void setup()
{
  size(550,550);    // set screen size.
  background(255);  // set background color.
  fill(0);          // I want to fill objects with black.
  noStroke();       // objects with no outlines.
}

// declare and initialize variables.
int xspeed=1, yspeed=3;
int x=0, y=0;


void draw()
{
  x = x + xspeed;
  y = y + yspeed;

  // screen boundary check
  if( (x < 0) || (x > 550) ) 
    xspeed = -xspeed;

  if( (y < 0) || (y > 550))
    yspeed = -yspeed;  
  
  //background(255);
  //rect(x,y,10,10);
  ellipse(x,y,5,5);
  ellipse(y,x,5,5);
  
}