Exception in thread "main" java.lang.NullPointerException? What does this mean? - arrays

Question

I have an error when trying to run this program. This is not the entire program just the few details I think where the problem is. The error points out
if(board[randSpace].isEmpty()) and
cl.makeChutes(5);"
Error is :
"Exception in thread "main" java.lang.NullPointerException
at ChutesAndLadders.makeChutes(ChutesAndLadders.java:29)
at ChutesAndLadders.main(ChutesAndLadders.java:67)"
This is my program:
import java.util.Random;
public class ChutesAndLadders{
Cell [] board = new Cell [100];
Random rand = new Random ();
public int chutes, ladders;
public void makeChutes (int a ){
int makeChutes = a;
for (int i = 0; i < a; i++){
int randSpace = rand.nextInt(99);
if(board[randSpace].isEmpty())
board[randSpace] = new Cell (-10,"C");
else
i--;
}
}
public static void main(String[] args) {
// Create an instance of ChutesAndLadders
ChutesAndLadders cl = new ChutesAndLadders(10,10);
// Randomly place 5 more chutes and 5 more ladders
cl.makeChutes(5);
cl.makeLadders(5);
Here is my isEmpty:
public boolean isEmpty(){
for(int i = 0; i < board.length; i++)
{
if (board[i].isEmpty())
return true;
}
return false;
}
I could be entirely wrong on my coding. I'm still learning so please be patient. Thank you!

Answer 1

Following on what Dewfy said, board is an array of Cell objects. That means it is a placeholder for Cell objects --a bunch of nulls waiting to be replaced by a Cell.
If say, the object at position 1 (board[1]) has not been assigned a Cell object (i.e. you haven't issued a command like board[1]=new Cell() or something like that) then board[1] is null. Therefore if you were to ask board[1].isEmpty() you would get a NullPointerException because you are trying to call a method on a null object.
Now, your error is likely to come from board[randSpace] being null. So, the question is: does it contain a Cell object? Have you initialized your array with Cell objects?
One way to check this would be to say:
if (board[randSpace]==null || board[randSpace].isEmpty()) .....

Answer 2

Problem appears with this line:
if(board[randSpace].isEmpty())
so it is supposed that obect at randSpace already exists, if not you create object (Cell). But object is not yet created, how you can ask it if null isEmpty(). Just check against null first