float x,y,dir,speed;

void setup() {
  size(300,300);
  x=width/2;
  y=height/2;
  dir=random(360);
  speed=3;
  rectMode(CENTER_DIAMETER);
}

void loop() {
  background(200);
  x+=cos(radians(dir))*speed;
  y+=sin(radians(dir))*speed;
  
//  dir+=0.5; // circular motion
  dir+=random(-10,10);
  
  rect(x,y, 30,30);
  if(x<0) x=width;
  else if(x>width) x=0;
  if(y<0) y=height;
  else if(y>height) y=0;
}