Java Multi-Dimentional Arrays. Null Pointer

Question

Sorry if I'm not explaining good for it's 12:34 am and i'm doing late night programming but I need help. Btw this is in LWJGL Here's my Code:

I keep getting a null pointer error for the addAt() and the draw(); Basicly there is a couple classes that make it so when I click it will run addAt(mousex,mousey); and in the render loop it will keep drawing. The class that is new Block(x,y) is a class that will draw the QUAD.

//beggining
public class Grid {

    Block[][] blocks = new Block[25][25];
    public Grid(){
        for (int x = 0; x < 25 - 1; x++) {
            for (int y = 0; y < 16 - 1; y++) {
                blocks[x][y] = new Block(x,y);
            }
        }
    }
    public void draw(){

        for (int x = 0; x < 25;x++){
            for (int y = 0; y < 25;y++){
                blocks[x][y].draw();
            }   
        }

    }
    public void addAt(int x,int y){

        blocks[x][y] = new Block(x,y);

    }
}
//end

basicly the Main is just making a Display and running the draw loop and the input listener.

Then the Block class is just making a quad at the defined x and y.

Sorry if I disobeyed a stack overflow rule. This is my First post and it's at a late time.:) Thanks in advance!

Can you print the parameters passed to addAt() call. It will help in debugging further. — Vivek Viswanathan
– Vivek Viswanathan, Commented Apr 20, 2012 at 4:54
Are you sure you're getting an exception on the addAt call? It shouldn't make any problems unless blocks is null, and it can't be. The problem probably occurs on draw. — user684934
– user684934, Commented Apr 20, 2012 at 4:58

krishnakumarp · Accepted Answer · 2012-04-20 04:55:15Z

2

While adding to block array your looping is from 0 to (25-1) and (16-1). While processing the block the looping is from 0 to 25. That would most probably lead to NPE. Try initializing you blocks from 0 to 25 (for both x and y values).

answered Apr 20, 2012 at 4:55

krishnakumarp

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1 Comment

user684934 Over a year ago

More specifically, both the loops should read like for(int x=0; x<25; x++)

Dieter Rehbein · Accepted Answer · 2012-04-20 05:00:42Z

0

I tried your code and it works fine (I don't get any exception). Here is my code:

public class Test {

    private class Block {
        int x, y;

        private Block(int x, int y) {
            this.x = x;
            this.y = y;
        }


        void draw() {

        }
    }


    Block[][] blocks = new Block[25][25];

    public Test(){
        for (int x = 0; x < 25 - 1; x++) {
            for (int y = 0; y < 16 - 1; y++) {
                blocks[x][y] = new Block(x,y);
            }
        }
    }
    public void draw(){

        for (int x = 0; x < 25;x++){
            for (int y = 0; y < 25;y++){
                blocks[x][y].draw();
            }
        }

    }
    public void addAt(int x,int y){
        blocks[x][y] = new Block(x,y);
    }



    public static void main(String[] args) {
        Test t = new Test();
        t.addAt(4,5);
    }
}

=> The problem seems to be the late night programming ;-)

answered Apr 20, 2012 at 5:00

Dieter Rehbein

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1 Comment

JavaMan111 Over a year ago

well the problem is in the game loop I run grid.draw so it updates properly. That is what's causing the null pointer and I have no clue how to fix that.

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