Unable rectify NullPointerException

[John Eipe]

Hello,

The below program is to develop an accessibility heuristic for Knight's Tour problem. (Let me know if I need to explain KT problem )

 
public class KnightsCleverTour {
    
    public static void main(String[] args){
        
        ChessBoard[][] cells = new ChessBoard[8][8];
        
        for(int m=0; m<8; m++){
            for(int l=0; l<8; l++){
                cells[m][l].knight=true;
                cells[m][l].access = cells[m][l].getaccessValue(m,l);
            }
        }
        
        for(int m=0; m<8; m++){
            for(int l=0; l<8; l++){
                System.out.print(cells[m][l].access);
            }
            System.out.println();
        }
        
        
    }
    
}

class ChessBoard {
    Boolean knight;
    int access;
    
    public int getaccessValue(int r, int c){
        this.checkPosition(r-1,c+2);
        this.checkPosition(r+1,c+2);
        this.checkPosition(r-1,c-2);
        this.checkPosition(r+1,c-2);
        this.checkPosition(r-2,c-1);
        this.checkPosition(r-2,c+1);
        this.checkPosition(r+2,c-1);
        this.checkPosition(r+2,c+1);
        
        return this.access;
    }
    public void checkPosition(int rp, int cp){
        if((rp>=0&&rp<8)&&(cp>=0&&cp<8)){
            this.access=this.access+1;
        }
    }
}

This is throwing java.lang.NullPointerException
at KinghtsTour.KnightsCleverTour.main(KnightsCleverTour.java:22)

Please help.

[Henry Wong]

Well, this stacktrace...

This is throwing java.lang.NullPointerException
at KinghtsTour.KnightsCleverTour.main(KnightsCleverTour.java:22)

Gives you the exact file, method, and line, where the exception occurred. It would be a good idea to start there.

However... just looking at your application for a few seconds... here is a hint. Where did you instantiate any chessboard objects?

Henry

[John Eipe]

ChessBoard object is instantiated at line 6.

Note:
Line 22 is actually

cells[m][l].knight=true;

[N. Priore]

john eipe wrote:ChessBoard object is instantiated at line 6.

Note:
Line 22 is actually

cells[m][l].knight=true;

On line 6 you are instantiating an array.

[Rob Spoor]

When you create a new array this way, all of its elements are null (or 0 for numbers, '\0' for chars, false for booleans). You'll need to initialize each element separately:

for (int i = 0; i < cells.length; i++)
{
    for (int j = 0; j < cells[i].length; j++)
    {
        cells[i][j] = new ChessBoard();
    }
}

[John Eipe]

Yes, you are right! I should have remembered that about arrays of objects.

Thanks,
John Eipe