注意添加的标志：
import java.awt.BasicStroke;import java.awt.BorderLayout;import java.awt.Graphics;import java.awt.Graphics2D;import java.awt.Label;import java.awt.Point;import java.awt.event.MouseEvent;import java.awt.event.MouseListener;import java.awt.event.MouseMotionListener;import java.util.ArrayList; import javax.swing.Jframe;import javax.swing.JPanel;public class WorkingLines { public WorkingLines() { Jframe jf = new Jframe("Free Hand Drawing Example"); JPanel draw = new DrawBoard(); jf.add(new Label("Click....move mouse, click again"), BorderLayout.NORTH); jf.add(draw); jf.setDefaultCloseOperation(Jframe.EXIT_ON_CLOSE); jf.setSize(600, 500); jf.setVisible(true); } public static void main(String a[]) { new WorkingLines(); }}class DrawBoard extends JPanel implements MouseListener, MouseMotionListener { ArrayList<pts> list = new ArrayList<>(); Point start, end; private boolean isDrawing = false; //a flag public DrawBoard() { start = null; end = null; addMouseListener(this); addMouseMotionListener(this); } @Override public void paintComponent(Graphics g) { super.paintComponent(g); Graphics2D g2 = (Graphics2D) g; g2.setStroke(new BasicStroke(3, BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND)); super.paintComponent(g2); for (pts p : list) { g.drawLine((int) p.getStart().getX(), (int) p.getStart().getY(), (int) p.getEnd().getX(), (int) p.getEnd().getY()); } if ((start != null) && (end != null)) { g.drawLine(start.x, start.y, end.x, end.y); } } @Override public void mouseClicked(MouseEvent arg0) {} @Override public void mouseEntered(MouseEvent arg0) {} @Override public void mouseExited(MouseEvent arg0) {} @Override public void mousePressed(MouseEvent me) { if(! isDrawing) { start = me.getPoint(); isDrawing = ! isDrawing; }else { pts pt = new pts(start, end); list.add(pt); start = end; } } @Override public void mouseReleased(MouseEvent me) { if (me.getClickCount() == 2) { start = null; end = null; isDrawing = false; } } @Override public void mouseDragged(MouseEvent me) {} @Override public void mouseMoved(MouseEvent me) { end = me.getPoint(); repaint(); }}class pts { Point start = null, end = null; public pts(Point start, Point end) { this.start = start; this.end = end; } public Point getStart() { return start; } public Point getEnd() { return end; }}
