Hello,
i would like to draw the five last position of each rectangles under them. It  looks like a sandwich of five stratum. More precisely when a rectangle move at t+6 i would like to draw the five previous position
with different alpha. I know that i have to store it in an array but how can i update the cycle ? how can i refresh my array with new position ? After being drawn, the oldest position of the five previous position disappear from the rendering and from the array
For example :
t+4
t+3
t+2
t+1
t+0 // when the t+5 will be drawn t+0 will dispappear and so on.

i try to implement this code :

Obj[] o = new Obj[20];
int Height = 50;
int Width =  50;

void setup() {
//size(screen.width,screen.height, P3D);
 size(500, 500, P3D);
 colorMode(HSB);      
 noStroke();
 smooth();            
 frameRate(30);
 for(int i=0; i < o.length; i++) { // initialize x, y for each object
   float objectSize = 2;
   float x = random(Width);
   float y = random(Height);
   o[i] = new Obj(x, y, objectSize);
 }}

void draw() {
  background(0);
 translate(width/2.1, height/2, 350);
 //translate(width/2.1, height/2, 750);
 rotateX(1);
 for(int i=0; i < o.length; i++) {
   o[i].draw();
 }}

class Obj {

 color colors;
 float x, y, objectSize;

 Obj(float x, float y, float s) {
   this.x = x;
   this.y = y;
   objectSize = s;
   colors = color(random(255), 255, 255); // hue, saturation, brightness
 }
 void draw() {
   move();
   render();
 }
 void move() {
     // move randomly
     x += random(-objectSize/2,objectSize/2);
     y += random(-objectSize/2,objectSize/2);
     // limits
     if (x > Width + objectSize) x = -objectSize;
     if (x < -objectSize) x = Width + objectSize;
     if (y > Height + objectSize) y = -objectSize;
     if (y < -objectSize) y = Height + objectSize;
 }
void render() {
   pushMatrix();
   fill(colors);
   translate(x,y);
   rect(objectSize/2, 0, objectSize, objectSize);
   popMatrix();
 }}

Isn't what you're trying to do similar to the storing input example?

http://processing.org/learning/basics/storinginput.html