在第一次鼠标单击和 cursor 之间显示一条线

Question

I'm trying to write a program that will show a line between the first click and mouse position after the 1st click. Then after the 2nd click, it shows a line.

I know I will have to use MouseListener getX() , getY() to get the position where the mouse click is at, but the part where I am confused is to show a line between the first click and mouse position then after the second click it shows a line. The tutorial I found online only show me how to draw a line between two mouse clicks.

Would greatly appreciate if anyone can point me in the right direction.

Answer 1

You need to know three points.

• First click point
• Second click point
• The current mouse point

If both first and second click points are null , then basically do nothing

If the first click point and current mouse point are not null and the second point is null , draw a line between the first and transient point

If both the first and second click points are not null , then draw a line between those two instead

To make this work, you'll need a MouseListener and a MouseMotionListener

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.geom.Line2D;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Test {

    public static void main(String[] args) {
        new Test();
    }

    public Test() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                JFrame frame = new JFrame();
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private Point startPoint;
        private Point endPoint;
        private Point transientPoint;

        public TestPane() {
            MouseAdapter ma = new MouseAdapter() {
                @Override
                public void mouseClicked(MouseEvent e) {
                    if (startPoint == null) {
                        startPoint = e.getPoint();
                    } else if (endPoint == null) {
                        endPoint = e.getPoint();
                    } else {
                        endPoint = null;
                        startPoint = e.getPoint();
                    }
                    repaint();
                }

                @Override
                public void mouseMoved(MouseEvent e) {
                    if (startPoint != null && endPoint == null) {
                        transientPoint = e.getPoint();
                        repaint();
                    } else {
                        transientPoint = null;
                    }
                }
            };

            addMouseListener(ma);
            addMouseMotionListener(ma);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(400, 400);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            if (startPoint != null && endPoint != null) {
                g2d.setColor(Color.BLUE);
                g2d.draw(new Line2D.Double(startPoint, endPoint));
            } else if (startPoint != null && transientPoint != null) {
                g2d.setColor(Color.RED);
                g2d.draw(new Line2D.Double(startPoint, transientPoint));
            }
            g2d.dispose();
        }

    }
}