Processing nullPointerException - nullpointerexception

Question

I have an error nullPointerException at the line if (stringReceive.contains(tabStock[j])){ but I don't understand why there is an error with the index j of the array 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 // ");
}

Answer 1

Either tabStock, tabStock[j] or stringReceive is null.
Print out their values before that line to check which one it is.
Once you know which one it is, trace back through your code to figure out why it's null.