Hello everyone,
I found this code:

PFont testo;
String insieme[] = {"4 - 3 =", "5 - 2 ="};
ArrayList charHolder;
ArrayList buffer;

void setup(){

  background (0);
  size (800,800);
  charHolder = new ArrayList();
  buffer = new ArrayList();
  for(int i = 0; i< insieme.length;i++){
   charHolder.add(insieme[i]);
  }
   testo = createFont("FFScala", 32);
   textFont (testo,100);
   textAlign (CENTER,CENTER);
   frameRate(10);
}
void draw (){
  if (keyPressed == true) {
    int index = floor(random(charHolder.size()));
    text ((String)charHolder.get(index),200 , 100);
    buffer.add(charHolder.remove(index));
  }
  if(charHolder.size() == 0){
   charHolder = (ArrayList)buffer.clone();
   buffer.clear();
  }
}

The problem that you overwrite the new string. It is possible to press key to remove the old string?

thanks