Boundary Collisions - example code

int xPos;
int yPos;
int xSpeed;
int ySpeed;

void setup() {
  size(600, 600);
  xPos = width/2;
  yPos = height/2;
  xSpeed = 1;
  ySpeed = 2;
}

void draw() {
  background(0);
  ellipse(xPos, yPos, 30, 30);
  xPos = xPos + xSpeed;
  yPos = yPos + ySpeed;
  boundaryCollision();
}

void boundaryCollision(){
  if(xPos > width-15){
    xSpeed = xSpeed*-1;
  }
  if(xPos < 15){
    xSpeed = xSpeed*-1;
  }
  if(yPos > height-15){
    ySpeed = ySpeed*-1;
  }
  if(yPos < 15){
    ySpeed = ySpeed*-1;
  }
}