How can I drag points ?

Question

hi, all
I am trying to achieve the following: I want to draw two points on a picture in the Jpanel with mouse, and wenn I release the mouse, there must be a line between the two points. if I just use mouse draged anyone of the point , it should be move with the mouse, and the line too. Can someone tell me how can I do it?
thank you very much!

Answer 1

import javax.swing.JFrame;
import java.awt.event.MouseListener;
import java.awt.event.MouseEvent;
import java.awt.Point;
import java.awt.event.MouseMotionListener;
public class TrivialApplication extends JFrame implements MouseListener, MouseMotionListener {
     private static MyPanel myPanel;
     private static PointGroupList pointList;
     private static Point currentPoint = null;
     public static void main(String args[]) {
          JFrame f = new TrivialApplication();
          f.show();
     public TrivialApplication() {
          setSize(400,400);
          pointList = new PointGroupList();
          myPanel = new MyPanel(400,400,pointList);
          getContentPane().add(myPanel);
          myPanel.addMouseListener(this);
          myPanel.addMouseMotionListener(this);
     public void mouseClicked(MouseEvent e) {
     public void mouseExited(MouseEvent e) {
     public void mouseEntered(MouseEvent e) {
     public void mousePressed(MouseEvent e) {
          int x = e.getX();
          int y = e.getY();
          currentPoint = pointList.getPointAt(x,y);
          if (currentPoint == null) {
               pointList.addPoint(x,y);
               myPanel.repaint();
     public void mouseReleased(MouseEvent e) {
     public void mouseMoved(MouseEvent e) {
     public void mouseDragged(MouseEvent e) {
          if (currentPoint != null) {
               currentPoint.x = e.getX();
               currentPoint.y = e.getY();
               myPanel.repaint();
import javax.swing.JPanel;
import java.awt.Graphics;
import java.awt.Dimension;
import java.awt.Color;
import java.awt.Point;
public class MyPanel extends JPanel {
     private PointGroupList pointList;
     public MyPanel(int w, int h, PointGroupList pointList) {
          setPreferredSize(new Dimension(w,h));
          this.pointList = pointList;
     public void paintComponent(Graphics g) {
          super.paintComponent(g);
          g.setColor(Color.white);
        g.fillRect(0,0,getSize().width, getSize().height);
          for (int i = 0; i < pointList.size(); i++) {
               PointGroup group = pointList.getPointGroupAt(i);
               g.setColor(Color.red);
               Point one = group.getFirstPoint();
               g.fillRect(one.x-3,one.y-3,6,6);
               if (group.size() == 2 ) {
                    Point two = group.getSecondPoint();
                    g.fillRect(two.x-3,two.y-3,6,6);
                    g.setColor(Color.blue);
                    g.drawLine(one.x,one.y,two.x,two.y);
import java.util.Vector;
import java.awt.Point;
public class PointGroupList {
     private Vector list;
     private int counter = 0;
     public PointGroupList() {
          list = new Vector();
     public void addPoint(int x, int y) {
          if (counter == 0) {
               PointGroup group = new PointGroup();
               group.addPoint(x,y);
               list.add(group);
          } else {
               PointGroup group = getPointGroupAt(size()-1);
               group.addPoint(x,y);
          counter = (counter + 1) % 2;
     public PointGroup getPointGroupAt(int index) {
          PointGroup group = null;
          if (index < size() && size() != 0) group = (PointGroup) list.elementAt(index);
          return group;
     public int size() {
          return list.size();
     public Point getPointAt(int x, int y) {
          Point returner = null;
          for (int i = 0; i < size(); i++) {
               PointGroup group = getPointGroupAt(i);
               for (int j = 0; j < group.size(); j++) {
                    Point point = group.getPointAt(j);
                    if ( ( (point.x <= x+5) && (point.x >= x-5) ) &&
                          ( (point.y <= y+5) && (point.y >= y-5) ) ) {
                         returner = point;     
          return returner;
import java.awt.Point;
import java.util.Vector;
public class PointGroup {
     private Vector list;
     public PointGroup() {
          list = new Vector();
     public void addPoint(int x, int y) {
          Point point = new Point(x,y);
          list.add(point);
     public Point getPointAt(int index) {
          Point point = (Point) list.elementAt(index);
          return point;
     public Point getFirstPoint() {
          return getPointAt(0);
     public Point getSecondPoint() {
          return getPointAt(1);
     public int size() {
          return list.size();
}