public class ATile
{
    public double g; // from start to this tile
    public double h; // heuristic to finish
    public double f = g + h;
    public ATile(){}
}

//to w innej klasie
private ATile aTiles[][];
aTiles = new ATile[dimension][dimension];

for (int i = 0; i < dimension; i++)
{
    for (int j = 0; j < dimension; j++)
    {
        aTiles[i][j].g = (double)Math.round(countDistance(3, 3, i, j)*100)/100; // NullPointerException tutaj
    }
}