At last I decided to take the time to write some code to test wether ArrayList of regular arrays are the fastest for stuff like particles (my own objects, with an expanding array size).

My test code is not very interesting but I think the answer will be for those who have the same needs and who never tested it. It turns out that with a growing array (at the end I have >30 000 items in the array), ArrayList is way faster than regular arrays.

With the ArrayList, walking through 30162 instances of my class takes around 400 milliseconds, when the same amout of instances require over 2000 milliseconds.

ArrayList wins, then.

Here is the ArrayList tryout :

```
ArrayList<poin> allPoints;
int cx,cy;
int begining;
void setup(){
begining = millis();
size(640,640);
cx=width/2;cy=height/2;
allPoints = new ArrayList(0);
}
void draw(){
background(255);
int t=millis();
ArrayList<poin> newAllPoints=new ArrayList();
for(poin p:allPoints){
p.avance();
if(p.alive){
newAllPoints.add(p);
}
}
float ang=TWO_PI/frameCount;
for(float a=0;a<TWO_PI;a+=ang){
newAllPoints.add(new poin(cx, cy, a));
}
allPoints=newAllPoints;
float ecouled=millis()-t;
println(allPoints.size(), ecouled, allPoints.size()/(ecouled/1000));
if(allPoints.size()>30000){println("total time : "+(millis()-begining));noLoop();}
}
class poin{
float x,y,a;boolean alive=true;
poin(float x, float y, float a){
this.x=x;this.y=y;this.a=a;
}
void avance(){
if(x<0||y<0||x>width||y>height)alive=false;
x+=cos(a)*2;y+=sin(a)*2;
point(x,y);
}
}
```

and now the regular Array tryout :

```
poin[] allPoints;
int cx,cy;
int begining;
void setup(){
begining = millis();
size(640,640);
cx=width/2;cy=height/2;
allPoints = new poin[0];
}
void draw(){
background(255);
int t=millis();
poin[] newAllPoints = new poin[0];
for(poin p:allPoints){
p.avance();
if(p.alive){
newAllPoints = (poin[]) append(newAllPoints, p);
}
}
float ang=TWO_PI/frameCount;
for(float a=0;a<TWO_PI;a+=ang){
newAllPoints = (poin[]) append(newAllPoints, new poin(cx, cy, a));
}
allPoints=newAllPoints;
float ecouled=millis()-t;
println(allPoints.length, ecouled,allPoints.length/(ecouled/1000));
if(allPoints.length>30000){println("total time : "+(millis()-begining));noLoop();}
}
class poin{
float x,y,a;boolean alive=true;
poin(float x, float y, float a){
this.x=x;this.y=y;this.a=a;
}
void avance(){
if(x<0||y<0||x>width||y>height)alive=false;
x+=cos(a)*2;y+=sin(a)*2;
point(x,y);
}
}
```

I can’t say if it stays true with P5js.