Question

I am attempting to create Conway’s game of life in Java, and seem to have hit a brick wall.
I created a class called “Cell,” which holds a boolean variable that essentially determines whether the cell is alive or dead, and methods to kill or create a cell when needed. In my main method, I take the number of rows and columns the user wants in their game and am trying to create an array of Cell objects each named “location.” When I attempt to run the code and print the initial value of each cell, I get a null pointer exception error and am not sure how to fix it. As far a I know, each array should not have a null value, however I am very new to this…

//Create a Cell class for the purpose of creating Cell objects.

public class Cell

{


private boolean cellState;

//Cell Constructor. Initializes every cell's state to dead.
public Cell()
{
    cellState = false;
}

//This function kills a cell.
//Should be called using objectName[x][y].killCell.
public void killCell()
{
    cellState = false;
}

//This function creates a cell.
//Should be called using objectName[x][y]createCell.
public void createCell()
{
    cellState = true;
}

public void printCell()
{
    if (cellState == true)
            {  
               System.out.print("1");
            }
            else if
            {
               System.out.print("0");
            }

}


//End Class Cell//
}

This is my Cell class. If a cell is alive, a one will be printed in its place. If dead, a 0 will be in its place.

Here is my main method. The error is occurring at the line where I create an array of Cell objects. Am I doing this all wrong?

//GAME OF LIFE//


import java.util.Scanner;
import java.lang.*;
public class GameOfLife
{
    public static void main(String[] args)
    {
    Scanner scan = new Scanner(System.in);
    System.out.println("\t\tWelcome to the Game of Life!");
    System.out.println("\n\nDeveloped by Daniel Pikul");

    System.out.print("\n\nHow many rows would you like your game to have?");
    int numRows = scan.nextInt();

    System.out.print("How many columns would you like your game to have?");
    int numColumns = scan.nextInt();

    //Create an array of cell objects.
    Cell[][] location = new Cell[numColumns][numRows];

    //This for loop will print out the cell array to the screen.//
    for (int i = 0; i < numRows; i++)
    {
        for (int j = 0; j < numColumns; j++)
        {
            location[i][j].printCell();
        }
        System.out.print("\n");
    }

    //Prompt the user to enter the coordinates of cells that should live.
    System.out.println("Input coordinates tocreate active cells.");

    int xCo, yCo;
    char userChoice;

    //This do loop takes coordinates from the user. Every valid coordinate
    //creates a living cell in that location.
    do
    {
        System.out.print("Enter an x coordinate and a y coordinate: ");
        xCo = scan.nextInt();
        yCo = scan.nextInt();
        location[xCo][yCo].createCell();
        System.out.print("Enter another coordinate? (Y/N) ");
        String tempString = scan.next();
        userChoice = tempString.charAt(0);
    }while(userChoice == 'Y');

    //THIS IS AS FAR AS I HAVE GOTTEN IN THE PROGRAM THUS FAR//
}

}

Answer 1

//This for loop will print out the cell array to the screen.//
for (int i = 0; i < numRows; i++)
{
    for (int j = 0; j < numColumns; j++)
    {
        location[i][j].printCell();  // location[i][j] not instantiated
    }
    System.out.print("\n");
}

In the above for loop: – you have not instantiated your Cell object in your array: –

location[i][j].printCell();

Before this code you need to do: –

location[i][j] = new Cell();