【求助】这个简单的画图程序画不出来

Question

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;
        }
    }
}

Answer 1

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();
}

Answer 2

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

Answer 3

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

Answer 4

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;
        }
    }
}