//********************************************************************
// Angle computed by the arctanget function (inverse tangent)
// Using Adapter class instead of listener interfaces
//********************************************************************
import java.util.ArrayList;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class AnglePanel extends JPanel
{
   private Point p1 = null, p2 = null;
   private double alpha;
   int x, y;
   
   public AnglePanel()
   {
      addMouseMotionListener (new AngleListener());
      addMouseListener (new ZeroListener());
      
      setBackground (Color.black);
      setPreferredSize (new Dimension(400, 300));
   }

   public void paintComponent (Graphics page)
   {
      super.paintComponent(page);

      page.setColor (Color.green);
      if (p1 != null && p2 != null) {
         page.drawLine (p1.x, p1.y, p2.x, p2.y);
         alpha = Math.atan((double)(p2.y-p1.y)/(p2.x-p1.x));
         if (p1.x > p2.x) alpha = alpha + Math.PI;
         if (p1.x < p2.x && p1.y > p2.y) alpha = alpha + 2*Math.PI;
         page.drawString("Alpha = "+String.valueOf(alpha),5,15);
         page.drawLine (p1.x-100, p1.y, p1.x+100, p1.y);
         page.drawLine (p1.x, p1.y-100, p1.x, p1.y+100);
         page.drawString("0", p1.x-8, p1.y+13);
         page.drawString("X", p1.x+105, p1.y+5);
         page.drawString("Y", p1.x-3, p1.y+115);
      }
   }

   private class ZeroListener extends MouseAdapter    
   {
      public void mousePressed (MouseEvent event)
      {
         p1 = event.getPoint();
      }
   }    
    
   private class AngleListener extends MouseMotionAdapter
   {
      public void mouseDragged (MouseEvent event)
      {
        p2 = event.getPoint();
        repaint();
      }
   }
}