Обработка исключения nullPointerException

У меня ошибка nullPointerException в строке if (stringReceive.contains(tabStock[j])){ но я не понимаю почему ошибка с индексом j массива tabstock.

import oscP5.*;
import netP5.*;

OscP5 oscP5;
NetAddress myRemoteLocation;

String[] tabStock = new String[6];
String[] tabReceive = new String[6];
String stringReceive;
String oldStringReceive;
int cptEssai = 0;
int cpt = 0;

void setup() {
size(400,400);
frameRate(25);
/* start oscP5, listening for incoming messages at port 12000 */
oscP5 = new OscP5(this,12000);

myRemoteLocation = new NetAddress("127.0.0.1",12001);

tabStock[0] = "1";
tabStock[1] = "2";
tabStock[2] = "3";
tabStock[3] = "4";
tabStock[4] = "5";
tabStock[5] = "6";
//stringReceive = "A3E8F6";
tabReceive[0] = "1";
tabReceive[1] = "2";
tabReceive[2] = "3";
tabReceive[3] = "4";
tabReceive[4] = "3";
tabReceive[5] = "6";


}


void draw() {
   background(0);
   compare();

}

void mousePressed() {

    OscMessage myMessage = new OscMessage("/test");

    myMessage.add(123); /* add an int to the osc message */

  /* send the message */
oscP5.send(myMessage, myRemoteLocation);
}


/* incoming osc message are forwarded to the oscEvent method. */
void oscEvent(OscMessage theOscMessage) {
/* print the address pattern and the typetag of the received OscMessage */
print("### received an osc message.");
print(" addrpattern: "+theOscMessage.addrPattern());
println(" typetag: "+theOscMessage.typetag());
stringReceive = theOscMessage.addrPattern();
println(stringReceive);
if (oldStringReceive != stringReceive){
    oldStringReceive = stringReceive;
}
}

void compare() {
    println(stringReceive);
    println("compare()");
    boolean failed=false;
    int j = 0;
    while (cptEssai < 3 && !failed){ 
    for (int i= 0; i < tabStock.length; i++){

    while(j<6){
         if (stringReceive.contains(tabStock[j])){
         print("tag existe et bien placé   //   ");
         print("allumage de la lampe qui correspond à ce tag   //   ");
         j++;
    }
         else {
              print("tag n'existe pas ou mal placé   //   ");
              print("extinction de toutes les lampes   //   ");
              oldStringReceive = stringReceive;
         if (oldStringReceive != stringReceive){
             cptEssai++;
             failed = true;
             break;
         }
         else
         {
            if(j>0){
            j--;
         }
         }

    }
   }
  }
  }
  print("GAME OVER ! Désactivation de l'épreuve   //   ");
}