the same type of path for other views and I don't have problem so far.

I have uploaded the project here in case someone wants to run the program.

Project structure is as it follows:

Thanks in advance and any help would be greatly appreciated!

I have never seen a `JavaFX Main` structured in this fashion. — SedJ601, Dec 28 '17 at 18:06
`mainWindow` is not initialized in the controller; it is initialized in the `Application` instance that is created when the application is launched (the instance on which `start()` is called). Bottom line here is you should not use the application class as the controller class; it is too confusing (and violates all sorts of basic OOP design principles, such as "Single Responsibility"). — James_D, Dec 28 '17 at 18:10
Thanks for the answers. I'm pretty new to software designs so it's kind of confusing for me at first. So it should work if I separate the Main from the controller classes? — ihavenoidea, Dec 28 '17 at 18:19
Yes: create a separate controller class. You can access the border pane in it in exactly the same way you access the other elements, i.e. give the border pane a `fx:id` in the FXML file, and create a corresponding `@FXML`-annotated field in the controller. — James_D, Dec 28 '17 at 18:20
Nominating for reopening. The Null Pointer Exception here is caused because the OP is unaware that he or she is initializing the field in a different instance than the instance in which is it being accessed. The question marked as duplicate doesn't address this scenario. — James_D, Dec 28 '17 at 20:06

James_D · Accepted Answer · 2017-12-28T20:17:33.577

When you launch a JavaFX application, an instance of your application class is created, and (among other things that happen), the start() method is invoked on that Application instance on the FX Application Thread.

When you load an FXML file, if the FXML file specifies a controller class, an instance of that controller class is created, the @FXML-annotated fields are injected into that instance, and the initialize() method is called. All this happens during the call to FXMLLoader.load().

Consequently, in your code, you end up with two different instances of MainWindow. The start() method is invoked on one instance, which initializes the mainWindow field, and the initialize() method is invoked on the other instance. Since mainWindow was never initialized in that second instance, you get a null pointer exception when you try to dereference it in the initialize() method.

Separate the controller class from the Application class. The Application class should do nothing other than manage the lifecycle of the application as a whole (typically, it should implement the code needed to start the application):

package projecterror.controller;

import java.io.IOException;

import javafx.application.Application;
import javafx.fxml.FXMLLoader;
import javafx.geometry.Rectangle2D;
import javafx.scene.Scene;
import javafx.scene.layout.BorderPane;
import javafx.stage.Screen;
import javafx.stage.Stage;

public class MainWindow extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception {
        primaryStage.setTitle("Test Project");

        initMainWindow(primaryStage);

    }

    public void initMainWindow(Stage primaryStage) {
        try {
            FXMLLoader loader = new FXMLLoader();
            loader.setLocation(getClass().getResource("../view/MainWindow.fxml"));
            BorderPane mainWindow = loader.load();

            Scene scene = new Scene(mainWindow);
            primaryStage.setScene(scene);

            //Fullscreen
            Screen screen = Screen.getPrimary();
            Rectangle2D bounds = screen.getVisualBounds();
            primaryStage.setX(bounds.getMinX());
            primaryStage.setY(bounds.getMinY());
            primaryStage.setWidth(bounds.getWidth());
            primaryStage.setHeight(bounds.getHeight());

            primaryStage.setResizable(false);
            primaryStage.show();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

}

package projecterror.controller;

import java.io.IOException;

import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.scene.control.Menu;
import javafx.scene.control.MenuItem;
import javafx.scene.layout.AnchorPane;
import javafx.scene.layout.BorderPane;

public class MainWindowController {

    @FXML
    private BorderPane mainWindow;

    @FXML
    Menu menuFile, menuAnalysis;
    @FXML
    MenuItem menuNew;


    @FXML
    private void initialize() {
        menuNew.setOnAction((event) -> {
            try {
                FXMLLoader loader = new FXMLLoader();
                loader.setLocation(getClass().getResource("../view/AnchorTest.fxml"));
                AnchorPane anchorTest = (AnchorPane) loader.load();
                mainWindow.setCenter(anchorTest);
            } catch (IOException e) {
                e.printStackTrace();
            }
        });
    }



}

In order for the controller to access the BorderPane, you need to inject it from the FXML (note the annotation on it in the controller code above). Add an fx:id to the element in the FXML file:

<BorderPane style="-fx-background-color: #DCDCDC;" fx:id="mainWindow"
    xmlns="http://javafx.com/javafx/9.0.1" xmlns:fx="http://javafx.com/fxml/1" 
    fx:controller="projecterror.controller.MainWindowController">

Just for completeness, I think you forgot the `private Stage primaryStage;` inside MainWindow class. It works perfectly, thanks for your explanation. — ihavenoidea, Dec 28 '17 at 20:14
@leandrocoutom Actually, I intended to omit it, since it is only needed in the `start()` and `initMainWindow()` methods, but I didn't complete the change. Let me fix.... — James_D, Dec 28 '17 at 20:17

Getting Null Pointer Exception with setCenter() method in JavaFX with Scene Builder 8.0

1 Answers1