Why can't I draw an ellipse with this code?

package test;

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.Ellipse2D;
import java.awt.image.BufferedImage;
import javax.swing.*;
public class test_bmp extends JPanel implements MouseListener,MouseMotionListener,ActionListener
{
 static BufferedImage image;
 Color color;
 Point start=new Point();
 Point end =new Point();
 JButton elipse=new JButton("Elipse开发者_JS百科");
 JButton rectangle=new JButton("Rectangle");
 JButton line=new JButton("Line");
 String selected;
 public test_bmp()
    {
  color = Color.black; 
  setBorder(BorderFactory.createLineBorder(Color.black));   
  addMouseListener(this);
  addMouseMotionListener(this);
    }
 public void paintComponent(Graphics g) 
 {
  //super.paintComponent(g);
  g.drawImage(image, 0, 0, this);
  Graphics2D g2 = (Graphics2D)g;
  g2.setPaint(Color.black);
  if(selected=="elipse")
        {
         g2.drawOval(start.x, start.y, (end.x-start.x),(end.y-start.y));
         System.out.println("Start : "+start.x+","+start.y);
         System.out.println("End   : "+end.x+","+end.y);
        }
        if(selected=="line")
         g2.drawLine(start.x,start.y,end.x,end.y);
 }
 //Draw on Buffered image
 public void draw()
    {
        Graphics2D g2 = image.createGraphics();
        g2.setPaint(color);
      System.out.println("draw");
        if(selected=="line")
         g2.drawLine(start.x, start.y, end.x, end.y);
        if(selected=="elipse")
        {
         g2.drawOval(start.x, start.y, (end.x-start.x),(end.y-start.y));
            System.out.println("Start : "+start.x+","+start.y);
         System.out.println("End   : "+end.x+","+end.y);
        }
        repaint();
        g2.dispose();
        }  
 public JPanel addButtons()
 {
  JPanel buttonpanel=new JPanel();
  buttonpanel.setBackground(color.lightGray);
  buttonpanel.setLayout(new BoxLayout(buttonpanel,BoxLayout.Y_AXIS));
  elipse.addActionListener(this);
  rectangle.addActionListener(this);
  line.addActionListener(this);
  buttonpanel.add(elipse);
  buttonpanel.add(Box.createRigidArea(new Dimension(15,15)));
  buttonpanel.add(rectangle);
  buttonpanel.add(Box.createRigidArea(new Dimension(15,15)));
  buttonpanel.add(line);
  return buttonpanel;
 }
 public static void main(String args[]) 
 {
   test_bmp application=new test_bmp();
   //Main window
   JFrame frame=new JFrame("Whiteboard");   
   frame.setLayout(new BorderLayout());
   frame.add(application.addButtons(),BorderLayout.WEST);
   frame.add(application);
   //size of the window
   frame.setSize(600,400);
   frame.setLocation(0,0);
   frame.setVisible(true);
   int w = frame.getWidth();
      int h = frame.getHeight();
      image = new BufferedImage(w, h, BufferedImage.TYPE_INT_RGB);
      Graphics2D g2 = image.createGraphics();
      g2.setPaint(Color.white);
      g2.fillRect(0,0,w,h);
      g2.dispose();
   frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 }
 @Override
 public void mouseClicked(MouseEvent arg0) {
  // TODO Auto-generated method stub
 }
 @Override
 public void mouseEntered(MouseEvent arg0) {
  // TODO Auto-generated method stub
 }
 @Override
 public void mouseExited(MouseEvent arg0) {
  // TODO Auto-generated method stub
 }
 @Override
 public void mousePressed(MouseEvent event) 
 {
  start = event.getPoint();
 }
 @Override
 public void mouseReleased(MouseEvent event) 
 {
  end = event.getPoint();
  draw();
 }
 @Override
 public void mouseDragged(MouseEvent e) 
 {
  end=e.getPoint();
  repaint();
 }
 @Override
 public void mouseMoved(MouseEvent arg0) {
  // TODO Auto-generated method stub

 }
 @Override
 public void actionPerformed(ActionEvent e) 
 {
  if(e.getSource()==elipse)
   selected="elipse";
  if(e.getSource()==line)
   selected="line";
  draw();

 }
}

I need to create a paint application. When I draw ellipse by dragging mouse from left to right it displays nothing. Why? Should I use any other function here?

---

Your program does draw an ellipse when you drag the mouse down and to the right. It's dragging up and/or left that does not work, because Graphics.drawOval does not work with a negative width or height.

Try adding a method like this:

private Shape createEllipse() {
    Ellipse2D e = new Ellipse2D.Double();
    e.setFrameFromDiagonal(start, end);
    return e;
}

Then call it from draw and paintComponent like this:

if(selected=="elipse") {
    g2.draw(createEllipse());
}

Also you probably do not need the call to draw() at the end of actionPerformed. If you switch between line and ellipse mode it will draw an ellipse with the same coordinates as the most recent line or vice-versa.

And one coding style issue: Using string literals for selected is confusing (although it does work.) I would define an enum instead.