# Pong - problems with boundary check

**URL:** <https://discourse.processing.org/t/pong-problems-with-boundary-check/11367>\
**Category:** Coding Questions\
**Created:** [May 18, 2019, 8:05pm UTC](https://discourse.processing.org/t/pong-problems-with-boundary-check/11367 "2019-05-18T20:05:50Z")\
**Posts on this page:** 12\
**Page:** 1

<div class="post-metadata">

**Author:** ![marcholman23](https://avatars.discourse-cdn.com/v4/letter/m/d26b3c/32.png) [@marcholman23](https://discourse.processing.org/u/marcholman23)\
**Post date:** [May 18, 2019, 8:05pm UTC](https://discourse.processing.org/t/pong-problems-with-boundary-check/11367/1 "2019-05-18T20:05:50Z")

</div>

Hi,

I’m new to processing and working on implementing a simple pong like program. At this point I have created a Ball and a Paddle. The Paddle is simply a rectangle that tracks the mouseX variable.

Now I am working on the ball. Eventually I will want it to bounce off the paddle at an angle. But **before that** I am just trying to get the ball to bounce off the screen boundaries. Later I will handle the case where the paddle misses the ball or bounces off the paddle.

So for now basically I just want my ball to bounce at an angle off the four walls (screen boundaries).

```auto
This is my code so far:
Ball ball;
Paddle paddle;

void setup() {
  size(640, 480);
  paddle = new Paddle();
  ball = new Ball(paddle);
}

void draw() {
  background(0);
  paddle.draw();
  paddle.move();
  
  
  
  ball.checkBoundaries();
  paddle.checkBoundaries();
  
  ball.draw();
  ball.move();
}

```

```auto

class Ball {
  float xPos;
  float yPos;
  
  float xSpeed = 2.8;
  float ySpeed = 2.2;
  
  int xDirection = 1;
  int yDirection = 1;
  
  int rad = 16;
  
  final float speed = 5;
  
  Paddle paddle;
  
  public Ball(Paddle paddle){
    paddle = paddle;
    
    // set starting position of the ball, start in center of screen
    xPos = width / 2;
    yPos = height / 2;
  }
  
  
  public void move() {
    // update the ball's position
    xPos += (xSpeed) + (xSpeed * xDirection);
   // yPos += (ySpeed) + (ySpeed * yDirection);
 
  }
  
  public void draw() {
    ellipse(xPos, yPos, rad, rad);
    checkBoundaries();
  }
  
  public void checkBoundaries() {
    if(xPos > width - rad || xPos < rad) {
      xDirection *= -1;
    }
    
    if(yPos > height - rad || yPos < rad) {
     yDirection *= -1;
     println("yDirection: " + yDirection + "yPos: " + yPos);
    }
  }
  
  public void checkCollision() {
    
  }
}

```

And finally my Paddle:

```auto
class Paddle {
  float x;
  final float y = 440;
  
  float speedX;
  
  public Paddle() {
    
  }
  
  public void move() {
    x = mouseX;
  }
  
  public void draw() {
    fill(255);
    rect(x, y, 100, 20);
  }
  
  public void checkBoundaries(){
    if(x < 0){
      x = 0;
    }
    
    if(x > (width - 100)){
     x = width - 100; 
    }
  }
}

```

I don’t understand why the ball is simply falling diagonally to the right and then disappearing off the screen instead of bouncing back at an angle.

Any help would be much appreciated.

Thanks,  
Marc

---

<div class="post-metadata">

**Author:** ![paulgoux](https://avatars.discourse-cdn.com/v4/letter/p/b9bd4f/32.png) [@paulgoux](https://discourse.processing.org/u/paulgoux)\
**Post date:** [May 18, 2019, 8:27pm UTC](https://discourse.processing.org/t/pong-problems-with-boundary-check/11367/2 "2019-05-18T20:27:48Z")

</div>

```auto
class Ball {
  float xPos;
  float yPos;
  
  float xSpeed = 2.8;
  float ySpeed = 2.2;
  
  int xDirection = 1;
  int yDirection = 1;
  
  int rad = 16;
  
  final float speed = 5;
  
  Paddle paddle;
  
  public Ball(Paddle paddle){
    paddle = paddle;
    
    // set starting position of the ball, start in center of screen
    xPos = width / 2;
    yPos = height / 2;
  }
  
  
  public void move() {
    // update the ball's position
    //xPos += (xSpeed) + (xSpeed * xDirection);
    yPos += (ySpeed) + (ySpeed * yDirection);
 
  }
  
  public void draw() {
    ellipse(xPos, yPos, rad, rad);
    checkBoundaries();
  }
  
  public void checkBoundaries() {
    if(xPos > width - rad ) {
      xSpeed -= 2.2;
    }
    if(xPos <rad ) {
      xSpeed = 2.2;
    }
    
    if(yPos > height) {
     ySpeed = -2.2*yDirection;
     println("yDirection: " + yDirection + "yPos: " + yPos);
    }
    if(yPos<rad) {
     ySpeed = 2.2*yDirection;
     println("yDirection: " + yDirection + "yPos: " + yPos);
    }
  }
  
  public void checkCollision() {
    
  }
}

```

---

<div class="post-metadata">

**Author:** ![Chrisir](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/chrisir/32/45_2.png) [@Chrisir](https://discourse.processing.org/u/Chrisir)\
**Post date:** [May 18, 2019, 8:29pm UTC](https://discourse.processing.org/t/pong-problems-with-boundary-check/11367/3 "2019-05-18T20:29:37Z")

</div>

> [@marcholman23](#):
>
> += (xSpeed) + (xSpeed \* xDirection);

looks suspicious

Why not

+= (xSpeed \* xDirection);

---

<div class="post-metadata">

**Author:** ![paulgoux](https://avatars.discourse-cdn.com/v4/letter/p/b9bd4f/32.png) [@paulgoux](https://discourse.processing.org/u/paulgoux)\
**Post date:** [May 18, 2019, 8:31pm UTC](https://discourse.processing.org/t/pong-problems-with-boundary-check/11367/4 "2019-05-18T20:31:25Z")

</div>

not sure why but that approach doesnt work in my sketch.  
😕

---

<div class="post-metadata">

**Author:** ![Chrisir](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/chrisir/32/45_2.png) [@Chrisir](https://discourse.processing.org/u/Chrisir)\
**Post date:** [May 18, 2019, 8:40pm UTC](https://discourse.processing.org/t/pong-problems-with-boundary-check/11367/5 "2019-05-18T20:40:52Z")

</div>

also the double checkBoundaries(); was confusing it.

Use this in the ball class

```auto
  public void draw() {
    ellipse(xPos, yPos, 
      rad, rad);
    // checkBoundaries();
  }

```

---

<div class="post-metadata">

**Author:** ![Chrisir](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/chrisir/32/45_2.png) [@Chrisir](https://discourse.processing.org/u/Chrisir)\
**Post date:** [May 18, 2019, 8:47pm UTC](https://discourse.processing.org/t/pong-problems-with-boundary-check/11367/6 "2019-05-18T20:47:06Z")

</div>

I always advice to check the boundaries separately because the \*-1  
can lead to stuttering (and lead to the problem with the double call to checkBoundaries() imho)

`abs()` gives always the positive value (e.g. 2.8 without the minus - sign); with \*-1 this gives always a negative value. So no stuttering can occur.

```auto

  public void checkBoundaries() {
    if (xPos > width - rad) {
      xSpeed = abs(xSpeed) * -1;
    }

    if ( xPos < rad ) {
      xSpeed = abs(xSpeed);
    }

    if (yPos > height - rad) {
      ySpeed = abs(ySpeed) * -1;
      println("yDirection: " + yDirection + "yPos: " + yPos);
    }
    if ( yPos < rad ) {
      ySpeed = abs(ySpeed);
    }
  }

```

---

<div class="post-metadata">

**Author:** ![marcholman23](https://avatars.discourse-cdn.com/v4/letter/m/d26b3c/32.png) [@marcholman23](https://discourse.processing.org/u/marcholman23)\
**Post date:** [May 18, 2019, 9:44pm UTC](https://discourse.processing.org/t/pong-problems-with-boundary-check/11367/7 "2019-05-18T21:44:43Z")

</div>

Wow! Thank you for your quick reply and detailed help. I really appreciate it. Now I have the ball bouncing off the edges of the screen.

Yeah, those extra calls to checkBoundaries were redundant.

I am not sure I understand how using abs() avoids stuttering - can you elaborate on that a bit.

Once I get that I will move on to adding logic to detect collisions between the ball and the paddle and have the ball bounce off the paddle. My paddle is 100 pixels wide. So if the x position of the ball - radius is equal to any point between the x position of the paddle and the x position of the paddle + 100 the ball would bounce off of it.

Does this make sense? I will try writing the code for that.

I am thinking that if, for example I want the player to have 5 lives I can use a counter. I’d decrement the counter each time a ball drops off the screen and then start off with a new ball.

However one problem I can think of in advance is that when balls drop off the screen they don’t really disappear - there is still a ball taking up memory and moving through coordinates that are off screen, right? So how do I destroy a ball after it falls off the screen? Would I be better off using an arraylist and removing the ball from it each time it drops off screen?

Thanks again for your time and help.  
Marc

---

<div class="post-metadata">

**Author:** ![Chrisir](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/chrisir/32/45_2.png) [@Chrisir](https://discourse.processing.org/u/Chrisir)\
**Post date:** [May 18, 2019, 9:52pm UTC](https://discourse.processing.org/t/pong-problems-with-boundary-check/11367/8 "2019-05-18T21:52:57Z")

</div>

You are on a good track

No need for an arraylist just re-init the ball with a new random position and speed

---

<div class="post-metadata">

**Author:** ![marcholman23](https://avatars.discourse-cdn.com/v4/letter/m/d26b3c/32.png) [@marcholman23](https://discourse.processing.org/u/marcholman23)\
**Post date:** [May 18, 2019, 9:59pm UTC](https://discourse.processing.org/t/pong-problems-with-boundary-check/11367/9 "2019-05-18T21:59:06Z")

</div>

I have added this to checkBoundaries to check for collisions between the ball and paddle: (100 is the length of the paddle, I’ll move that to a constant later )

```auto
for(float index = paddle.x; index < paddle.x + 100; index++) {
      if(xPos == index) {
       ySpeed = abs(ySpeed) * -1;
      }
    }

```

But I am getting null pointer exception. Where did I go astray here?

Does processing have a simpler way to check collisions?

Thanks,  
Marc

---

<div class="post-metadata">

**Author:** ![Chrisir](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/chrisir/32/45_2.png) [@Chrisir](https://discourse.processing.org/u/Chrisir)\
**Post date:** [May 18, 2019, 10:09pm UTC](https://discourse.processing.org/t/pong-problems-with-boundary-check/11367/10 "2019-05-18T22:09:40Z")

</div>

That’s not correct

First kill the boundary check for the lower screen border

Instead just put

if(ball.x\>=paddle.x&& ball.x\<paddle.x plus paddle width && ball.y \> paddle.y)

bounce

Pseudo-code

---

<div class="post-metadata">

**Author:** ![marcholman23](https://avatars.discourse-cdn.com/v4/letter/m/d26b3c/32.png) [@marcholman23](https://discourse.processing.org/u/marcholman23)\
**Post date:** [May 18, 2019, 10:30pm UTC](https://discourse.processing.org/t/pong-problems-with-boundary-check/11367/11 "2019-05-18T22:30:19Z")

</div>

Thank you!

I am having a problem accessing paddle.x.

In my main program I create a paddle and then I create a ball, passing to it the paddle.

I can access paddle.x fine in the main program and in my Ball constructor. But when I try to access it elsewhere in my Ball class it throws null pointer exception.

Here is my current code:

```auto
Ball ball;
Paddle paddle;

void setup() {
  size(640, 480);
  paddle = new Paddle();
  ball = new Ball(paddle);
 // println(paddle.x); prints paddle.x
}

void draw() {
  background(0);
  paddle.draw();
  paddle.move();
  
  paddle.checkBoundaries();
  
  ball.draw();
  ball.move();
}

```

```auto

class Paddle {
  public float x;
  public final float y = 440;
  
  float speedX;
  
  public Paddle() {
    x = 0;
    
  }
  
  public void move() {
    x = mouseX;
  }
  
  public void draw() {
    fill(255);
    rect(x, y, 100, 20);
  }
  
  public void checkBoundaries(){
    if(x < 0){
      x = 0;
    }
    
    if(x > (width - 100)){
     x = width - 100; 
    }
  }
}

class Ball {
  float xPos =0;
  float yPos = 0;
  
  float xSpeed = 2.8;
  float ySpeed = 2.2;
  
  int xDirection = 1;
  int yDirection = 1;
  
  int rad = 16;
  
  final float speed = 5;
  
  Paddle paddle;
  
  public Ball(Paddle paddle){
    paddle = paddle;
    
    // set starting position of the ball, start in center of screen
    xPos = width / 2;
    yPos = height / 2;
   // println(paddle.x); prints paddle.x
    
  }
  
  
  public void move() {
    // update the ball's position
    xPos += (xSpeed * xDirection);
    yPos += (ySpeed * yDirection);
    // println(paddle.x); throws null pointer exception
 
  }
  
  public void draw() {
    ellipse(xPos, yPos, rad, rad);
    checkBoundaries();
  }
  
  public void checkBoundaries() {
    //// check collision between ball and paddle, 100 = paddle length
    //if(xPos > paddle.x){
    ////if(xPos > paddle.x && xPos < (paddle.x + 100) && (yPos + 20) >= paddle.y) {
    // xSpeed = abs(xSpeed) * -1; 
    // ySpeed = abs(ySpeed) * -1; 
    //}
    println(paddle.x);
    
     
    
    if(xPos > width - rad) {
     xSpeed = abs(xSpeed) * -1; 
    }
    
    if(xPos < rad) {
     xSpeed = abs(xSpeed); 
    }
    
    if(yPos > height - rad) {
     ySpeed = abs(ySpeed) * -1; 
    }
    
    // check lower screen boundary
    //if(yPos < rad) {
    // ySpeed = abs(ySpeed); 
    //}
  }
  
  public void checkCollision() {
    
  }
}

```

Paddle and Ball are included together above.

So I am confused about passing objects in processing. I had to use globals to make this work. Instead - as above - what I really want to do is have the ball know about the paddle. But after passing to the constructor when I try to access paddle.x I get null pointer.

Note: I am using all public and no getter/setters for simplicities sake.

---

<div class="post-metadata">

**Author:** ![Chrisir](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/chrisir/32/45_2.png) [@Chrisir](https://discourse.processing.org/u/Chrisir)\
**Post date:** [May 19, 2019, 3:32am UTC](https://discourse.processing.org/t/pong-problems-with-boundary-check/11367/12 "2019-05-19T03:32:08Z")

</div>

> [@marcholman23](#):
>
> Paddle paddle;

That’s wrong when you have it in the ball class

You want the ball class to check against the normal paddle that was declared before setup