# What does add() do?
**URL:** https://discourse.processing.org/t/what-does-add-do/19356
**Category:** Beginners
**Created:** [April 3, 2020, 1:49am UTC](https://discourse.processing.org/t/what-does-add-do/19356 "2020-04-03T01:49:24Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Cybernet](https://discourse.processing.org/u/Cybernet)
#### Post date: [April 3, 2020, 1:49am UTC](https://discourse.processing.org/t/what-does-add-do/19356/1 "2020-04-03T01:49:24Z")
So I have a code:
```auto
class Projectile extends PVector
{
PVector velocity;
Projectile(PVector loc, PVector vel)
{
super(loc.x, loc.y);
velocity = vel;
}
void Display()
{
add(velocity); // what does this do?
fill(0, 0, 255);
ellipse(x, y, 3, 3);
}
}
```
I found this on a example I saw. It creates a bullet, and shoots it towards the mouse. The example didn’t explain the code, so I had to figure out what I does. But I’m stuck on this part. When it says `add(velocity)`, it confuses me. What are you adding velocity too?
---
### Author:  [@kfrajer](https://discourse.processing.org/u/kfrajer)
#### Post date: [April 3, 2020, 2:08am UTC](https://discourse.processing.org/t/what-does-add-do/19356/2 "2020-04-03T02:08:11Z")
Since this is inside the Class Projectile which extends from PVector, the line in questions is invoking the [add()](https://processing.org/reference/PVector.html) associated to PVector. This is a core concept in object oriented programming where your Projectile class is inheriting fields, functions defined in PVector.
In other words, you can think of that line as `this.add(velocity);` so it is adding velocity to the member variables x,y,z, associated to your instantiated Projectile _object_, variables inherited from the PVector class.
Kf
---
### Author:  [@Cybernet](https://discourse.processing.org/u/Cybernet)
#### Post date: [April 3, 2020, 2:33am UTC](https://discourse.processing.org/t/what-does-add-do/19356/3 "2020-04-03T02:33:09Z")
Okay. Thank you. One more question too. Is it also the same idea with `ellipse(x, y, 3, 3)`? So the x and y are from PVector?
---
### Author:  [@kfrajer](https://discourse.processing.org/u/kfrajer)
#### Post date: [April 3, 2020, 3:04am UTC](https://discourse.processing.org/t/what-does-add-do/19356/4 "2020-04-03T03:04:07Z")
That is correct. You can see this in the docs and it the actual code:
- [https://processing.github.io/processing-javadocs/core/processing/core/PVector.html](https://processing.github.io/processing-javadocs/core/processing/core/PVector.html)
- [https://github.com/processing/processing/blob/master/core/src/processing/core/PVector.java](https://github.com/processing/processing/blob/master/core/src/processing/core/PVector.java)
Kf