1. boolean cells[][];
2. boolean newCells[][];
3. int rows, cols;
4. void setup() {
5.   size(400, 400);
6.   rows = height / 10;
7.   cols = width / 10;
8.   cells = new boolean[cols][rows];
9.   newCells = new boolean[cols][rows];
10.   for (int i = 0; i < cols; i++) {
11.     for (int j = 0; j < rows; j++) {
12.       if (int(random(0, 3)) == 1)
13.         cells[i][j] = true;
14.       else
15.         cells[i][j] = false;
16.     }
17.   }
18.   frameRate(30);
19. }
20. void draw() {
21.   background(0);
22.   for (int i = 0; i < cols; i++) { //loop through x of array
23.     for (int j = 0; j < rows; j++) { //loop through y of array
24.       if (cells[i][j])
25.         rect(i * 10, j * 10, 10, 10); //draw a cell if it's active
26.       int adjacent = 0; //the number of adjacent cells
27.       for (int k = i - 1; k <= i + 1; k++) { //Loop through adjacent cells on the x axis
28.         if (k < cols && k >= 0) { //no arrayIndexOutOfBoundsExceptions
29.           for (int l = j - 1; l <= j + 1; l++) { //Loop through adjacent cells on the y axis
30.             if (l < rows && l >= 0) {
31.               if (cells[k][l])
32.                 adjacent++; //that's one more adjacent cell! Yay!
33.             }
34.           }
35.         }
36.       }
37.       //now for rule calculation...
38.       if (cells[i][j]) {
39.         if (adjacent < 2 || adjacent > 3) //lonely or overcrowded?
40.           newCells[i][j] = false;
41.         else
42.           newCells[i][j] = true; //two or three friends is good :D
43.       } else { //dead :(
44.         if (adjacent == 3)
45.           newCells[i][j] = true; //reincarnation! :D
46.         else
47.           newCells[i][j] = false; //still dead. :(
48.       }
49.     }
50.   }
51.   arrayCopy(newCells, cells);
52. }

Thanks in advance.