First remark: it is a bad idea to name a class "Object", as it is the name of the base Java class, root of the hierarchy of all classes. Beside, it is better to use a more descriptive name, like Ball or similar.

Second remark: you also your best attempt at putting these objects in a list. There are several example sketches in Processing showing such process.

Thanks for your answer

about your first remark , it's because it's a lesson code from my teacher ...
and abour your second remark : i tried to create a list but with the pvectors i'm confused ...

this is now :

Balle [] balle =  new Balle [200] ;
 
void setup() {
  size(300, 300);
  PVector pos = new PVector(width/2, height/2);
  PVector vel = new PVector(0, 0);
  PVector acc =new PVector(0, 0);
  balle = new Balle(pos, vel, acc, 1.5);
 
}

void draw() {
  background(0);
  balle.draw();
  PVector gravite = new PVector(0, 0.08);
  balle.appliqueForce(gravite);
  PVector vent = new PVector ( 0.01, 0);
  balle.appliqueForce(vent);
       
 
}



in class :
class Balle {
  PVector pos;
  PVector vel;
  PVector acc;
  float masse;

  Balle (PVector p, PVector v, PVector a, float m) {
    pos = p.get();
    vel = v.get();
    acc = a.get();
    masse = m;
  }

  void update() {
    border();
    vel.add(acc);
    pos.add(vel);
    acc.mult(0);
  }

  void draw() {
    update();
    fill(255);
    ellipse (pos.x, pos.y, masse*5, masse*5);
 
}
 
  void border() {
    if (pos.x <0) {
      pos.x = 0;
      vel.x*=-1;
    }
    if ( pos.x > width) {
      pos.x = width;
      vel.x*=-1;
    }
    if (pos.y <0) {
      pos.y = 0;
      vel.y*=-1;
    }
    if ( pos.y > height) {
      pos.y = height;
      vel.y*=-1;
    }
  }
  void appliqueForce(PVector force) {
  force.div(masse);  
  acc.add(force);
}
PVector getGraviteForce (Balle balle) {
    PVector direction = PVector.sub(pos,balle.pos);
    float distance = direction.mag();
    distance = constrain(distance,5.0,25.0);
    direction.normalize();
    float force = (G * masse * balle.masse)/ (distance*distance);
    direction.mult(force);
    return direction;
  }
}

he says he can't convert ... i'm completely lost..

Look at how to initialize Arrays.
In this case, you have to use

1. for(int i = 0; i < balle.length; i ++) {
2.         balle[i] = new Balle(pos, vel, acc, 1.5);
3. }

That's why I ask for code, because you wrote about a list, but you show an array, which is a different beast in Java...
Beside, calsign explained well why you have this error (it is better to report full messages, it helps in helping people).

Okay thank you Calsign for your explainations ...
I begin to understand how it works ...
Before posting , i tried to write :

1. for(int i = 0; i < 300; i ++) {
2.         balle[i] = new Balle;

instead of yours :

1. for(int i = 0; i < balle.length; i ++) {
2.         balle[i] = new Balle(pos, vel, acc, 1.5);

Try putting a t between the g and the h...

Side note: it is good practice to name data with size (arrays, ArrayList, etc.) with a plural name: balles shows it has several elements in it, reducing the risk of confusing with a single ball.

Oups xd
Thanks a lot !