java - repaint() 不会调用paintComponent()

Question

我正在尝试用 Java 制作乒乓球游戏，但它不起作用。我已经做了一些测试，似乎变量正在更新，但是当我这样做时计时器中的 repaint(); actionPerformed(ActionEvent e) 不会调用 paintComponent() 方法

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class PongGame extends JComponent implements ActionListener, MouseMotionListener{

    public int state = 1;
    public int paddleX;
    public String buttonColor = "blue";
    public int mouseX, mouseY;
    private int ballX = 400;
    private int ballY = 150;

    public static void main(String[] args){

        JFrame window = new JFrame("Pong");
        PongGame game = new PongGame();
        window.add(new PongGame());
        window.pack();
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.setLocationRelativeTo(null);
        window.setResizable(false);
        window.setVisible(true);

        Timer t = new Timer(20, game);
        t.start();
    }

    public Dimension getPreferredSize(){
        return new Dimension(800, 600);
    }

    public void paintComponent(Graphics g){
        paddleX = mouseX;

        g.setColor(Color.WHITE);
        g.fillRect(0,0, 800, 600);

        g.setColor(Color.BLACK);

        g.fillRect(paddleX, 550, 150, 15);
        g.fillOval(ballX, ballY, 30, 30);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        ballX = ballX + 10;
        ballY = ballY + 10;
        System.out.println(ballX + " " + ballY);

    }

    @Override
    public void mouseDragged(MouseEvent e) {

    }

    @Override
    public void mouseMoved(MouseEvent e) {
        mouseX = e.getX();
        repaint();
    }
}

Answer 1

• 您尚未将已实现的 MouseMotionListener 注册到任何组件:

  game.addMouseMotionListener(game);
  

• 您没有将第一个创建的 PongGame 实例添加到框架中，而是添加了一个产生错误的新实例:

   PongGame game = new PongGame();
    window.add(new PongGame());  // <<--- why creating the new instance ?
                // it should be window.add(game);
  

• 作为一种良好的编程实践:尝试将添加监听器代码放入组件自己的创建上下文中，即放入其构造函数中，以使代码更具可读性。