Question

我查看了旧代码中的所有注释和评论，但我不明白为什么它不起作用。

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;
import java.util.Stack;
import javafx.scene.layout.VBox;

public class Main extends Application{

    Stage window;//makes window a stage
    Scene scene1, scene2;//makes scene1 and scene2 into Scene's
        public static void main(String []args) {

        //launches the main 
        launch(args);
    }
    @Override//Overrides Application
    public void start(Stage primaryStage) throws Exception {
        window = primaryStage;//Makes window primary stage
        Label label1 = new Label ("This is the first scene!");//Makes a label
        Button button1 = new Button();//Declares button as a Button
        button1.setText("Click me for Scene 2");//Sets the text of the button
        button1.setOnAction(e -> window.setScene(scene2));
        //This say, on the action, change the scene

        VBox layout1 = new VBox(20);//Makes layout1 into a VBox 
        layout1.getChildren().addAll(label1, button1);//Adds the 'Children'
        scene1 = new Scene(layout1, 500, 400);//Sets up layout1

        Button button2 = new Button();//makes a second button
        button2.setText("Click me for scene 1");//sets the text for button2
        button2.setOnAction(e -> window.setScene(scene1));//When button 2 is click, it changes scene

        StackPane layout2 = new StackPane();//Makes a new StackPane layout
        layout2.getChildren().add(button2);//Adds button2 to the layout
        scene2 = new Scene (layout2, 500, 400);//Gives arguemnts for Scene2

        window.setScene(scene1);//Sets scene of the window stage 
        window.setTitle("This is a title");//Sets title
        window.show();//Shows the window

    }
}

Vs以上。

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;
import java.util.Stack;
import javafx.scene.layout.VBox;

public class Main extends Application{

    Stage window;//makes window a stage
    Scene scene1, scene2;//makes scene1 and scene2 into Scene's
        public static void main(String []args) {

        //launches the main 
        launch(args);
    }
    @Override//Overrides Application
    public void start(Stage primaryStage) throws Exception {
        window = primaryStage;//Makes window primary stage
        Label label1 = new Label ("This is the first scene!");//Makes a label
        Button button1 = new Button();//Declares button as a Button
        button1.setText("Click me for Scene 2");//Sets the text of the button
        button1.setOnAction(e -> window.setScene(scene2));
        //This say, on the action, change the scene

        VBox layout1 = new VBox(20);//Makes layout1 into a VBox 
        layout1.getChildren().addAll(label1, button1);//Adds the 'Children'
        Scene scene1 = new Scene(layout1, 500, 400);//Sets up layout1

        Button button2 = new Button();//makes a second button
        button2.setText("Click me for scene 1");//sets the text for button2
        button2.setOnAction(e -> window.setScene(scene1));//When button 2 is click, it changes scene

        StackPane layout2 = new StackPane();//Makes a new StackPane layout
        layout2.getChildren().add(button2);//Adds button2 to the layout
        Scene scene2 = new Scene (layout2, 500, 400);//Gives arguemnts for Scene2

        window.setScene(scene1);//Sets scene of the window stage 
        window.setTitle("This is a title");//Sets title
        window.show();//Shows the window

    }
}

我在两者之间唯一改变的地方是添加“场景＆＃39;

”这个词

scene1 =新场景....作品

VS

场景scene2 =新场景....不起作用。这是为什么？

Answer 1

您是shadowing your member variables并在第二个代码中将它们保留为空。

换句话说，第一个代码中只有Scene的2个实例，而另一个代码中只有4个

执行此操作时，它使用Main.this.scene2，而不是本地实例。

button1.setOnAction(e -> window.setScene(scene2));

如果确实需要使用本地实例，则需要声明它final并在设置动作侦听器之前进行初始化。

Answer 2

将场景放在它们之前的原因不起作用是因为你已经声明了它们，但没有初始化它们，因为行中的场景 Scene scene1, scene2 所以你不需要再次指定。这就像创建一个整数int a然后初始化它int a = 1。这没有意义，因为a已被定义为int。实际上，它不是“它不起作用”而是“它没有意义”。

为什么这会弄乱我的程序？

2 个答案: