I am getting Exception in thread "main" java.lang.NullPointerException in my code when ever i try to store a new object in my array

Question

I am building an A* algorithm where i am suppose to add nodes to the grid[][] first ( in my case it is cells[i][j] ). So what i did is i run a loop in order to fill my 2D array with nodes with x and y coordinate filled inside it. But when ever i try to run my code it throws this exception java.lang.NullPointerException, i know this exception happens when object == null , but somehow i am not able to figure out the reason behind this problem, Here is my code :-

 private Node[][] cells;

 private void fill(){

    for(int i=0;i<rows;i++){
        for(int j=0 ; j<cols;j++){
            cells[i][j] = new Node(i,j,rows,cols); //throws exception this 
                                                     line
        }
    }
}

Please help me out to find the reason behind this.

Yes, you have `declared` but **not** specified how long the array will be. Try `Node[][] cells = new Node[row][cell];` — Scary Wombat, Dec 06 '18 at 05:24
@ScaryWombat yeah that was the problem , thank you so much :) — Jas world, Dec 06 '18 at 05:38
actually the problem is that the array(s) were not **initialized**, which is done by `new Node...` (the size must also be specified for that) — user85421, Dec 06 '18 at 05:52

score 1 · Accepted Answer · answered Dec 06 '18 at 05:27

1

it is because you are not initializaing cells array. Just inititalize it to proper size , then it should not be a problem. For example:

cells = new Node[5][5];

answered Dec 06 '18 at 05:27

codeLover

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`cells = new Node[rows][cols];` precisely – shriyog Dec 06 '18 at 05:29
oops, Thanks it is working now :) – Jas world Dec 06 '18 at 05:37

sushildlh · Answer 2 · 2018-12-06T05:49:15.527

Just try ArrayList<ArrayList<Node>> if you don't know the size of array.

private ArrayList<ArrayList<Node>> nodes = new ArrayList<>();

for(int i=0;i<rows;i++){
       ArrayList<Node> n  = new ArrayList<>();
        for(int j=0 ; j<cols;j++){
           Node node =  new Node(i,j,rows,cols); //throws exception this 
                                                               line
           n.add(node )

        }
      nodes.add(n);
    }

If you know the size then use Node[][] nodes = new Node[10][10].

I am getting Exception in thread "main" java.lang.NullPointerException in my code when ever i try to store a new object in my array

2 Answers2