it = particles.iterator();
while(it.hasNext()){
Particle p = it.next();
p.run();
if(p.isDead()){
it.remove();
}
}
}
}
class Particle{
PVector location;
PVector velocity;
PVector acceleration;
float lifespan;
Particle(PVector l){
acceleration = new PVector(0,0.05);
velocity = new PVector(random(-1,1),random(-2,0));
location = l.copy();
lifespan = 255.0;
}
void run(){
update();
display();
}
void update(){
velocity.add(acceleration);
location.add(velocity);
lifespan -= 2.0;
}
void display(){
stroke(0,lifespan);
fill(0,lifespan);
ellipse(location.x,location.y,8,8);
}
boolean isDead(){
if(lifespan < 0.0){
return true;
}else{
return false;
}
}
}
```
---
### Author:  [@SomeOne](https://discourse.processing.org/u/SomeOne)
#### Post date: [April 22, 2022, 4:12am UTC](https://discourse.processing.org/t/the-particle-is-undefined/36510/2 "2022-04-22T04:12:45Z")
Yes it is undefined. You have defined Particle constructor with PVector parameter `Particle(PVector l){`, but you haven’t defined constructor without parameters `Particle(){` as you try to use it.
---
### Author:  [@jiajia](https://discourse.processing.org/u/jiajia)
#### Post date: [April 22, 2022, 4:55am UTC](https://discourse.processing.org/t/the-particle-is-undefined/36510/3 "2022-04-22T04:55:30Z")
OH.I was negligent about this and your help allowed me to solve the error in an instant.very thanks 🙂
---
### Author:  [@jiajia](https://discourse.processing.org/u/jiajia)
#### Post date: [April 22, 2022, 5:02am UTC](https://discourse.processing.org/t/the-particle-is-undefined/36510/4 "2022-04-22T05:02:50Z")
A little problem arose and I asked you again.
---
### Author:  [@SomeOne](https://discourse.processing.org/u/SomeOne)
#### Post date: [April 22, 2022, 5:10am UTC](https://discourse.processing.org/t/the-particle-is-undefined/36510/5 "2022-04-22T05:10:09Z")
You have misspelled `void setup()` and ps is not created before you call it.
---
### Author:  [@jiajia](https://discourse.processing.org/u/jiajia)
#### Post date: [April 22, 2022, 7:35am UTC](https://discourse.processing.org/t/the-particle-is-undefined/36510/7 "2022-04-22T07:35:04Z")
But how do I create it 🙂 🤔
```auto
import java.util.Iterator;
ParticleSystem ps;
Particle p;
void setup(){
size(400,400);
ps = new ParticleSystem();
}
void draw(){
background(255);
ps.run();
}
class ParticleSystem{
ArrayList particles;
ParticleSystem(){
particles = new ArrayList();
}
void addParticle(){
particles.add(new Particle());
}
void run(){
Iteratorit = particles.iterator();
while(it.hasNext()){
Particle p = it.next();
p.run();
if(p.isDead()){
it.remove();
}
}
}
}
class Particle{
PVector location;
PVector velocity;
PVector acceleration;
float lifespan;
Particle(){
acceleration = new PVector(0,0.05);
velocity = new PVector(random(-1,1),random(-2,0));
location = location.copy();
lifespan = 255.0;
}
void run(){
update();
display();
}
void update(){
velocity.add(acceleration);
location.add(velocity);
lifespan -= 2.0;
}
void display(){
stroke(0,lifespan);
fill(0,lifespan);
ellipse(location.x,location.y,8,8);
}
boolean isDead(){
if(lifespan < 0.0){
return true;
}else{
return false;
}
}
}
```
---
### Author:  [@SomeOne](https://discourse.processing.org/u/SomeOne)
#### Post date: [April 22, 2022, 8:41am UTC](https://discourse.processing.org/t/the-particle-is-undefined/36510/8 "2022-04-22T08:41:37Z")
> [@jiajia](#):
>
> `ps = new ParticleSystem();`
This creates an empty storage for Particles. To get something going on you need to add Particles the storage. You can add Particle with command `ps.addParticle()`
> [@jiajia](#):
>
> ```auto
> void addParticle(){
> particles.add(new Particle());
> }
>
> ```
---
### Author:  [@jiajia](https://discourse.processing.org/u/jiajia)
#### Post date: [April 22, 2022, 9:57am UTC](https://discourse.processing.org/t/the-particle-is-undefined/36510/9 "2022-04-22T09:57:58Z")
yea very thankyou，I learning 🙂