Why am I getting this Null Pointer Exception?

[Mikelo Vatki]

I was trying to model a board game called "Risk" in Java when I came upon this persistent error. I can't seem to figure out why java is throwing a null pointer exception. I have three classes, risk.class, territory.class and terrHandler.class but I have removed some of the excess code that is not related to this problem. Below is the code.

risk

 
import java.util.*;
import java.lang.Math.*;
 
public class risk{
public static void main(String[] args) {
 
    //initialize territories
int numOfTerritories = 10;
 
territory[] territories = new territory[numOfTerritories];
 
terrHandler helper = new terrHandler();
   
int terrConnectRange = 2;
 
helper.connectTerritories(terrConnectRange, territories);
 
}
}

territory

public class territory {

boolean[] isConnectedTo;
 
//getters
 
public boolean[] getConnections(){
return isConnectedTo;
}
 
//setters
 
public void setConnection(int thatTerritory){
isConnectedTo[thatTerritory] = true;
}
 
}

terrHandler

import java.util.*;
import java.lang.Math.*;
 
public class terrHandler{

Random rand = new Random();
 
public void connectTerritories(int range, territory[] territories){
 
System.out.println("connectTerritories has been invoked");
 
int magicNumber = 3;
int q = 0;
int r;

int numTerritories = territories.length;

while(q < numTerritories){
 
System.out.println("while loop started, connecting territory number "+ q);
 
int numLinks = 0;
int j = 0;
while(numLinks==0){
numLinks = Math.abs(Math.round(magicNumber+rand.nextInt(2*range)-range)); 
}

System.out.println("numLinks="+numLinks);

while(j < numLinks){
r = rand.nextInt(numTerritories);

while(r==q){
r = rand.nextInt(numTerritories);
}
 
System.out.println("setting a connection");
                        
territories[q].setConnection(r);

j++;
}

q++;
}
 
}
}

"territories[q].setConnection(r);" is what is causing the exception but I don't understand what could be null here. Thanks in advance to anyone who responds.

Mikelo Vatki

[Eric Daly]

public void setConnection(int thatTerritory){
isConnectedTo[thatTerritory] = true;
}

How many elements did you assign (if any) to the isConnectedTo array? It is never initialized... this will throw a NullPointerException unless you initialize your array.

[Ilja Preuss]

More specifically,

new territory[numOfTerritories];

doesn't instantiate any territory objects. All it does is instantiating an array of references to territory objects. All those references will initially be set to null.

As an aside, general coding convention is to have class names start with a capital letter. Not doing so will make it much harder for others to read your code.

[Mikelo Vatki]

Alright, I found my mistake. Thanks to both of you for responding. I simply added a while loop to properly initialize each element in each of the arrays. I added a function to cycle through isConnectedTo[] and set each element to false. I have renamed all of my classes so that they conform to the convention as well (Ilja Preuss, thank you for pointing that out).

[Ilja Preuss]

Originally posted by Mikelo Vatki:
I added a function to cycle through isConnectedTo[] and set each element to false.

Oh, *that*'s redundant. All array elements are automatically initialized to their default value - for object references, that's null, for booleans false (for numerical primitives zero).

[Ronald Schild]

Would you consider for loops in your code?

The way you've written your loop:

int x = 0 ; 
 
while( x < 10 ) {
  // Things and code and stuff
  x++ ; 
}

The way you could consider a loop like that:

 
for( int x = 0 ; x < 10 ; x++ ) {
  // Things and code and stuff
}