java - 在 swing 中使用双缓冲区时出现 NullPointerException

Question

在下面的代码中，我使用双缓冲区来避免图像闪烁，如 this question 中所建议的那样我的

import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.awt.image.BufferedImage;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;


public class TestProgram extends JFrame implements KeyListener {
    private Image doubleBuffer;
    private Graphics myGraphics;
    private BufferedImage TestImage;
    private int cordX = 100;
    private int cordY = 100;

    public TestProgram() {
        setTitle("Testing....");
        setSize(500,500);
        imageLoader();
        setVisible(true);
    }

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

    public void imageLoader() {
        try {
            String testPath = "test.png";
            TestImage = ImageIO.read(getClass().getResourceAsStream(testPath));

        } catch (IOException ex) {
            ex.printStackTrace();
        }

        addKeyListener(this);

        doubleBuffer = createImage(getWidth(), getHeight());
        myGraphics = doubleBuffer.getGraphics();
        drawImages();
    }

    @Override
    public void update(Graphics g) {
        drawImages();
        g.drawImage(doubleBuffer, 0, 0, this);        
    }

    public void drawImages() {
        myGraphics.drawImage(TestImage, cordX, cordY, this);
    }



    public void keyPressed(KeyEvent ke) {
        switch (ke.getKeyCode()) {
            case KeyEvent.VK_RIGHT: {
                cordX+=5;
            }
            break;
            case KeyEvent.VK_LEFT: {
                cordX-=5;
            }
            break;
            case KeyEvent.VK_DOWN: {
                cordY+=5;
            }
            break;
            case KeyEvent.VK_UP: {
                cordY-=3;
            }
            break;
        }
        repaint();
    }

    public void keyTyped(KeyEvent ke) {}

    public void keyReleased(KeyEvent ke) {}
}

问题是我在这一行得到了一个 nullPointerException

myGraphics = doubleBuffer.getGraphics();

我的做法是否正确？请帮忙。谢谢

Answer 1

1. 不要直接画到JFrame，放在JPanel或JComponent

2. Swing GUI 应该从 Initial Thread 开始

3. 什么是 TestImage 和 Image 的路径？？？，

4. KeyListener未指定 Swing JComponents , 使用 KeyBindings相反

5. 坐标改变后你必须调用repaint()

6. 把那个放在一起

.

import java.awt.*;
import java.awt.event.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.*;
import java.util.HashMap;
import java.util.Map;
import javax.imageio.ImageIO;
import javax.swing.*;

public class MoveIcon extends JPanel {

    private static final long serialVersionUID = 1L;
    private static final String IMAGE_PATH = "http://duke.kenai.com/misc/Bullfight.jpg";
    private static final String IMAGE_PATH_PLAYER = "http://duke.kenai.com/iconSized/duke4.gif";
    public static final int STEP = 3;
    private static final int TIMER_DELAY = STEP * 8;
    private BufferedImage bkgrndImage = null;
    private BufferedImage playerImage = null;
    private Map<Direction, Boolean> directionMap = new HashMap<Direction, Boolean>();
    private int playerX = 0;
    private int playerY = 0;

    enum Direction {

        UP(KeyEvent.VK_UP, 0, -1), DOWN(KeyEvent.VK_DOWN, 0, 1),
        LEFT(KeyEvent.VK_LEFT, -1, 0), RIGHT(KeyEvent.VK_RIGHT, 1, 0);
        private int keyCode;
        private int xDirection;
        private int yDirection;

        private Direction(int keyCode, int xDirection, int yDirection) {
            this.keyCode = keyCode;
            this.xDirection = xDirection;
            this.yDirection = yDirection;
        }

        public int getKeyCode() {
            return keyCode;
        }

        public int getXDirection() {
            return xDirection;
        }

        public int getYDirection() {
            return yDirection;
        }
    }

    public MoveIcon() {
        try {
            URL bkgrdImageURL = new URL(IMAGE_PATH);
            URL playerImageURL = new URL(IMAGE_PATH_PLAYER);
            bkgrndImage = ImageIO.read(bkgrdImageURL);
            playerImage = ImageIO.read(playerImageURL);
            setPreferredSize(new Dimension(bkgrndImage.getWidth(), bkgrndImage.getHeight()));
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        for (Direction direction : Direction.values()) {
            directionMap.put(direction, false);
        }
        setKeyBindings();
        Timer timer = new Timer(TIMER_DELAY, new TimerListener());
        timer.start();
    }

    private void setKeyBindings() {
        InputMap inMap = getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW);
        ActionMap actMap = getActionMap();
        for (final Direction direction : Direction.values()) {
            KeyStroke pressed = KeyStroke.getKeyStroke(direction.getKeyCode(), 0, false);
            KeyStroke released = KeyStroke.getKeyStroke(direction.getKeyCode(), 0, true);
            inMap.put(pressed, direction.toString() + "pressed");
            inMap.put(released, direction.toString() + "released");
            actMap.put(direction.toString() + "pressed", new AbstractAction() {

                private static final long serialVersionUID = 1L;

                @Override
                public void actionPerformed(ActionEvent e) {
                    directionMap.put(direction, true);
                }
            });
            actMap.put(direction.toString() + "released", new AbstractAction() {

                private static final long serialVersionUID = 1L;

                @Override
                public void actionPerformed(ActionEvent e) {
                    directionMap.put(direction, false);
                }
            });
        }

    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        if (bkgrndImage != null) {
            g.drawImage(bkgrndImage, 0, 0, null);
        }
        if (playerImage != null) {
            g.drawImage(playerImage, playerX, playerY, null);
        }
    }

    private class TimerListener implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent e) {
            boolean moved = false;
            for (Direction direction : Direction.values()) {
                if (directionMap.get(direction)) {
                    playerX += STEP * direction.getXDirection();
                    playerY += STEP * direction.getYDirection();
                    moved = true;
                }
            }
            if (moved) {
                int x = playerX - 2 * STEP;
                int y = playerY - 2 * STEP;
                int w = playerImage.getWidth() + 4 * STEP;
                int h = playerImage.getHeight() + 4 * STEP;
                MoveIcon.this.repaint(x, y, w, h); // !! repaint just the player
            }
        }
    }

    private static void createAndShowUI() {
        JFrame frame = new JFrame("MoveIcon");
        frame.getContentPane().add(new MoveIcon());
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        java.awt.EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                createAndShowUI();
            }
        });
    }
}