【求助】这个简单的画图程序画不出来

[捏造的信仰]

下面这个程序不知道为什么画不出来。求指教，谢谢！

import javax.swing.*;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Component;
import java.awt.Graphics;
import java.awt.HeadlessException;
import java.awt.event.HierarchyEvent;
import java.awt.event.HierarchyListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.image.BufferedImage;

/**
 * 简单的 Java 铅笔
 */
public class JPencil extends JFrame {

    // 程序执行入口
    public static void main(String[] args) throws Exception {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (Exception e) {
            e.printStackTrace();
        }

        new JPencil("JPencil").setVisible(true);
    }

    public JPencil(String title) throws HeadlessException {
        super(title);
        setupWindow();
        setupControls();
    }

    private void setupControls() {
        this.setLayout(new BorderLayout());

        final DrawPanel drawPanel = new DrawPanel();
        this.add(drawPanel, BorderLayout.CENTER);

        final JPencil jPencil = this;

        // 当窗口打开的时候初始化 drawPanel（否则无法获取 width 和 height）
        this.addHierarchyListener(new HierarchyListener() {

            public void hierarchyChanged(HierarchyEvent e) {
                boolean isShowingUp = jPencil.isVisible() &&
                        e.getChangeFlags() == HierarchyEvent.SHOWING_CHANGED;

                if (isShowingUp) {
                    drawPanel.init();
                }
            }
        });
    }

    private void setupWindow() {
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
        this.setResizable(false);
        this.setSize(800, 600);
        this.setLocation(50, 50);
    }

    /////////////////////////////////////////

    /**
     * 绘图板
     */
    private class DrawPanel extends JPanel {

        private BufferedImage image;  // 缓存 Image 对象

        private Pencil pencil;

        public void init() {
            this.image = new BufferedImage(getWidth(), getHeight(), BufferedImage.TYPE_INT_ARGB);
            this.pencil = new Pencil(this, this.image);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.drawImage(this.image, 0, 0, null);  // 在这里绘图
        }
    }

    /////////////////////////////////////////

    /**
     * 铅笔工具
     */
    private class Pencil extends MouseAdapter {

        private BufferedImage image;

        private Component component;

        private int lastX = -1, lastY = -1;

        /**
         * 构造函数
         *
         * @param component 可绘图的 Component 对象
         * @param image     要绘制的 Image 对象
         */
        public Pencil(Component component, BufferedImage image) {
            this.image = image;
            this.component = component;
            this.clear();

            component.addMouseListener(this);       // MouseListener 负责处理鼠标键按下和松开事件
            component.addMouseMotionListener(this); // MouseMotionListener 负责处理鼠标拖拽事件
        }

        private void clear() {
            this.image.getGraphics().setColor(Color.white);
            this.image.getGraphics().fillRect(0, 0, getWidth(), getHeight());
            this.image.getGraphics().setColor(Color.black);
        }

        private void draw(int x, int y) {
            this.image.getGraphics().drawLine(lastX, lastY, x, y);
            this.component.repaint();
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            if (this.lastX != -1 && this.lastY != -1) {
                draw(e.getX(), e.getY());
            }

            this.lastX = e.getX();
            this.lastY = e.getY();
        }

        @Override
        public void mousePressed(MouseEvent e) {
            // 设置绘图的起始点
            this.lastX = e.getX();
            this.lastY = e.getY();
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            // 结束绘图
            this.lastX = -1;
            this.lastY = -1;
        }
    }
}

[xiongyu2006]

结贴率101.92%，这是怎么搞出来的！

[YBBPS1109]

private void draw(int x, int y) {
Graphics2D graphics2d = this.image.createGraphics();
graphics2d.setColor(Color.black);
graphics2d.drawLine(lastX, lastY, x, y);
this.component.repaint();
}

[捏造的信仰]

[Quote=引用 1 楼 xiongyu2006 的回复:]

结贴率101.92%，这是怎么搞出来的！
[/Quote]

被删帖删多了就这样。

[捏造的信仰]

[Quote=引用 2 楼 ybbps1109 的回复:]
private void draw(int x, int y) {
Graphics2D graphics2d = this.image.createGraphics();
graphics2d.setColor(Color.black);
graphics2d.drawLine(lastX, lastY, x, y);
……
[/Quote]

知道怎么回事了。每次调用 getGraphics() 都是获得一个新的对象。

[jlu_lamp_lamp]

private void draw(int x, int y) {
this.image.getGraphics().drawLine(lastX, lastY, x, y);
this.component.repaint();
}
把你的draw方法改成这样就行了
private void draw(int x, int y) {
this.component.getGraphics().drawLine(lastX, lastY, x, y);
}
因为是要在你的组件上画，所以要从组件中获取graphics

[lfp001]

没有保存绘图对象。应该这样改：
（1）在Pencil类中private BufferedImage image;下一行增加一行申明一个内在绘图对象：
private java.awt.Graphics imageGraphics;

（2）Pencil构造方法的this.image = image;后增加一行：
imageGraphics = this.image.getGraphics(); // 获取绘图对象

（3）Pencil.clear方法内的3条语句改为：
imageGraphics.setColor(Color.white);
imageGraphics.fillRect(0, 0, getWidth(), getHeight());
imageGraphics.setColor(Color.black);

（4）Pencil.draw方法内this.image.getGraphics().drawLine(lastX, lastY, x, y);改为：
imageGraphics.drawLine(lastX, lastY, x, y); // 内存绘图，传说中的双缓冲

修改后的代码是这样的：

import javax.swing.*;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Component;
import java.awt.Graphics;
import java.awt.HeadlessException;
import java.awt.event.HierarchyEvent;
import java.awt.event.HierarchyListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.image.BufferedImage;

/**
 * 简单的 Java 铅笔
 */
public class JPencil extends JFrame {

    // 程序执行入口
    public static void main(String[] args) throws Exception {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (Exception e) {
            e.printStackTrace();
        }

        new JPencil("JPencil").setVisible(true);
    }

    public JPencil(String title) throws HeadlessException {
        super(title);
        setupWindow();
        setupControls();
    }

    private void setupControls() {
        this.setLayout(new BorderLayout());

        final DrawPanel drawPanel = new DrawPanel();
        this.add(drawPanel, BorderLayout.CENTER);

        final JPencil jPencil = this;

        // 当窗口打开的时候初始化 drawPanel（否则无法获取 width 和 height）
        this.addHierarchyListener(new HierarchyListener() {

            public void hierarchyChanged(HierarchyEvent e) {
                boolean isShowingUp = jPencil.isVisible() &&
                        e.getChangeFlags() == HierarchyEvent.SHOWING_CHANGED;

                if (isShowingUp) {
                    drawPanel.init();
                }
            }
        });
    }

    private void setupWindow() {
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
        this.setResizable(false);
        this.setSize(800, 600);
        this.setLocation(50, 50);
    }

    /////////////////////////////////////////

    /**
     * 绘图板
     */
    private class DrawPanel extends JPanel {

        private BufferedImage image;  // 缓存 Image 对象

        private Pencil pencil;

        public void init() {
            this.image = new BufferedImage(getWidth(), getHeight(), BufferedImage.TYPE_INT_ARGB);
            this.pencil = new Pencil(this, this.image);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.drawImage(this.image, 0, 0, null);  // 在这里绘图
        }
    }

    /////////////////////////////////////////

    /**
     * 铅笔工具
     */
    private class Pencil extends MouseAdapter {

        private BufferedImage image;
        private java.awt.Graphics imageGraphics;

        private Component component;

        private int lastX = -1, lastY = -1;

        /**
         * 构造函数
         *
         * @param component 可绘图的 Component 对象
         * @param image     要绘制的 Image 对象
         */
        public Pencil(Component component, BufferedImage image) {
            this.image = image;
            imageGraphics = this.image.getGraphics();
            this.component = component;
            this.clear();

            component.addMouseListener(this);       // MouseListener 负责处理鼠标键按下和松开事件
            component.addMouseMotionListener(this); // MouseMotionListener 负责处理鼠标拖拽事件
        }

        private void clear() {
        	imageGraphics.setColor(Color.white);
        	imageGraphics.fillRect(0, 0, getWidth(), getHeight());
        	imageGraphics.setColor(Color.black);
        }

        private void draw(int x, int y) {
            imageGraphics.drawLine(lastX, lastY, x, y);
            this.component.repaint();
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            if (this.lastX != -1 && this.lastY != -1) {
                draw(e.getX(), e.getY());
            }

            this.lastX = e.getX();
            this.lastY = e.getY();
        }

        @Override
        public void mousePressed(MouseEvent e) {
            // 设置绘图的起始点
            this.lastX = e.getX();
            this.lastY = e.getY();
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            // 结束绘图
            this.lastX = -1;
            this.lastY = -1;
        }
    }
}

[edward168855]

都是高手啊!