更改JavaFX中的场景NullPointerException [英] Changing Scenes in JavaFX NullPointerException

问题描述

我想要换到另一个班级的场景，但我遇到了很大困难。

现在我可以转到第二个屏幕了问题，但回到第一个屏幕给我 NullPointerException 。

非常感谢帮助。非常感谢提前。

主类

  public class Main扩展Application {
 
 Stage ps; 
 Group root = new Group（）; 
场景s =新场景（root，300,300，Color.AQUA）; 
 Controller con = new Controller（）; 
 
 
 public void start（Stage primaryStage）抛出异常{
 ps = primaryStage; 
 con.buttonLayout（）; 
 buttonLayout（）; 
 
 primaryStage.setTitle（Hello World）; 
 ps.setScene（s）; 
 primaryStage.show（）; 
} 
 
 public void buttonLayout（）{
 Button但= = new Button（first）; 
 
 but.setOnMouseClicked（new EventHandler< MouseEvent>（）{
 @Override 
 public void handle（MouseEvent event）{
 ps.setScene（con.s） ; 
} 
}）; 
 
 root.getChildren（）。add（but）; 
} 
 
 
 public static void main（String [] args）{
 launch（args）; 
} 
} 
  

其他课程

  public class Controller {
 Group root = new Group（）; 
场景s =新场景（root，300,300，Color.BLACK）; 
 
 
 public void buttonLayout（）{
 Button但= = new Button（back to first）; 
 
 but.setOnMouseClicked（new EventHandler< MouseEvent>（）{
 @Override 
 public void handle（MouseEvent event）{
 Main main = new Main（）; 
 main.ps.setScene（main.s）; 
} 
}）; 
 
 root.getChildren（）。add（but）; 
} 
} 
  

解决方案

重组你的应用程序，创建两个控制器而不是一个。所以每个场景都有一个控制器。在您的主应用程序中，只需调用第一个控制器来设置您的第一个场景。

以下是一个示例：

• 
  
  

  Main.java

    import javafx.application.Application; 
   import javafx.scene.Scene; 
   import javafx.scene.paint.Color; 
   import javafx.stage.Stage; 
   
  公共类Main扩展Application {
   public void start（Stage stage）throws Exception {
   Controller1 controller1 = new Controller1（）; 
  场景场景=新场景（
   controller1.getContent（），300,300，Color.AQUA 
  ）; 
   
   stage.setScene（场景）; 
   stage.show（）; 
  } 
   
   public static void main（String [] args）{
   launch（args）; 
  } 
  } 
    

  Controller.java

    import javafx.scene.Parent; 
   
   public interface Controller {
   Parent getContent（）; 
  } 
    

  Controller1.java

    import javafx.scene.Parent; 
   import javafx.scene.control.Button; 
   import javafx.scene.layout.StackPane; 
   
  公共类Controller1实现Controller {
   private final StackPane root; 
   
   public Controller1（）{
   Button navToSecond = new Button（second）; 
   
   navToSecond.setOnMouseClicked（event  - > {
   Controller2 controller2 = new Controller2（）; 
   navToSecond.getScene（）。setRoot（controller2.getContent（））; 
  }）; 
   
   root = new StackPane（navToSecond）; 
  } 
   
   @Override 
   public Parent getContent（）{
   return root; 
  } 
  } 
    

  Controller2.java

    import javafx.scene.Parent; 
   import javafx.scene.control.Button; 
   import javafx.scene.layout.StackPane; 
   
  公共类Controller2实现Controller {
   private final StackPane root; 
   
   public Controller2（）{
   Button navToFirst = new Button（first）; 
   
   navToFirst.setOnMouseClicked（event  - > {
   Controller1 controller1 = new Controller1（）; 
   navToFirst.getScene（）。setRoot（controller1.getContent（））; 
  }）; 
   
   root = new StackPane（navToFirst）; 
  } 
   
   @Override 
   public Parent getContent（）{
   return root; 
  } 
  } 
    

  
  

  I'm wanting to change to a scene in another class but I'm having great difficulty.

  Now the I can move to the to the second screen no problem, but moving back to the first screen gives me the NullPointerException.

  Help would be much appreciated. Many thanks in advance.

  Main Class

  public class Main extends Application {
  
  Stage ps;
  Group root = new Group();
  Scene s = new Scene(root, 300, 300, Color.AQUA);
  Controller con = new Controller();
  
  
  public void start(Stage primaryStage) throws Exception {
      ps = primaryStage;
      con.buttonLayout();
      buttonLayout();
  
      primaryStage.setTitle("Hello World");
      ps.setScene(s);
      primaryStage.show();
  }
  
  public void buttonLayout() {
      Button but = new Button("first");
  
      but.setOnMouseClicked(new EventHandler<MouseEvent>() {
          @Override
          public void handle(MouseEvent event) {
              ps.setScene(con.s);
          }
      });
  
      root.getChildren().add(but);
  }
  
  
  public static void main(String[] args) {
      launch(args);
  }
  }
  

  Other class

  public class Controller{
  Group root = new Group();
  Scene s = new Scene(root, 300, 300, Color.BLACK);
  
  
  public void buttonLayout() {
      Button but = new Button("back to first");
  
      but.setOnMouseClicked(new EventHandler<MouseEvent>() {
          @Override
          public void handle(MouseEvent event) {
              Main main = new Main();
              main.ps.setScene(main.s);
          }
      });
  
      root.getChildren().add(but);
  }
  }
  

  解决方案

  Restructure your application, create two controllers instead of just one. So you have one controller for each scene. And in your main application, just invoke the first controller to set your first scene.

  Here is a sample:

  • Small JavaFX framework for swapping in and out child panes in a main FXML container.
  • Loading new fxml in the same scene

  ---

  An additional simplified example which does not cache scenes or controllers, does not use FXML and completely replaces the content of the scene on each navigation:

  Main.java

  import javafx.application.Application;
  import javafx.scene.Scene;
  import javafx.scene.paint.Color;
  import javafx.stage.Stage;
  
  public class Main extends Application {
      public void start(Stage stage) throws Exception {
          Controller1 controller1 = new Controller1();
          Scene scene = new Scene(
                  controller1.getContent(), 300, 300, Color.AQUA
          );
  
          stage.setScene(scene);
          stage.show();
      }
  
      public static void main(String[] args) {
          launch(args);
      }
  }
  

  Controller.java

  import javafx.scene.Parent;
  
  public interface Controller {
      Parent getContent();
  }
  

  Controller1.java

  import javafx.scene.Parent;
  import javafx.scene.control.Button;
  import javafx.scene.layout.StackPane;
  
  public class Controller1 implements Controller {
      private final StackPane root;
  
      public Controller1() {
          Button navToSecond = new Button("second");
  
          navToSecond.setOnMouseClicked(event -> {
              Controller2 controller2 = new Controller2();
              navToSecond.getScene().setRoot(controller2.getContent());
          });
  
          root = new StackPane(navToSecond);
      }
  
      @Override
      public Parent getContent() {
          return root;
      }
  }
  

  Controller2.java

  import javafx.scene.Parent;
  import javafx.scene.control.Button;
  import javafx.scene.layout.StackPane;
  
  public class Controller2 implements Controller {
      private final StackPane root;
  
      public Controller2() {
          Button navToFirst = new Button("first");
  
          navToFirst.setOnMouseClicked(event -> {
              Controller1 controller1 = new Controller1();
              navToFirst.getScene().setRoot(controller1.getContent());
          });
  
          root = new StackPane(navToFirst);
      }
  
      @Override
      public Parent getContent() {
          return root;
      }
  }
  

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