Javafx Gui更新

Question

My problem is basically from Java language which I cannot figure out. This is just a basic code to help me understand the concept of updating GUI using Platform.runlater method.

package practise;

import java.util.Random;
import javafx.application.Application;
import javafx.application.Platform;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Line;
import javafx.stage.Stage;

public class Practise extends Application {

    @Override
    public void start(Stage primaryStage) {
        Button btn = new Button();
        final Line l1 = new Line(200,80,200,0);
        final int c = 0;        

        btn.setText("Say 'Hello World'");
        btn.setOnAction(new EventHandler<ActionEvent>() {

            @Override
            public void handle(ActionEvent event) {
                System.out.println("Hello World!");

                Platform.runLater(new Runnable() {
                    Random rn = new Random();
                    @Override

                    public void run() {
                        if(c==0)
                        {
                            l1.setStartX(rn.nextInt(400));
                            l1.setStartY(rn.nextInt(400));
                            c = 1;
                        }
                        else
                        {
                            l1.setEndX(rn.nextInt(400));
                            l1.setEndY(rn.nextInt(400));
                            c = 0;
                        }
                    }
                });
            }
        });


        l1.setStroke(Color.YELLOW);
        l1.setStrokeWidth(2);

        StackPane root = new StackPane();
        root.getChildren().add(btn);
        root.getChildren().add(l1);

        Scene scene = new Scene(root, 400, 400, Color.WHITE);

        primaryStage.setTitle("Hello World!");
        primaryStage.setScene(scene);
        primaryStage.show();
    }

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

}

This is my code. I wish to alternatingly change either the start point or the end points of the lines. For that I created a variable 'c' which alternates it's values every time I press the button.

But the issue is that the compiler forces me to change int c into final. Which means it becomes a constant and no longer serves my purpose.

Why is this happening and how can I get around it?

Answer 1

You're using a variable with local scope in an anonymous inner class. You can only do this if the variable is final . If you need to change the value of the variable, you could define it as an instance variable.

public class MainApp extends Application {

    private int c;

    @Override
    public void start(Stage primaryStage) {
        Button btn = new Button();
        final Line l1 = new Line(200,80,200,0);

        btn.setText("Say 'Hello World'");
        btn.setOnAction(new EventHandler<ActionEvent>() {

            @Override
            public void handle(ActionEvent event) {
                System.out.println("Hello World!");

                Platform.runLater(new Runnable() {
                    Random rn = new Random();
                    @Override

                    public void run() {
                        if(c==0)
                        {
                            System.out.println("c=" + c);
                            l1.setStartX(rn.nextInt(400));
                            l1.setStartY(rn.nextInt(400));
                            c = 1;
                        }
                        else
                        {
                            System.out.println("c=" + c);
                            l1.setEndX(rn.nextInt(400));
                            l1.setEndY(rn.nextInt(400));
                            c = 0;
                        }
                    }
                });
            }
        });


        l1.setStroke(Color.YELLOW);
        l1.setStrokeWidth(2);

        StackPane root = new StackPane();
        root.getChildren().add(btn);
        root.getChildren().add(l1);

        Scene scene = new Scene(root, 400, 400, Color.WHITE);

        primaryStage.setTitle("Hello World!");
        primaryStage.setScene(scene);
        primaryStage.show();
    }

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

}

Output after a couple of button clicks:

Hello World!
c=0
Hello World!
c=1
Hello World!
c=0