# Can you help me understand why b = null in draw function is incorrect

**URL:** <https://discourse.processing.org/t/can-you-help-me-understand-why-b-null-in-draw-function-is-incorrect/20713>\
**Category:** Coding Questions\
**Created:** [May 10, 2020, 3:06pm UTC](https://discourse.processing.org/t/can-you-help-me-understand-why-b-null-in-draw-function-is-incorrect/20713 "2020-05-10T15:06:05Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![Arsenii](https://avatars.discourse-cdn.com/v4/letter/a/ed8c4c/32.png) [@Arsenii](https://discourse.processing.org/u/Arsenii)\
**Post date:** [May 10, 2020, 3:06pm UTC](https://discourse.processing.org/t/can-you-help-me-understand-why-b-null-in-draw-function-is-incorrect/20713/1 "2020-05-10T15:06:05Z")

</div>

# Can you help me understand why b = null in draw function is incorrect

```auto
import java.util.List;
import java.util.ArrayList;
spaceShip ship;

boolean left = false;
boolean right = false;
boolean forward = false;

int maxQuantity = 7;
float inercia;

ArrayList<Bolder> bmas = new ArrayList<Bolder>();
ArrayList<Integer> brem = new ArrayList<Integer>();
ArrayList<Laser> las = new ArrayList<Laser>();

PVector Rand = new PVector(random(50), random(50));
PVector shipPos;
void setup() {
  size(1280, 720);
  ship = new spaceShip();
  shipPos = ship.pos;
  for (int j = 0; j < maxQuantity; ++j) {
    bmas.add(new Bolder(random( width/2, width ) * random(3), random(height/2, height) * random(3)));
  }
}
void draw() {
  int j =0;
  background(0);

  for (Laser l : las) {
    l.show();
    l.move();
  }
  ship.render();
  ship.edges();

  // все связанное с астеройдами
  for (Bolder b : bmas) {
    b.show();
    b.move();
    b.edges();
    for (Laser l : las) {
      if (b.destroy(l, b, l.pos.x, l.pos.y))
        b = null;
    }
    ++j;
  }
  //for (int h = 0; h < brem.size(); ++h)
    //bmas.remove(brem.get(h));
    
  //добавление новых астеройдов 
  if (bmas.size() < maxQuantity) {
    for (int i = bmas.size(); i < maxQuantity; ++i)
      bmas.add(new Bolder(random(width), random(height)));
  }
}
void keyPressed() {
  if (keyCode == RIGHT) {
    ship.turn(0.75);
  } else if (keyCode == LEFT) {
    ship.turn(-0.75);
  } else if (keyCode == UP) {
    ship.forward(-24);
  } else if (keyCode == DOWN) {
    las.add(new Laser(ship.pos.x, ship.pos.y, ship.heading, -4));
  }
}

```

---

<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 10, 2020, 3:50pm UTC](https://discourse.processing.org/t/can-you-help-me-understand-why-b-null-in-draw-function-is-incorrect/20713/2 "2020-05-10T15:50:57Z")

</div>

I think you need to tell the ArrayLists of which class the objects in it are:

You do this with `<Bolder>`

```auto
ArrayList<Bolder> bmas = new ArrayList();
ArrayList<Bolder> brem = new ArrayList();
ArrayList<Laser> las = new ArrayList();

```

---

<div class="post-metadata">

**Author:** ![noahbuddy](https://avatars.discourse-cdn.com/v4/letter/n/73ab20/32.png) [@noahbuddy](https://discourse.processing.org/u/noahbuddy)\
**Post date:** [May 10, 2020, 7:34pm UTC](https://discourse.processing.org/t/can-you-help-me-understand-why-b-null-in-draw-function-is-incorrect/20713/3 "2020-05-10T19:34:43Z")

</div>

@Chrisir is correct on initializing ArrayLists.  
Additionally, the assignment of “b = null;” in this case is only setting the local variable “b” to null.  
This will not affect the contents of “bmas”.

You can verify this by adding the following after the loop:

```auto
  String s = "";
  for (Bolder b : bmas) {
    s += b + "\n";
  }
  text(s, 10, 20); // will not show "null" in any of the objects listed

```

I recommend using a state variable inside the “Bolder” class.  
For example:

```auto
b.alive = false; // initialize "alive = true;" in constructor

```

Then you can remove any objects that were marked “false”.  
Take a look at the note about modifying an ArrayList on the [reference page](https://processing.org/reference/ArrayList.html).