Conway's Game of Life Update(Next Generation)

Question

I am working on Conway's game of life java code and I am having a struggle with my update method also known as the next generation creator. I will post my code I have written so far and please let me know what I can do to fix the update method.

A cell is born if there was none at time T 1 and exactly three of its neighbors were alive.

An existing cell remains alive if at time T 1 there were either two or three neighbors

A cell dies from isolation if at time T 1 there were fewer than two neighbors.

A cell dies from overcrowding if at time T 1 there were more than three neighbors.

public class GameOfLife {

    private char [][] grid;
    private int rows;
    private int columns;

    public GameOfLife(int rows, int columns) {
        grid=new char[rows][columns];
        for(int i=0;i<grid.length;i++)
        {
            for(int j=0;j<grid[i].length;j++)
                grid[i][j]=' ';
        }

    }

    public int numberOfRows() {
         int countRows=0;
          for(int i=0;i<grid.length;i++){
             countRows++;
             rows=countRows;
          }
          return rows;

    }

    public int numberOfColumns() {
        int countColumns=0;
          for(int i=0;i<1;i++){
             for(int j=0;j<grid[i].length;j++)
                countColumns++;
                columns=countColumns;
          }
          return columns;
    }

    public void growCellAt(int row, int col) {
        for(int i=0;i<grid.length;i++){
            for(int j=0;j<grid[i].length;j++) 
                   grid[row][col]='O';
        }
    }

    public boolean cellAt(int row, int col) {
        for(int i=0;i<grid.length;i++){
            for(int j=0;j<grid[i].length;j++)
                if(grid[row][col]=='O')
                    return true;
        }
        return false;
    }

    public String toString() {
        String result="";
        for(int i=0;i<rows;i++){
            for(int j=0;j<columns;j++)
                result+=grid[i][j];
        }
        return result;
    }

    public int neighborCount(int row, int col) {
        int count=0;
        int i=row;
        int j=col;
        int left;
        int right;
        int up;
        int down;
        if(i > 0)
            up = i-1;
        else
            up = grid.length-1;

        if(i < (grid.length-1))
            down = i+1;
        else
            down = 0;

        if(j > 0) 
            left = j-1;
        else
            left = grid[i].length - 1;

        if(j < (grid[i].length-1))
            right = j+1;
        else
            right = 0;

        if(grid[up][left] == 'O')
            count++;

        if(grid[up][j] == 'O')
            count++;

        if(grid[up][right] == 'O')
            count++;

        if(grid[i][left] == 'O')
            count++;

        if(grid[i][right] == 'O')
            count++;

        if(grid[down][left] == 'O')
            count++;

        if(grid[down][j] == 'O')
            count++;

        if(grid[down][right] == 'O')
            count++;

        return count;
    }

    public void update() {

        for(int i=0;i<grid.length;i++){
            for(int j=0;j<grid[i].length;j++){
                if(grid[i][j]==' ' && neighborCount(i,j)==3)
                    grid[i][j]='O';
                if(neighborCount(i,j)<2 || neighborCount(i,j)>3)
                    grid[i][j]= ' ';
                if(grid[i][j]=='O' && neighborCount(i,j)==2 || neighborCount(i,j)==3)
                    grid[i][j]='O';
            }
        }
    }
}

Ok regarding making a new array in the update method, is this all that needs to be done? Also, how would I go about making assertion tests for the update method?

public void update() {
    char[][] newGrid = new char[grid.length][grid[0].length];
    for(int i=0;i<grid.length;i++){
        for(int j=0;j<grid[i].length;j++){
            if(grid[i][j]==' ' && neighborCount(i,j)==3)
                newGrid[i][j]='O';
            if(neighborCount(i,j)<2 || neighborCount(i,j)>3)
                newGrid[i][j]= ' ';
            if(grid[i][j]=='O' && neighborCount(i,j)==2 || neighborCount(i,j)==3)
                newGrid[i][j]='O';
        }
    }
}

Answer 1

It looks like you are trying to modify the same grid you are looping through. As you loop through your grid, changes should be made based on the previous state of the grid. Try constructing a new grid instead of writing over the old one.