创建对象时代码崩溃，因此可以从另一个类调用重绘

Question

我目前正在做一个新项目，长话短说我正在尝试在矩形对象上实现动作侦听器，以便它可以上下移动。

但是在这里，当在 test3 类中创建一个对象时，代码完全崩溃，在这种情况下 test2 类在 test3 类中（是的，我知道类需要大写）。 因为我已经复制了制作新项目的问题并且只使用了所需的代码。

public class test {

    test2 board = new test2();

    public void frame() {
        JFrame b = new JFrame("test");
        b.setSize(905,705);
        b.setLocation(300,60);
        b.setResizable(false);
        b.setVisible(true);
        b.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);         
        b.add(board);
    }

    public static  void main(String[] args) {
        test start = new test();
        start.frame();
    }
}

public class test2 extends JPanel {

    public int playerScore = 0;
    public int opponentScore = 0;
    test3 player = new test3();
    
    public void paint(Graphics g) {
        Graphics2D g2d = (Graphics2D) g;

        board(g);
        g.setColor(Color.WHITE);

        g2d.fill(player.player);
    }

    public void board(Graphics g) {
        g.setColor(Color.black);
        g.fillRect(0, 0, 900, 900);
    }
}

public class test3 {

    public boolean down = false;
    public boolean up = false;
    public int playerXpos = 45;
    public int playerYpos = 300;
    public int playerWidth = 15;
    public int playerHeight = 80;
    Rectangle player = new Rectangle(playerXpos, playerYpos, playerWidth, playerHeight);
    test2 theBoard = new test2();

    public void actionPerformed(ActionEvent e) {
        if (down) {
            down = true;

            if (up != true) {
                down = true;
            } else {
                up = true;
                down = false;
            }
        }
        
        if (up) {
            up = true;

            if (down != true) {
                up = true;
            } else {
                up = false;
                down = true;
            }
        }
    }

    public void keyTyped(KeyEvent e) {
        // TODO Auto-generated method stub
    }

    public void keyPressed(KeyEvent e) {
        if (e.getKeyCode() == KeyEvent.VK_UP) {
            for (int r = playerYpos; r >=0; r--) {
                if (r == 0) {
                    playerYpos = playerYpos - 20;
                } else {
                    playerYpos = playerYpos - 1;
                }
            
                if (playerYpos < 50) {
                    playerYpos = 50;
                }
            }

            theBoard.repaint();
        }

        if (e.getKeyCode() == KeyEvent.VK_DOWN) {
            for (int r = playerYpos; r >= 0; r--) {
                if (r == 0) {
                    playerYpos = playerYpos + 20;   
                } else {
                    playerYpos = playerYpos - 1;
                }
            
                if (playerYpos > 800){
                    playerYpos = 800;
                }
            }

            theBoard.repaint();
        }
    }

    public void keyReleased(KeyEvent e) {
        // TODO Auto-generated method stub
    }
}

Answer 1

您的代码无法编译。

我使用了您的一些代码并创建了以下 GUI。

这是移动播放器矩形后的 GUI。

Oracle 有一个有用的教程，用 JFC/Swing 创建 GUI 。 跳过 Netbeans 部分。

我做的第一件事是通过调用SwingUtilities invokeLater方法来启动 Swing 应用程序。 此方法可确保在Event Dispatch Thread上创建和执行 Swing 组件。

我创建了一个Player类来保存播放器矩形。 这样，您可以在一个地方更改播放器形状。

我创建了一个JFrame 。 JFrame方法必须按特定顺序调用。 这是我用于 Swing 应用程序的顺序。

我创建了一个绘图JPanel 。 这个JPanel与你的相似，除了我覆盖了paintComponent方法。 我将Player一个实例传递给绘图JPanel以便JPanel可以绘制播放器矩形。

我使用键绑定而不是键侦听器。 无论绘图JPanel是否处于焦点，这都会使键起作用。 键绑定还使我可以轻松添加 WASD 键和箭头键。

这是完整的可运行代码。

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;

import javax.swing.AbstractAction;
import javax.swing.ActionMap;
import javax.swing.InputMap;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.KeyStroke;
import javax.swing.SwingUtilities;

public class ExampleGUI implements Runnable {

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new ExampleGUI());
    }
    
    private final DrawingPanel drawingPanel;
    
    private final Player player;
    
    public ExampleGUI() {
        this.player = new Player();
        this.drawingPanel = new DrawingPanel(player);
    }

    @Override
    public void run() {
        JFrame frame = new JFrame("Example GUI");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        frame.add(drawingPanel, BorderLayout.CENTER);
        
        frame.pack();
        frame.setLocationByPlatform(true);
        setKeyBindings(drawingPanel);
        frame.setVisible(true);
    }
    
    private void setKeyBindings(JPanel panel) {
        String up = "up";
        String down = "down";
        String left = "left";
        String right = "right";
        
        InputMap inputMap = panel.getInputMap();
        ActionMap actionMap = panel.getActionMap();
        inputMap.put(KeyStroke.getKeyStroke(KeyEvent.VK_UP, 0, false), up);
        inputMap.put(KeyStroke.getKeyStroke(KeyEvent.VK_DOWN, 0, false), down);
        inputMap.put(KeyStroke.getKeyStroke(KeyEvent.VK_LEFT, 0, false), left);
        inputMap.put(KeyStroke.getKeyStroke(KeyEvent.VK_RIGHT, 0, false), right);
        
        inputMap.put(KeyStroke.getKeyStroke(KeyEvent.VK_W, 0, false), up);
        inputMap.put(KeyStroke.getKeyStroke(KeyEvent.VK_S, 0, false), down);
        inputMap.put(KeyStroke.getKeyStroke(KeyEvent.VK_A, 0, false), left);
        inputMap.put(KeyStroke.getKeyStroke(KeyEvent.VK_D, 0, false), right);
        
        actionMap.put(up, new PlayerAction(this, player, 0, -10));
        actionMap.put(down, new PlayerAction(this, player, 0, +10));
        actionMap.put(left, new PlayerAction(this, player, -10, 0));
        actionMap.put(right, new PlayerAction(this, player, +10, 0));
    }
    
    public void repaint() {
        drawingPanel.repaint();
    }
    
    public class DrawingPanel extends JPanel {

        private static final long serialVersionUID = 1L;
        
        private final Player player;
        
        public DrawingPanel(Player player) {
            this.player = player;
            this.setPreferredSize(new Dimension(600, 600));
        }
        
        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);

            Graphics2D g2d = (Graphics2D) g;
            drawBoard(g2d);
            drawPlayer(g2d);
        }

        private void drawBoard(Graphics2D g2d) {
            g2d.setColor(Color.BLACK);
            g2d.fillRect(0, 0, getWidth(), getHeight());
        }

        private void drawPlayer(Graphics2D g2d) {
            g2d.setColor(player.getColor());
            g2d.fill(player.getPosition());
        }
        
    }
    
    public class PlayerAction extends AbstractAction {
        
        private static final long serialVersionUID = 1L;

        private final ExampleGUI frame;
        
        private final Player player;
        
        private final Point moveDirection;
        
        public PlayerAction(ExampleGUI frame, Player player, int x, int y) {
            this.frame = frame;
            this.player = player;
            this.moveDirection = new Point(x, y);
        }

        @Override
        public void actionPerformed(ActionEvent event) {
            player.move(moveDirection);
            frame.repaint();
        }
        
    }
    
    public class Player {
        
        private final Color color;
        
        private Rectangle position;
        
        public Player() {
            this.color = Color.WHITE;
            this.position = new Rectangle(300, 300, 20, 80);
        }
        
        public void move(Point point) {
            position.x += point.x;
            position.y += point.y;
        }

        public Color getColor() {
            return color;
        }

        public Rectangle getPosition() {
            return position;
        }
        
    }

}