Preventing exceptions

[ieoj31]

Hi,

I'm having some trouble preventing various exceptions in an implementation of Conway's Game of Life. Here's the code.

import java.util.*;

public class Life {
    private final boolean[][] cells;
    
    public static void main( String[] args ) {
        Life generation = new Life( 70 );
        for (int i = 0; i != 100; i++) {
            System.out.println( generation );
            generation.next();
        }
    }
    
    // Constructors
    // Initialise size * size grid with random cells.
    public Life( int size ) {
        final Random rand = new Random( );
        cells = new boolean[ size ][ ];
        for (int row = 0; row < size; row ++) {
            cells[ row ] = new boolean[ size ];
            for (int col = 0; col < size; col ++) {
                cells[ row ][ col ] = (rand.nextInt( 2 ) == 0);
            }
        }
    }
       
    // Public methods and helper methods.
    @Override
    public String toString( ) {
    String result = "";
        for (int row = 0; row < cells.length; row ++) {
            final boolean[] column = cells[ row ];
            for (int col = 0; col < column.length; col ++) {
                result = result + (column[ col ] ? "x" : ".");
            }
            result = result + "\n";
        }
    return result;
    }
    
    public void next() {
    //apply the rules to each cell
    //output the new grid
    //counter to count number of live cells  
        boolean[][] newGrid;
        newGrid  = new boolean[cells.length][];
        int counter = 0;  
        for (int row = 0; row < cells.length; row++) {  
            cells[ row ] = new boolean[ cells.length ];            
            for (int col = 0; col < cells[row].length; col++) {               
                
                if (cells[row + 1][col - 1] == true) {
                       counter++;      
                   }
                    
                if (cells[row - 1][col - 1] == true) {
                    counter++;      
                }
                    
                if (cells[row - 1][col] == true) {
                    counter++;      
                }
                    
                if (cells[row - 1][col + 1] == true) {
                    counter++;      
                }
                    
                if (cells[row][col - 1] == true) {
                    counter++;      
                }  
                
                if (cells[row][col] == true) {
                    counter++;      
                }
                    
                if (cells[row][col + 1] == true) {                              
                   counter++;      
                }
                
                if (cells[row + 1][col] == true) {
                    counter++;      
                }
                    
                if (cells[row + 1][col + 1] == true) {
                    counter++;      
                }

                    
                if (counter > 2 && counter < 3) {
                    newGrid[row][col] = true;      
                } else {
                    newGrid[row][col] = false;  
                }
                
                cells[row][col] = newGrid[row][col];
            }
        }
    }      
}

The exception that I'm getting with this is an arrayindexoutofboundsexception: -1 at line 52

I know this is related to the various [row - 1] and [col - 1] references I have and how they will sometimes be null, but how do I resolve this?

Thanks in advance!

[JosAH]

The exception that I'm getting with this is an arrayindexoutofboundsexception: -1 at line 52

I know this is related to the various [row - 1] and [col - 1] references I have and how they will sometimes be null, but how do I resolve this?

You have to check the values for row and col, e.g. if row == 0 you can't refer to cell[row-1]. That complicates all the conditions in the various if statements somewhat.

kind regards,

Jos

[ieoj31]

I think I've sorted that out, but i'm still getting a null pointer exception at line 92 (newGrid[row][col] = false;)

Any idea where this is coming from?

[JosAH]

I think I've sorted that out, but i'm still getting a null pointer exception at line 92 (newGrid[row][col] = false;)

Any idea where this is coming from?

Yep, newGrid is a two dimensional array, i.e. newGrid[i] is the i-th row and it contains all the column values of that row (it's a one dimensional array). You only allocated the row references but none of the columns of your two dimensional array (see above your loop). All the one dimensional arrays representing the columns are null. Hence the exception.

kind regards,

Jos