NullPointerException, need help

Question

I have a problem when I run my program. The error message is:
Java.lang.NullPointerException at LangtonsAnt.LangtonsAnt.main(LangtonsAnt.java:11)
As far as I know means a NullPointerException that the programs tries to access an opject which is not there. Unfortunatly I have not the faintest idea what I'm doing wrong...
package LangtonsAnt;
public class LangtonsAnt{   
public static void main(String[] args){ 
       Grid newGrid = new Grid();
       newGrid.initGrid();
       System.out.println(newGrid.grid[5][5].white); 
package LangtonsAnt;
public class Grid {
    private static final int sizeOfGrid = 100; 
    static Tiles [][] grid;
    public void initGrid(){   
        Tiles[][] grid = new Tiles[sizeOfGrid][sizeOfGrid];
        for(int loop1 = 0; loop1 < sizeOfGrid; loop1++){
            for(int loop2 = 0; loop2 < sizeOfGrid; loop2++){ 
               grid[loop1][loop2] = new Tiles();
               grid[loop1][loop2].white = true;
}

Answer 1

These are the easiest errors to find. The JVM has told you the exact source file and line number at which it occurs. Turn on line number display in your editor, go to line 11, and examine every reference you see. Put in System.out.println() statements to print out the references and see which one is null.
In this case it's not too difficult to see. You sure did write this code in an odd way. You call a Grid default constructor and then an initGrid method right after that which takes no arguments. Isn't initializing a Grid the constructor's job? I could see why you'd have a separate init method if you passed arguments that could change the Grid, but you don't.
You declare a static Tiles 2D array (another mystery), and then in the initGrid you declare a second, local Tiles 2D array of the same name. The local variable is initialized while the static instance of the same name remains null. When you leave the initGrid method the local variable is garbage collected and the static var remains null.
I think you meant it to look like this:
package LangtonsAnt;
public class LangtonsAnt
    public static void main(String[] args)
       Grid newGrid = new Grid();
       newGrid.initGrid();
       System.out.println(newGrid.grid[5][5].white);
class Grid
    private static final int sizeOfGrid = 100;
    static Tiles [][] grid;
    public void initGrid(){
        Grid.grid = new Tiles[sizeOfGrid][sizeOfGrid];
        for(int loop1 = 0; loop1 < sizeOfGrid; loop1++){
            for(int loop2 = 0; loop2 < sizeOfGrid; loop2++){
               grid[loop1][loop2] = new Tiles();
               grid[loop1][loop2].white = true;
}I'd recommend that you write it more like this:
package langstonsant; // follow the Sun coding standards.
public class Grid
    public static final int DEFAULT_GRID_SIZE = 100;
    private Tiles [][] grid;  // NOTE: not static.
    public Grid() { this(DEFAULT_GRID_SIZE); }
    public Grid(int sizeOfGrid)
        if (sizeOfGrid <= 0)
            throw new IllegalArgumentException("Grid size must be greater than zero");
        this.grid = new Tiles[sizeOfGrid][sizeOfGrid];
        for(int loop1 = 0; loop1 < sizeOfGrid; loop1++)
            for(int loop2 = 0; loop2 < sizeOfGrid; loop2++)
               this.grid[loop1][loop2] = new Tiles();
               this.grid[loop1][loop2].white = true;