关键内容,可以自行扩展

package main;import java.awt.BasicStroke;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import java.awt.geom.Point2D;
import java.util.Vector;import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.border.EmptyBorder;public class MainFrame extends JFrame implements MouseListener, MouseMotionListener {/*** */private static final long serialVersionUID = 1L;private JPanel contentPane;private Graphics2D g2;private static Vector<Point2D> pointInfo;/*** Launch the application.*/public static void main(String[] args) {SwingUtilities.invokeLater(() -> {MainFrame mainFrame = new MainFrame();mainFrame.setLocationRelativeTo(null);mainFrame.setVisible(true);});}/*** Create the frame.*/public MainFrame() {addMouseListener(this);addMouseMotionListener(this);setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);setBounds(100, 100, 951, 606);contentPane = new JPanel();contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));contentPane.setLayout(new BorderLayout(0, 0));setContentPane(contentPane);//初始化pointInfo=new Vector<>();}@Overridepublic void paint(Graphics g) {System.out.println("paint");g2 = (Graphics2D) g;int size = pointInfo.size();Point p1,p2;g2.setColor(Color.BLACK);g2.setStroke(new BasicStroke(WIDTH));for (int i = 0; i < size-1; i++) {p1 = (Point) pointInfo.elementAt(i);p2 = (Point) pointInfo.elementAt(i + 1);g2.drawLine(p1.x, p1.y, p2.x, p2.y);}}public void update(Graphics g) {System.out.println("update");paint(g);}@Overridepublic void mouseDragged(MouseEvent e) {System.out.println("mouseDragged");Point cutflag = new Point(e.getX(), e.getY());pointInfo.addElement(cutflag);repaint();}@Overridepublic void mousePressed(MouseEvent e) {Point cutflag = new Point(e.getX(), e.getY());pointInfo.addElement(cutflag);}@Overridepublic void mouseReleased(MouseEvent e) {Point cutflag = new Point(-1, -1);pointInfo.addElement(cutflag);pointInfo.clear();}@Overridepublic void mouseMoved(MouseEvent e) {}@Overridepublic void mouseClicked(MouseEvent e) {}@Overridepublic void mouseEntered(MouseEvent e) {}@Overridepublic void mouseExited(MouseEvent e) {}
}