source: liacs/cvp/java/eyes/Ogen.java@ 151

import java.awt.*;

public class Ogen
    extends java.applet.Applet{
    
    //blx holds previous x-value of of left eye, etc...
    double blx,bly,brx,bry;
    //new values of eyes
    double lx=150, ly=150, rx=260, ry=150;
    
    public boolean mouseMove(Event evt,int x,int y)
    {
        //move to corner of window and calc schale
        double k = schaallengte(x-150,y-150);
        lx = k * (double)(x-150) + 150;
        ly = k * (double)(y-150) + 150;
        //move to corner of window and calc scale
        k  = schaallengte(x-260,y-150);
        rx = k * (double)(x-260) + 260;
        ry = k * (double)(y-150) + 150;
        repaint();  // call to update
        return true;
    }
    
    //get the scale factor to make sure the eyeballs remains in the eye
    //itself, by the concept of Inversive geometry, see:
        //http://en.wikipedia.org/wiki/Inversive_geometry
    public double schaallengte(int x,int y)
    {
        return (double)(38) / Math.sqrt((double)(x*x+y*y+1444));
    }
    
    public void paint(Graphics g)
    {
        //out black circle of eyes (solid)
        g.setColor(Color.black);
        g.fillOval(100,100,100,100);
        g.fillOval(210,100,100,100);
        //put white circle in there to make the circle non-solid
        g.setColor(Color.white);
        g.fillOval(104,104,92,92);
        g.fillOval(214,104,92,92);
        //draw eyes, calling update might be more handy
        g.setColor(Color.black);
        g.fillOval((int)(blx=lx-8),(int)(bly=ly-8),16,16);
        g.fillOval((int)(brx=rx-8),(int)(bry=ry-8),16,16);
    }
    
    public void update(Graphics g)
    {
        // delete old eyes by painting them white again
        g.setColor(Color.white);
        g.fillOval((int)blx,(int)bly,16,16);
        g.fillOval((int)brx,(int)bry,16,16);
        // set new eyes, the minus 8 will make it a good center, else it
                // will pass the border and whipe the eyeball
        g.setColor(Color.black);
        g.fillOval((int)(blx=lx-8),(int)(bly=ly-8),16,16);
        g.fillOval((int)(brx=rx-8),(int)(bry=ry-8),16,16);
    }
}