How to do multi-forming from already-coded FXML files in JavaFX?

Question

So I'm creating a project in JavaFX which requires the use of multiple forms, and I decided to create 2 FXML files using SceneBuilder. However, when I need to manipulate the forms using the code, it works for the first form, but produces a NullPointerException for the second form. Currently, I have both forms linked to a single controller, which uses the following method to switch between forms (this method is in the controller):

private void launch(Stage primaryStage, String form, String title) throws Exception {
        Parent root = FXMLLoader.load(getClass().getResource(form));
        primaryStage.setTitle(title);
        primaryStage.setScene(new Scene(root, 600, 400));
        primaryStage.show();
    }

And the stage is basically obtained from the Main class. The result is that the form will load, but a NullPointerException is produced as follows:

java.lang.NullPointerException
    at Optimizer.Controller2.displayData(Controller2.java:70)
    at Optimizer.Controller.displayData(Controller.java:190)
    at Optimizer.Controller.loadHandler(Controller.java:63)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at com.sun.javafx.reflect.Trampoline.invoke(MethodUtil.java:76)
    at jdk.internal.reflect.GeneratedMethodAccessor2.invoke(Unknown Source)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at javafx.base/com.sun.javafx.reflect.MethodUtil.invoke(MethodUtil.java:275)
    at javafx.fxml/com.sun.javafx.fxml.MethodHelper.invoke(MethodHelper.java:83)
    at javafx.fxml/javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1782)
    at javafx.fxml/javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1670)
    at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at javafx.base/com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at javafx.base/com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
    at javafx.base/javafx.event.Event.fireEvent(Event.java:198)
    at javafx.graphics/javafx.scene.Node.fireEvent(Node.java:8863)
    at javafx.controls/javafx.scene.control.Button.fire(Button.java:200)
    at javafx.controls/com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:206)
    at javafx.controls/com.sun.javafx.scene.control.inputmap.InputMap.handle(InputMap.java:274)
    at javafx.base/com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
    at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
    at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
    at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at javafx.base/com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at javafx.base/com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
    at javafx.base/javafx.event.Event.fireEvent(Event.java:198)
    at javafx.graphics/javafx.scene.Scene$MouseHandler.process(Scene.java:3876)
    at javafx.graphics/javafx.scene.Scene$MouseHandler.access$1300(Scene.java:3604)
    at javafx.graphics/javafx.scene.Scene.processMouseEvent(Scene.java:1874)
    at javafx.graphics/javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2613)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:397)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:295)
    at java.base/java.security.AccessController.doPrivileged(Native Method)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$2(GlassViewEventHandler.java:434)
    at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:433)
    at javafx.graphics/com.sun.glass.ui.View.handleMouseEvent(View.java:556)
    at javafx.graphics/com.sun.glass.ui.View.notifyMouse(View.java:942)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:175)
    at java.base/java.lang.Thread.run(Thread.java:844)

I've tried using separate controllers for each form, but that will produce a LoadException and IntelliJ won't even show the icon which links the variables for the controls to the fx:id. The second form also won't even load.

For a minimal reproducible example, here's the Main class:

package sample;

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

public class Main extends Application {
    private static Stage stage;

    public static Stage getStage() {
        return stage;
    }

    @Override
    public void start(Stage primaryStage) throws Exception{
        stage = primaryStage;
        Parent root = FXMLLoader.load(getClass().getResource("sample.fxml"));
        primaryStage.setTitle("Hello World");
        primaryStage.setScene(new Scene(root, 300, 275));
        primaryStage.show();
    }


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

Here's the Controller class:

package sample;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.event.ActionEvent;
import javafx.scene.control.TextField;
import javafx.stage.Stage;

public class Controller {
    @FXML Button btnNew;
    @FXML TextField txt;

    public void switchHandler (ActionEvent evt) {
        try {
            launch(Main.getStage(), "sample2.fxml", "");
            txt.setText("Hello World");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private void launch(Stage primaryStage, String form, String title) throws Exception {
        Parent root = FXMLLoader.load(getClass().getResource(form));
        primaryStage.setTitle(title);
        primaryStage.setScene(new Scene(root, 600, 400));
        primaryStage.show();
    }
}

Here's the first form:

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

<?import javafx.scene.control.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>
<?import javafx.geometry.Insets?>
<?import javafx.scene.layout.GridPane?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>


<AnchorPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="sample.Controller">
   <children>
      <Button fx:id="btnNew" onAction="#switchHandler" layoutX="213.0" layoutY="145.0" mnemonicParsing="false" text="Button" />
   </children>
</AnchorPane>

And here's the second form:

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

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


<AnchorPane prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="sample.Controller">
   <children>
      <TextField fx:id="txt" layoutX="151.0" layoutY="156.0" />
   </children>
</AnchorPane>

So the program is supposed to first make the first form pop out, and then go to the second form at the click of the button and then change the text of the TextField in the second form to "Hello World". The second form pops out as normal, but the line which tries to change the text of the TextField creates a NullPointerException.

a) find the exact location of the NPE b) debug to find why it is null c) when stuck, come back with a [mcve] demonstrating the problem — kleopatra, Oct 19 '20 at 15:50
@kleopatra I have added a minimal reproducible example. The bug appears in the line `txt.setText("Hello World");` in the controller, but I do not know how to fix it. — Richard, Oct 19 '20 at 21:32
**Never** use the same controller class with multiple FXML files. Each controller _class_ should map one-to-one with an FXML file. If you do that then you're problem—which is that only some fields are injected depending on the which FXML file was loaded—will go away. If you need to communicate between the two controllers, check out [Passing Parameters JavaFX FXML](https://stackoverflow.com/questions/14187963/passing-parameters-javafx-fxml). — Slaw, Oct 20 '20 at 00:46
@Slaw Thanks! I've done as you said (and figured out another problem I faced along the way), but now there's a new NullPointerException (which I suspect has a different cause). I've posted a new question which you can see [here](https://stackoverflow.com/questions/64438499/how-to-call-a-method-in-another-controller-when-multiforming-with-javafx) and I would greatly appreciate it if you could help me again! — Richard, Oct 20 '20 at 04:31

How to do multi-forming from already-coded FXML files in JavaFX?

0 Answers0