能否从Panel（0,0）到（3000,1000）画条线

Question

javaJPanel绘图

Answer 1

重载Jpanel的paintComponent(Graphics g)方法就好了import java.awt.Graphics;import javax.swing.JFrame;
import javax.swing.JPanel;class DemoPanel extends JPanel {
    public DemoPanel() {
    }
    @Override
    protected void paintComponent(Graphics g) {
        g.drawLine(0, 0, 100, 200);
    }
    
}
public class Test extends JFrame{
    Test(){
        setSize(300, 300);
        add(new DemoPanel());
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
    
    public static void main(String[] args) {
        new Test().setVisible(true);
    }
}

Answer 2

查了下资料，用java.awt.robot这个类的createScreenCapture可以截屏，然后用ImageIO.write成文件。
但截屏的坐标和大小需要自己算，有点不好弄。

Answer 3

自己写了一个示例，备忘public class Demo extends JFrame {
    private JButton btn1 = new JButton("截屏");    Demo() {
        setTitle("截屏demo");
        setSize(300, 300);
        add(btn1, BorderLayout.SOUTH);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);        btn1.addActionListener(new ActionListener() {            @Override
            public void actionPerformed(ActionEvent e) {
                new Thread(new Runnable() {                    @Override
                    public void run() {
                        Robot robot;
                        try {
                            robot = new Robot();
                            Rectangle rect = Demo.this.getBounds();
                            BufferedImage img = robot.createScreenCapture(rect);
                            ImageIO.write(img, "jpg", new File("a.jpg"));
                        } catch (Exception e) {
                            e.printStackTrace();
                        }                    }
                }).start();
            }
        });
    }    public static void main(String[] args) {
        new Demo().setVisible(true);
    }
}

Answer 4

我试了一下，我发现createScreenCapture方法截的是电脑的显示屏，不是截当前panel中的图像。

Answer 5

这个问题我已经解决了，代码如下：
public void drawPathShowImage(){ 
 
  Image pathImage = Toolkit.getDefaultToolkit().getImage("./source/THU_2Dmap.jpg");
        //首先创建一个BufferedImage变量，
        BufferedImage bi = new BufferedImage((int)map2Dimension.getWidth(), (int)map2Dimension.getHeight(),
         BufferedImage.TYPE_3BYTE_BGR);         //再创建一个Graphics变量，用来画出来要保持的图片 
        Graphics2D g = (Graphics2D) bi.getGraphics(); 
        try { 
            g.drawImage(pathImage, 0, 0, null);
            g.setColor(Color.RED);
            g.setStroke(new BasicStroke(8.0f));
            g.drawLine(0, 0, 5000, 4000);
            
            //将BufferedImage变量写入文件中。 
            ImageIO.write(bi,"jpg",new File("d:\\test.jpg")); 
        } catch (IOException e) { 
            // TODO Auto-generated catch block 
            e.printStackTrace(); 
        } 
    }

Answer 6

可是有个问题，我的面板里的图片可以切换。
如果当前面板里显示的不是“./source/THU_2Dmap.jpg"这个图片，保存的就是一张有红线的黑底图片。如果当前显示的就是这个图片，保存的在这个图片上画了红线的图片。

Answer 7

写了一个可以随机切换的demo，备忘先随机加载一幅图，截图以后存传文件，然后再随机换图。
import java.awt.BasicStroke;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import java.io.File;
import java.util.Random;import javax.imageio.ImageIO;
import javax.swing.BorderFactory;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;class DPanel extends JPanel{
    private ImageIcon[] images = null;
    private JLabel lbl = null;
    
    private void initImages() {
        images = new ImageIcon[] { new ImageIcon("a.jpg"),
                new ImageIcon("b.jpg"), new ImageIcon("c.jpg") };
        lbl = new JLabel(images[new Random().nextInt(images.length)]);
    }
    
    DPanel(){
        initImages();
        add(lbl);
        setBorder(BorderFactory.createLineBorder(Color.BLUE));
    }
    
    JLabel getLbl(){
        return this.lbl;
    }
    
    ImageIcon[] getImgs(){
        return this.images;
    }
}public class Demo extends JFrame {
    private JButton btn1 = new JButton("截屏");
    private DPanel panel = new DPanel();    Demo() {
        setTitle("截屏demo");
        setSize(300, 300);
        add(panel, BorderLayout.CENTER);
        add(btn1, BorderLayout.SOUTH);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);        btn1.addActionListener(new ActionListener() {            @Override
            public void actionPerformed(ActionEvent e) {
                new Thread(new Runnable() {                    @Override
                    public void run() {
                        try {
                            BufferedImage img = new BufferedImage(
                                    panel.getLbl().getIcon().getIconWidth(),
                                    panel.getLbl().getIcon().getIconHeight(),
                                    BufferedImage.TYPE_3BYTE_BGR);
                            Graphics2D g2 = img.createGraphics();
                            g2.drawImage(((ImageIcon)panel.getLbl().getIcon()).getImage(), 
                                    0, 0, null);
                            g2.setColor(Color.BLUE);
                            g2.setStroke(new BasicStroke(8.0f));
                            g2.drawLine(0, 0, 500, 500);
                            ImageIO.write(img, "jpg", new File("screenshot.jpg"));
                            
                            // 截屏以后随机选一幅图新加载到JLabel中
                            ImageIcon[] images = panel.getImgs();
                            JLabel lbl = panel.getLbl();
                            int n = new Random().nextInt(images.length);
                            lbl.setIcon(images[n]);
                        } catch (Exception e) {
                            e.printStackTrace();
                        }                    }
                }).start();
            }
        });
    }    public static void main(String[] args) {
        new Demo().setVisible(true);
    }
}