Rotate Polygon around center Example

This example I constructed after looking through sourcecode by Brain Postma. He has several good sourcecodes on his site that is perfect to learn things from. Here I made a polygon that rotates around its center and rotates with the a and d keys. At first I did not understand why the rotation was not around its center but it was that you need to build the polygon from the center. So that half of it is in negative numbers.
I will be studying Brian Postma's sourcecode further and will probably lead to more examples on this blog.

 

import java.applet.Applet;
import java.awt.*;
public class RotatePolygonaroundcenter001 extends Applet  implements Runnable{
 Graphics bufferGraphics;
 Image offscreen;
    int[] XArray = {0, -15, 15};
    int[] YArray = {-15, 15, 15};
 double polangle;
 double polx = 320/2-15;
 double poly = 240/2-15;
 int[] xcoord = new int[3];
   int[] ycoord = new int[3];
 boolean rotleft = false;
 boolean rotright = false;
 public void init() {
        setBackground(Color.black);
        offscreen = createImage(getSize().width,getSize().height);
        bufferGraphics = offscreen.getGraphics();
     makecoords(polx,poly,polangle);
  new Thread(this).start();
 }
    public void run() {
        for(;;) { // animation loop never ends
         repaint();
         try {
    if ( rotleft == true ) {
        polangle -= 3;
     if ( polangle < 0 ) {
      polangle = 360;
     }
    }
    if ( rotright == true ) {
     polangle += 3;
     if ( polangle > 360 ) {
      polangle = 0;
     }
    }
          makecoords(polx,poly,polangle);
             Thread.sleep(16);
             }
             catch (InterruptedException e) {
             }
     }
    }

 public void makecoords(double x, double y, double angle)
   {
     int  i;
  angle = Math.toRadians(angle);
     for (i=0; i<3; i++)
     {
        xcoord[i] = (int)(x+(XArray[i]*Math.sin(angle)-YArray[i]*Math.cos(angle)));
        ycoord[i] = (int)(y+(XArray[i]*Math.cos(angle)+YArray[i]*Math.sin(angle)));
     }
  }

     public void update(Graphics g){
     bufferGraphics.clearRect(0,0,getSize().width,getSize().height);
     bufferGraphics.setColor(Color.red);
        bufferGraphics.drawString("Rotate polygon around center.",10,10);
  bufferGraphics.drawString("a/d to rotate.",10,237);
     bufferGraphics.fillPolygon (xcoord, ycoord, 3);

        g.drawImage(offscreen,0,0,this);
     }
  public boolean keyDown (Event e, int key){
   if(key==97) // a key
        {
         rotleft = true;
        }
        if(key==100) // d key
        {
   rotright = true;
        }

   return true;
  }

 public boolean keyUp (Event e, int key){
    if( key == 97 ) // a key
        {
          rotleft = false;
        }
        if( key == 100 ) // d key
        {
          rotright = false;
        }

  return true;
 }

}