Java重绘不更新

Question

我在编码时遇到一个问题，当我在任何地方调用 repaint 时，它不会更新图像，但程序仍然继续运行:

import java.applet.*; 
import java.awt.*; 
import java.awt.event.*; 
import java.awt.image.ImageObserver;
import java.util.Random;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class mainMenu extends JPanel implements MouseListener{
    Graphics g;
    Image playButton = Toolkit.getDefaultToolkit().getImage("game sprites/play_button.png");
    private int part = 0;

    public static void main(String[] a) {
        mainMenu mm = new mainMenu();

        mm.makeJFrame();
    }
    public void makeJFrame(){
        JFrame f = new JFrame();
        f.setSize(1000, 600);
        f.add(new mainMenu());
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setVisible(true);
        f.addMouseListener(this);
        f.setTitle("MAIN MENU");
    }

   public void paintComponent(Graphics g) {
       if(part == 0)
           System.out.println("0");
           g.drawImage( playButton, 330, 200, 294, 102, this);
       if(part == 1)
       {
           System.out.println("1");
           g.setColor(Color.RED);
           g.fillRect(0, 0, 40, 40);
       }
   }

    public void mouseClicked(MouseEvent e) {}
    public void mouseEntered(MouseEvent e) {}
    public void mouseExited(MouseEvent e) {}
    public void mousePressed(MouseEvent e) {
        int x = e.getX();
        int y = e.getY();

        if(part == 0)
        {
            if(x > 330 && x < 624 && y > 232 && y < 334)
            {
                System.out.println("a");
                part = 1;
                repaint();
            }
        }
    }
    public void mouseReleased(MouseEvent e) {}
}

我测试了很多次，问题出在重绘上，程序可以工作，但图像在不应该的情况下保持不变。它打印出: 0 0 0 a(当我点击按钮时) 即使我单击按钮，也不会再次打印出“a”，因为部分设置为 1，所以不应该打印出“a”。

Answer 1

图片加载了吗？这看起来很可疑:Image playButton = Toolkit.getDefaultToolkit().getImage("game sprites/play_button.png");。图像存储在哪里？

这看起来很可疑......

if (part == 0)
    System.out.println("0");
    g.drawImage(playButton, 330, 200, 294, 102, this);

并且可能意味着......

if (part == 0) {
    System.out.println("0");
    g.drawImage(playButton, 330, 200, 294, 102, this);
}

您已经破坏了绘制链，您必须在进行任何自定义绘制之前调用 super.paintComponent...

@Override
protected void paintComponent(Graphics g) {
    super.paintComponent(g);

最后，您有两个 mainMenu 实例...

第一个是屏幕上显示的......

f.add(new MainMenu());

第二个是响应鼠标事件并更新部分的状态

f.addMouseListener(this);

这与第一个无关。

您应该避免在组件中创建窗口(例如 JPanel)，而是创建 MainMenu 的实例并将其添加到 JFrame 的实例中>，例如...

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.Toolkit;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class MainMenu extends JPanel implements MouseListener {

    Image playButton = Toolkit.getDefaultToolkit().getImage("...");
    private int part = 0;

    public static void main(String[] a) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new MainMenu());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public MainMenu() {
        addMouseListener(this);
    }

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

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        System.out.println(part);
        if (part == 0) {
            System.out.println("0");
            g.drawImage(playButton, 0, 0, this);
        } else if (part == 1) {
            System.out.println("1");
            g.setColor(Color.RED);
            g.fillRect(0, 0, 40, 40);
        }
    }

    public void mouseClicked(MouseEvent e) {
    }

    public void mouseEntered(MouseEvent e) {
    }

    public void mouseExited(MouseEvent e) {
    }

    public void mousePressed(MouseEvent e) {
        int x = e.getX();
        int y = e.getY();

        if (part == 0) {
            System.out.println("a");
            part = 1;
            repaint();
        }
    }

    public void mouseReleased(MouseEvent e) {
    }
}