Question

我已经尽最大的努力来获取一个按钮来更改场景，但是我无法使其正常工作。

当我点击按钮时，我得到一个nullpointerexception。这与控制器类中的button2action方法有关。我知道它已与FXML正确链接，因为单击该消息时我正在打印调试消息。

我花了很多时间徒劳地尝试使它起作用，因为您可能可以从我在主要班级中评论过的所有废话中看到（其中一些可能已经进行了一些调整，所以我已经留下它，以防万一有人发现我在那个烂摊子里看不到的东西。

这是我的控制人：

package com.frogger.mainapp;

import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.scene.Scene;

public class SampleController {

    @FXML
    public void button2action(ActionEvent event) {
        System.out.println("This is horrible!");
        Main.primaryStage.setScene(Main.scene);
        Main.primaryStage.centerOnScreen();
        Main.animal2 = new Animal("file:resources/froggerUp.png", 400.00, 700.00, true);
        Main.background.add(Main.animal2);
    }
}

这是我的FXML：

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

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

<AnchorPane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="558.0" prefWidth="355.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="com.frogger.mainapp.SampleController">
   <children>
      <VBox alignment="CENTER" prefHeight="558.0" prefWidth="355.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
         <children>
<!--                  fitHeight="183.0" fitWidth="355.0"-->
            <ImageView pickOnBounds="true" preserveRatio="true" VBox.vgrow="ALWAYS">
               <image>
                  <Image url="@Frogger_background_top.png" />
               </image>
            </ImageView>
            <Button alignment="CENTER" mnemonicParsing="false" prefHeight="131.0" prefWidth="610.0" style="-fx-background-color: green;" text="Single Player Mode">
               <font>
                  <Font name="Comic Sans MS Bold" size="28.0" />
               </font>
            </Button>
            <Button fx:id="bt2" mnemonicParsing="false" onAction="#button2action" prefHeight="149.0" prefWidth="653.0" style="-fx-background-color: red;" text="Two Player Mode">
               <font>
                  <Font name="Comic Sans MS Bold" size="28.0" />
               </font>
            </Button>
            <Button mnemonicParsing="false" prefHeight="137.0" prefWidth="355.0" style="-fx-background-color: blue;" text="High Score">
               <font>
                  <Font name="Comic Sans MS Bold" size="28.0" />
               </font>
            </Button>
         </children>
      </VBox>
   </children>
</AnchorPane>

这是我的主要课程：

package com.frogger.mainapp;

import javafx.animation.AnimationTimer;
import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.Alert;
import javafx.scene.control.Alert.AlertType;
import javafx.scene.control.Button;
import javafx.scene.layout.*;
import javafx.stage.Stage;

public class Main extends Application {
    AnimationTimer timer;
    public static MyStage background;
    Animal animal;
    public static Animal animal2;
    public static Stage primaryStage;
    public static Scene scene;
    private boolean secondPlayer;

    @FXML private Button bt2;

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

    @Override
    public void start(Stage primaryStage) throws Exception {
        background = new MyStage();
        scene  = new Scene(background,600,800);
//      scene  = new Scene(background,600,800);
        //loginScene buttons
//      Label label1= new Label("Welcome to my Frogger Game");
//      Button button1= new Button("Single Player Mode");
//      Button button2= new Button("Two Player Mode");
//      bt2.addEventHandler(ActionEvent.ACTION, (e)->{
//          animal2 = new Animal("file:resources/froggerUp.png", 400.00, 700.00, true);
//          background.add(animal2);
//          primaryStage.setScene(scene);
//          primaryStage.centerOnScreen();
//      });
//
//      });
        //Todo: Get the buttons working
        //setting up loginScene
//      VBox layout1 = new VBox(20);
//      Image image = new Image("file:Frogger_background.png");
//      ImageView iv1 = new ImageView();
//      iv1.setImage(image);
//      Image image = new Image("file:Frogger_background.png");
//      BackgroundSize backgroundSize = new BackgroundSize(320, 480, true, true, true, false);
//      BackgroundImage loginBack = new BackgroundImage(image, BackgroundRepeat.REPEAT, BackgroundRepeat.NO_REPEAT, BackgroundPosition.CENTER, backgroundSize);
//      layout1.setBackground(new Background(loginBack));
//      layout1.getChildren().addAll(label1, button1, button2);
        Parent root = FXMLLoader.load(getClass().getResource("/Sample.fxml"));
        Scene loginScene = new Scene(root,320, 480);
//      button2action(primaryStage, scene);
//      String css = Main.class.getResource("application.css").toExternalForm();
//      loginScene.getStylesheets().add(css);
        primaryStage.setResizable(false);
        primaryStage.setScene(loginScene);
        primaryStage.show();

//      scene.getStylesheets().add(getClass().getResource("/application.css").toExternalForm());

        MyBackgroundImage froggerback = new MyBackgroundImage("file:resources/iKogsKW.png");

        background.add(froggerback);

        background.add(new Log("file:resources/log3.png", 150, 0, 166, 0.75));
        background.add(new Log("file:resources/log3.png", 150, 220, 166, 0.75));
        background.add(new Log("file:resources/log3.png", 150, 440, 166, 0.75));
        background.add(new Log("file:resources/logs.png", 300, 0, 276, -2));
        background.add(new Log("file:resources/logs.png", 300, 400, 276, -2));
        background.add(new Log("file:resources/log3.png", 150, 50, 329, 0.75));
        background.add(new Log("file:resources/log3.png", 150, 270, 329, 0.75));
        background.add(new Log("file:resources/log3.png", 150, 490, 329, 0.75));

        background.add(new Turtle(500, 376, -1, 130, 130));
        background.add(new Turtle(300, 376, -1, 130, 130));
        background.add(new WetTurtle(700, 376, -1, 130, 130));
        background.add(new WetTurtle(600, 217, -1, 130, 130));
        background.add(new WetTurtle(400, 217, -1, 130, 130));
        background.add(new WetTurtle(200, 217, -1, 130, 130));
        background.add(new End(13,96));
        background.add(new End(141,96));
        background.add(new End(141 + 141-13,96));
        background.add(new End(141 + 141-13+141-13+1,96));
        background.add(new End(141 + 141-13+141-13+141-13+3,96));
        animal = new Animal("file:resources/froggerUp.png", 150.00, 700.00, false);
        background.add(animal);


        background.add(new Obstacle("file:resources/truck1"+"Right.png", 0, 649, 1, 120, 120));
        background.add(new Obstacle("file:resources/truck1"+"Right.png", 300, 649, 1, 120, 120));
        background.add(new Obstacle("file:resources/truck1"+"Right.png", 600, 649, 1, 120, 120));
        background.add(new Obstacle("file:resources/car1Left.png", 100, 597, -1, 50, 50));
        background.add(new Obstacle("file:resources/car1Left.png", 250, 597, -1, 50, 50));
        background.add(new Obstacle("file:resources/car1Left.png", 400, 597, -1, 50, 50));
        background.add(new Obstacle("file:resources/car1Left.png", 550, 597, -1, 50, 50));
        background.add(new Obstacle("file:resources/truck2Right.png", 0, 540, 1, 200, 200));
        background.add(new Obstacle("file:resources/truck2Right.png", 500, 540, 1, 200, 200));
        background.add(new Obstacle("file:resources/car1Left.png", 500, 490, -5, 50, 50));
        background.add(new Digit(0, 30, 360, 25));

        background.start();
        start();
    }

//  @FXML
//  public void button2action(ActionEvent event){
//      primaryStage.setScene(scene);
//      primaryStage.centerOnScreen();
//      animal2 = new Animal("file:resources/froggerUp.png", 400.00, 700.00, true);
//      background.add(animal2);
//  }
//  @FXML
//  public void initialize(){
//      bt2.addEventHandler(ActionEvent.ACTION, (e)->{
//
//      });
//  }

    public void createTimer() {
        timer = new AnimationTimer() {
            //    @Override
            public void handle(long now) {
                if (animal.changeScore()) {
                    setNumber(animal.getPoints());
                }
                if (animal.getStop()) {
                    System.out.print("STOP");
                    background.stopMusic();
                    stop();
                    background.stop();
                    Alert alert = new Alert(AlertType.INFORMATION);
                    alert.setTitle("You Have Won The Game!");
                    alert.setHeaderText("Your High Score: " + animal.getPoints() + "!");
                    alert.setContentText("Highest Possible Score: 800");
                    alert.show();
                }
            }
        };
    }

    public void start() {
        background.playMusic();
        createTimer();
        timer.start();
    }

        public void stop() {
        timer.stop();
    }

    public void setNumber(int n) {
        int shift = 0;
        while (n > 0) {
              int d = n / 10;
              int k = n - d * 10;
              n = d;
              background.add(new Digit(k, 30, 360 - shift, 25));
              shift+=30;
            }
    }
    public static Stage getStage(){
        return primaryStage;
    }
//  public static Scene getScene(){
//      return scene;
//  }
    public void setSecondPlayer(boolean _secondPlayer) {
        this.secondPlayer = _secondPlayer;
    }

    public boolean getSecondPlayer(){
        return this.secondPlayer;
    }




}

这是堆栈跟踪：

"C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2019.3\jbr\bin\java.exe" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2019.3\lib\idea_rt.jar=56118:C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2019.3\bin" -Dfile.encoding=UTF-8 -p "C:\Users\Joshu\Documents\SECOND YEAR\Software Maintenance\Frogger\out\production\froggerSWM" -m com.frogger.mainapp/com.frogger.mainapp.Main
This is horrible!
Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
    at javafx.fxml/javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1787)
    at javafx.fxml/javafx.fxml.FXMLLoader$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:8865)
    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$2(GlassViewEventHandler.java:434)
    at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:395)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:433)
    at javafx.graphics/com.sun.glass.ui.View.handleMouseEvent(View.java:556)
    at javafx.graphics/com.sun.glass.ui.View.notifyMouse(View.java:942)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:175)
    at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: java.lang.reflect.InvocationTargetException
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at com.sun.javafx.reflect.Trampoline.invoke(MethodUtil.java:76)
    at jdk.internal.reflect.GeneratedMethodAccessor2.invoke(Unknown Source)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at javafx.base/com.sun.javafx.reflect.MethodUtil.invoke(MethodUtil.java:275)
    at javafx.fxml/com.sun.javafx.fxml.MethodHelper.invoke(MethodHelper.java:83)
    at javafx.fxml/javafx.fxml.FXMLLoader$MethodHandler.invoke(FXMLLoader.java:1782)
    ... 47 more
Caused by: java.lang.NullPointerException
    at com.frogger.mainapp/com.frogger.mainapp.SampleController.button2action(SampleController.java:12)
    ... 58 more

Answer 1

您的日志显示您在第12行收到NullPointerException：

at com.frogger.mainapp/com.frogger.mainapp.SampleController.button2action(SampleController.java:12)

现在，我们将转到第12行：

Main.primaryStage.setScene(Main.scene);

应用程序将从setScene对象访问primaryStage方法，该方法在Main类中是静态的。但是您是否已在Main class中对其进行了初始化？

不。如您所见，在主类中的primaryStage声明：

public static Stage primaryStage;

但是在代码的任何地方，我都找不到primaryStage =某些东西。我认为您将start方法的方法变量与静态变量混淆了。尝试将在primaryStage方法中收到的start设置为您创建的静态变量：

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

我认为这可以帮助您：如果您还有其他方法，请告诉我。

JavaFXML更改场景

1 个答案: