Java - NullPointerException after initializing array and when testing array with class method

Question

I've been trying to initialize an array and then test its values with a class method. I have initialized the array and have already tested it successfully within the constructor, so it looks like the array was populated, but as soon as I try to test this in the Main method it's throwing a NullPointer exception.

class PercolGrid {
    int sides;
    boolean[] grid;

    public PercolGrid (int inSides) {
        sides = inSides;
        //grid = new boolean[] {false,false,false,false,false,
        //false,false,false,false,false};
        boolean[] grid = new boolean[sides];
        for (int i = 0; i < sides; i++) {
            grid[i] = false;
            System.out.println("Setting " + i + " to " + grid[i]);
        }
        System.out.println("Checking outside FOR loop, first square is: " + grid[0]);
    }


    public boolean testSqr (int i) {
        System.out.println("Requested index is: " + i);
        return grid[i];
    }

    public static void main(String[] args){
        PercolGrid firstGrid = new PercolGrid(10);
        System.out.println("Grid created! Checking index ....");
        System.out.println("First square is :" + firstGrid.testSqr(0)); // NullPointerException
        System.out.println("First square is :" + firstGrid.grid[0]); // and here
    }
}

It's almost like the referenced data exists within the constructor but then doesn't exist outside of it. When I comment out the for loop and the boolean[] .... line above it, and uncomment my grid = new boolean[] .... line, it all works ok, but I want to choose the number of sides when I instantiate the object.

EDIT - This same error occurs if I comment out line 19 (firstGrid.testSqr(0)) and instead run line 20 (firstGrid.grid[0]).

This is a practice using a 1D array before I try the same thing with a 2D array. What am I missing?

My output looks like this:

Setting 0 to false
...
Setting 9 to false
Checking outside FOR loop, first square is: false
Grid created! Checking index ....
Requested index is: 0
java.lang.NullPointerException
    at PercolGrid.testSqr(PercolGrid.java:19)
    at PercolGrid.main(PercolGrid.java:25)

Answer 1

Your problem is with this line:

boolean[] grid = new boolean[sides];

This is initializing a local variable grid, not the field in the instance.

Change it to:

grid = new boolean[sides];

This initializes the field in the instance.

By putting the type in front you are declaring a new variable. When you declar a variable in a method its scope is limited to that method. Since your local variable is named the same as your instance variable it "hides" the instance variable.

Answer 2

boolean[] grid = new boolean[sides];

Here you create a new boolean array grid which hides the class field.

Just

grid = new boolean[sides];

and you will refer to grid class field.