World class for the Conway's Game Of Life tutorial on the SSaurel's Channel

World.java

package com.ssaurel.gameoflife;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;

public class World {
  
 public static final Random RANDOM = new Random();
 public int width,height;
 private Cell[][] board;
 
 public World(int width, int height) {
  this.width = width;
  this.height = height;
  board = new Cell[width][height];
  init();
 }
  
 private void init() {
  for(int i = 0; i < width; i++) {
    for (int j = 0; j < height; j++) {
     board[i][j] = new Cell(i, j, RANDOM.nextBoolean());
    }
  }
 }
  
 public Cell get(int i, int j) {
  return board[i][j];
 }
  
 public int nbNeighboursOf(int i, int j) {
  int nb = 0;
   
  for (int k = i -1; k <= i +1; k++) {
   for (int l = j - 1; l <= j +1; l++) {
    if ((k != i || l != j) && k >= 0
         && k < width && l >=0 && l < height) {
         Cell cell = board[k][l];
         if (cell.alive) {
          nb++;
         }
    }
   }
  }
  return nb;
 }
  
 public void nextGeneration() {
  List<Cell> liveCells = new ArrayList<Cell>();
  List<Cell> deadCells = new ArrayList<Cell>();
   
  for (int i = 0; i < width; i++) {
   for (int j = 0; j < height; j++) {
    Cell cell = board[i][j];
    int nbNeighbours = nbNeighboursOf(cell.x, cell.y);
     
    // rule 1 & rule 3
    if (cell.alive &&
        (nbNeighbours < 2 || nbNeighbours > 3)) {
         deadCells.add(cell);
    }
     
    // rule 2 & rule 4
    if ((cell.alive && (nbNeighbours == 3 || nbNeighbours == 2))
         ||
        (!cell.alive && nbNeighbours == 3)) {
         liveCells.add(cell);
    }
   }
  }
   
  // update future live and dead cells
  for (Cell cell : liveCells) {
   cell.reborn();
  }
  for (Cell cell : deadCells) {
   cell.die();
  }
 }
}