在 FXML 控制器中诊断空指针异常的问题作业

分类：编程问答

如何解决在 FXML 控制器中诊断空指针异常的问题作业

我是 JavaFX 和 FXML 的新手，我已经构建了我的主舞台，我能够更改文本框中的文本，但是每当我尝试在 TextField 中确认提交时，我都会收到空指针异常，即我一直未能成功诊断。

这是我的主课

package application;
    
import java.net.URL;

import javafx.application.Application;
import javafx.event.EventHandler;
import javafx.fxml.FXMLLoader;
import javafx.stage.Stage;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.input.KeyCode;
import javafx.scene.input.KeyEvent;


public class Main extends Application {
    static MainGuiController mainControllerHandle;
    @Override
    public void start(Stage primaryStage) {
        try {
            FXMLLoader fxmlloader = new FXMLLoader();
            URL fxmlUrl = getClass().getResource("CarRaceGuiFXML.fxml");
            mainControllerHandle = (MainGuiController)fxmlloader.getController();
            fxmlloader.setLocation(fxmlUrl);
            Parent root = fxmlloader.load();
            
            Scene scene = new Scene(root,1028,400);
            scene.getStylesheets().add(getClass().getResource("application.css").toExternalForm());
            primaryStage.setScene(scene);
            primaryStage.show();
        } catch(Exception e) {
            e.printStackTrace();
        }
        
        
    }   
    
    public static void main(String[] args) {
        launch(args);
    }
    
}

这是我的 FXML

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

<?import javafx.scene.control.*?>
<?import javafx.scene.image.*?>
<?import javafx.scene.*?>
<?import javafx.scene.text.*?>
<?import javafx.scene.shape.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.layout.GridPane?>

<Pane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="1028.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="application.MainGuiController">
   <children>
      <Rectangle arcHeight="5.0" arcWidth="5.0" fill="#9ca2a4" height="142.0" layoutY="125.0" stroke="#9ca2a4" strokeType="INSIDE" width="1028.0" />
      <Rectangle arcHeight="5.0" arcWidth="5.0" fill="#b6d36e" height="130.0" stroke="#b6d36e" strokeType="INSIDE" width="1028.0" />
      <Rectangle arcHeight="5.0" arcWidth="5.0" fill="#b6d36e" height="136.0" layoutY="264.0" stroke="#b6d36e" strokeType="INSIDE" width="1028.0" />
      <Group id="StartLine" layoutX="23.0" layoutY="128.0">
         <children>
            <Rectangle arcHeight="5.0" arcWidth="5.0" fill="#20ff1f" height="133.0" layoutX="7.0" layoutY="2.0" stroke="BLACK" strokeType="INSIDE" width="37.0" />
            <Text layoutX="-34.0" layoutY="80.0" rotate="90.0" strokeType="OUTSIDE" strokeWidth="0.0" text="Start" textAlignment="CENTER" wrappingWidth="120.0">
               <font>
                  <Font size="28.0" />
               </font>
            </Text>
         </children>
      </Group>
      <ImageView fx:id="playerCar" fitHeight="51.0" fitWidth="53.0" layoutY="212.0">
         <image>
            <Image url="@../../../../OneDrive%20-%20Saskatchewan%20Polytechnic/SaskPolytech/Year%201/Semester%202/COSC190/Asignments/Assignment%203/car1.png" />
         </image>
      </ImageView>
      <Group id="FinishLine" layoutX="957.0" layoutY="128.0">
         <children>
            <Rectangle arcHeight="5.0" arcWidth="5.0" fill="#ff1f23" height="136.0" stroke="BLACK" strokeType="INSIDE" width="37.0" />
            <Text layoutX="-37.0" layoutY="78.0" rotate="90.0" strokeType="OUTSIDE" strokeWidth="0.0" text="Finish" textAlignment="CENTER" wrappingWidth="112.0">
               <font>
                  <Font size="28.0" />
               </font>
            </Text>
         </children>
      </Group>
      <Rectangle arcHeight="5.0" arcWidth="5.0" fill="#f5f5f5" height="51.0" layoutX="382.0" layoutY="307.0" stroke="BLACK" strokeType="INSIDE" width="380.0" />
      <MenuBar prefHeight="25.0" prefWidth="1028.0">
        <menus>
          <Menu mnemonicParsing="false" text="File">
            <items>
              <MenuItem mnemonicParsing="false" text="Close" />
            </items>
          </Menu>
          <Menu mnemonicParsing="false" text="Help">
            <items>
              <MenuItem mnemonicParsing="false" text="About" />
            </items>
          </Menu>
        </menus>
      </MenuBar>
      <Text fx:id="equationDisplay" layoutX="386.0" layoutY="343.0" strokeType="OUTSIDE" strokeWidth="0.0" text="Text" textAlignment="CENTER" wrappingWidth="128.0">
         <font>
            <Font name="Times New Roman" size="31.0" />
         </font>
      </Text>
      <TextField fx:id="answerSubmission" layoutX="514.0" layoutY="320.0" onAction="#handleSubmission" prefHeight="25.0" prefWidth="141.0" />
      <ImageView fx:id="correctAnswer" fitHeight="38.0" fitWidth="38.0" layoutX="668.0" layoutY="313.0" opacity="0.0">
         <image>
            <Image url="@../../../../OneDrive/Pictures/Icons/green-checkmark1.png" />
         </image>
      </ImageView>
      <ImageView fx:id="wrongAnswer" fitHeight="38.0" fitWidth="38.0" layoutX="713.0" layoutY="314.0" opacity="0.0">
         <image>
            <Image url="@../../../../OneDrive/Pictures/Icons/red-x1.png" />
         </image>
      </ImageView>
   </children>
</Pane>

这是我的控制器

package application;


import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.TextField;
import javafx.scene.image.ImageView;
import javafx.scene.layout.GridPane;
import javafx.scene.text.Text;

import java.net.URL;
import java.util.ResourceBundle;

public class MainGuiController extends GridPane implements Initializable{
    
    
    @FXML
    
    private Text equationDisplay;{
        
    }
    
    @FXML
    public static TextField answerSubmission;{
        
}
    
    @FXML
    private ImageView correctAnswer;{
        
    }
    
    @FXML
    private ImageView wrongAnswer;{
        
    }
    
    @FXML
    private void handleSubmission(ActionEvent e) {
        String answer = answerSubmission.getText();
        System.out.println(answer);
    }

    @Override
    public void initialize(URL arg0,ResourceBundle arg1) {
        equationDisplay.setText(Equations.setEquationString(Equations.getEquationString()));

这里是 StackTrace

Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
    at javafx.fxml/javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1862)
    at javafx.fxml/javafx.fxml.FXMLLoader$ControllerMethodEventHandler.handle(FXMLLoader.java:1729)
    at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:234)
    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:8889)
    at javafx.controls/com.sun.javafx.scene.control.behavior.TextFieldBehavior.fire(TextFieldBehavior.java:154)
    at javafx.controls/com.sun.javafx.scene.control.behavior.TextInputControlBehavior.lambda$keyMapping$62(TextInputControlBehavior.java:332)
    at javafx.controls/com.sun.javafx.scene.control.inputmap.InputMap.handle(InputMap.java:274)
    at javafx.base/com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:247)
    at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
    at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:234)
    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$KeyHandler.process(Scene.java:4064)
    at javafx.graphics/javafx.scene.Scene.processKeyEvent(Scene.java:2123)
    at javafx.graphics/javafx.scene.Scene$ScenePeerListener.keyEvent(Scene.java:2591)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$KeyEventNotification.run(GlassViewEventHandler.java:217)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$KeyEventNotification.run(GlassViewEventHandler.java:149)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleKeyEvent$1(GlassViewEventHandler.java:248)
    at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:412)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.handleKeyEvent(GlassViewEventHandler.java:247)
    at javafx.graphics/com.sun.glass.ui.View.handleKeyEvent(View.java:547)
    at javafx.graphics/com.sun.glass.ui.View.notifyKey(View.java:971)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:174)
    at java.base/java.lang.Thread.run(Thread.java:832)
Caused by: java.lang.reflect.InvocationTargetException
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:64)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at com.sun.javafx.reflect.Trampoline.invoke(MethodUtil.java:76)
    at jdk.internal.reflect.GeneratedMethodAccessor2.invoke(Unknown Source)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at javafx.base/com.sun.javafx.reflect.MethodUtil.invoke(MethodUtil.java:273)
    at javafx.fxml/com.sun.javafx.fxml.MethodHelper.invoke(MethodHelper.java:83)
    at javafx.fxml/javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1857)
    ... 46 more
Caused by: java.lang.NullPointerException: Cannot invoke "javafx.scene.control.TextField.getText()" because "application.MainGuiController.answerSubmission" is null
    at application.MainGuiController.handleSubmission(MainGuiController.java:41)
    ... 57 more

我只是希望被推向正确的方向。任何帮助将不胜感激。

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