Outside-in driven by constraints "first class collections members" and "no getters"

No primitives outside-in Game of Life

Board.java

package gameOfLife;

import java.util.Set;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.Consumer;

class Board {
	private Set<Coordinate> livingCells;
	public Board(Set<Coordinate> livingCells) {
		this.livingCells = livingCells;
	}
	
	public void forEachLivingCell(Consumer<Coordinate> action) {
		livingCells.forEach(action);
	}
	public int countNeighbours(Coordinate coordinate) {
		AtomicInteger neighbourCounter = new AtomicInteger(0);
		coordinate.forEachNeighbour(neighbour -> {
			if (hasLivingCellAt(neighbour))
				neighbourCounter.incrementAndGet();
		});
		return neighbourCounter.get();
	}
	public boolean hasLivingCellAt(Coordinate coordinate) {
		return livingCells.contains(coordinate);
	}
}

Coordinate.java

package gameOfLife;

import java.util.function.Consumer;

public class Coordinate {

	private final int x;
	
	private final int y;

	public Coordinate(int x, int y) {
		this.x = x;
		this.y = y;
	}


	public void forEachNeighbour(Consumer<Coordinate> action) {
		action.accept(new Coordinate(x-1, y-1));
		action.accept(new Coordinate(x-1, y));
		action.accept(new Coordinate(x-1, y+1));
		action.accept(new Coordinate(x, y-1));
		action.accept(new Coordinate(x, y+1));
		action.accept(new Coordinate(x+1, y-1));
		action.accept(new Coordinate(x+1, y));
		action.accept(new Coordinate(x+1, y+1));
	}
	
	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + x;
		result = prime * result + y;
		return result;
	}
	
	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Coordinate other = (Coordinate) obj;
		if (x != other.x)
			return false;
		if (y != other.y)
			return false;
		return true;
	}
}

Game.java

package gameOfLife;

import java.util.Arrays;
import java.util.HashSet;

public class Game {

	private Board board;

	public Game(Coordinate... livingCells) {
		this.board = new Board(new HashSet<>(Arrays.asList(livingCells)));
	}

	public boolean hasLivingCellAt(Coordinate coordinate) {
		return board.hasLivingCellAt(coordinate);
	}

	public void nextIteration() {
		HashSet<Coordinate> livingCells = new HashSet<>();
		board.forEachLivingCell(coordinate -> {
			if(board.countNeighbours(coordinate) == 2)
				livingCells.add(coordinate);
			coordinate.forEachNeighbour(neighbour -> {
				if(board.countNeighbours(neighbour) == 3)
					livingCells.add(neighbour);});
		});
		board = new Board(livingCells);
	}
}

GameTest.java

package gameOfLife;
import static org.assertj.core.api.Assertions.assertThat;
import static org.junit.Assert.*;

import org.junit.Test;

import gameOfLife.Coordinate;
import gameOfLife.Game;

public class GameTest {
	@Test
	public void checksForDeadCell() throws Exception {
		Game game = new Game();
		assertThat(game.hasLivingCellAt(new Coordinate(0, 0))).isFalse();
	}

	@Test
	public void checksForLivingCell() throws Exception {
		Game game = new Game(new Coordinate(0, 0));
		assertThat(game.hasLivingCellAt(new Coordinate(0, 0))).isTrue();
	}

	@Test
	public void singleLivingCellDies() throws Exception {
		Game game = new Game(new Coordinate(0, 0));
		game.nextIteration();
		assertThat(game.hasLivingCellAt(new Coordinate(0, 0))).isFalse();
	}

	@Test
	public void livingCellAtX0Y0WithTwoNeighboursStaysAlive() throws Exception {
		Game game = new Game(new Coordinate(0, 0), new Coordinate(1, 0), new Coordinate(0, 1));
		game.nextIteration();
		assertThat(game.hasLivingCellAt(new Coordinate(0, 0))).isTrue();
	}


	@Test
	public void livingCellAtX1Y1WithTwoNeighboursStaysAlive() throws Exception {
		Game game = new Game(new Coordinate(1, 1), new Coordinate(1, 0), new Coordinate(0, 1));
		game.nextIteration();
		assertThat(game.hasLivingCellAt(new Coordinate(1, 1))).isTrue();
	}

	@Test
	public void deadCellAtX1Y1WithThreeNeighboursBecomesAlive() throws Exception {
		Game game = new Game(new Coordinate(0, 0), new Coordinate(1, 0), new Coordinate(0, 1));
		game.nextIteration();
		assertThat(game.hasLivingCellAt(new Coordinate(1, 1))).isTrue();
	}

}