2. ///questa sarà la classe per emettere 
3. class Emitter extends FBox
4.   {
5.   //emission rates, ogni quanti frame genero X body?  
6.   int emitEveryNFrame = 10;
7.   int emitNBody = 1;  
8.   
9.   //frame counter
10.   int emissionFrameCounter = 0;
11.   
12.   //particle name
13.   int id = 0; 
14.   String name = this.getName() + "particle";
15.   
16.   //constructor
17.   Emitter(float xDim, float yDim)
18.     {
19.     super(xDim, yDim);
20.     this.setStatic(true);
21.     }
22.     
23.   //methods  
24.   void emit()
25.     {
26.     //a ogni frame aggiungiamo  
27.     emissionFrameCounter +=1;
28.     
29.     float emitterPosX = this.getX();
30.     float emitterPosY = this.getY();
31.     //se sono passati i frame richiesti emetti altrimenti no..
32.     if (emissionFrameCounter == emitEveryNFrame);
33.       {
34.       for(int x=0; x< emitNBody; x++)
35.         { 
36.         //avanziamo con l id
37.         id +=1;  
38.         CustomFBox emesso = new CustomFBox(10,20);
39.         emesso.setName(name+id);
40.         emesso.setGroupIndex(int(random(0,3)));
41.         emesso.setFill(255.0,0.0,0.0);
42.         emesso.setPosition(random(emitterPosX-100,emitterPosX+100), random(emitterPosY-100,emitterPosY+100));
43.         sceneHandler.add(emesso);
44.         }//end loop
45.       }
46.     
47.     //puliamo il contatore
48.     emissionFrameCounter = 0;
49.     }  
50.    
51.   }//end emitter

You can either make new objects replace older objects' index position in their array;
or start assigning  null  to older objects to make room for newer ones,