書いてみた。executeメソッドのみ。元ネタ: http://bufferings.hatenablog.com/entry/2015/06/03/082323

Sample.java

	public Result execute(Input in) throws IllegalArgumentException{
		assertNotNull(in, "In must not be null.");
		try{
			Integer productId = in.getSelected();
			assertNotNull(productId, "ProductId must not be null.");
			assertInRange(productId, 1, 10, "ProductId must be in the range 1-10.");
			Product product = dao.findById(productId);
			assertNotNull(product, "Product doesn't exist.");

			List<Integer> coins = in.getCoins();
			assertNotNull(coins, "InputCoins must not be null.");
			assertInRange(coins.size(), 1, 100, "The number of InputCoins must be in the range 1-100.");

			List<Integer> supportedCoins = new ArrayList<Integer>();
			List<Integer> unsupportedCoins = new ArrayList<Integer>();
			filterUnsupportedCoins(coins, supportedCoins, unsupportedCoins);

			int sum = sumUpCoins(supportedCoins);
			int rest = sum - product.getPrice();
			assertTrue(rest >= 0, "Not enough money.");

			List<Integer> restCoins = new ArrayList<Integer>();
			divideToCoins(rest, restCoins);
			restCoins.addAll(unsupportedCoins);

			return createSucceededResult(product, restCoins);
		} catch(IllegalArgumentException | DaoException e){
			LOG.error(e);
			return createCanceledResult(coins);
		}
	}

	// 最適化版
	public Result execute2(Input in) throws IllegalArgumentException{
		assertNotNull(in, "In must not be null.");
		try{
			Integer productId = in.getSelected();
			assertNotNull(productId, "ProductId must not be null.");
			assertInRange(productId, 1, 10, "ProductId must be in the range 1-10.");
			Product product = dao.findById(productId);
			assertNotNull(product, "Product doesn't exist.");

			List<Integer> coins = in.getCoins();
			assertNotNull(coins, "InputCoins must not be null.");
			assertInRange(coins.size(), 1, 100, "The number of InputCoins must be in the range 1-100.");

			List<Integer> changeCoins = new ArrayList<>();
			final LongAdder sum = new LongAdder();
			Collection<Integer> sumCalculator = new AbstractCollection<Integer>(){
				@Override
				public Iterator<Integer> iterator() {
					return null;
				}
				@Override
				public int size() {
					return 0;
				}
				@Override
				public boolean add(Integer e) {
					sum.add(e);
					return true;
				}
			};
			filterUnsupportedCoins(coins, sumCalculator, changeCoins);

			int rest = sum.intValue() - product.getPrice();
			assertTrue(rest >= 0, "Not enough money.");

			divideToCoins(rest, changeCoins);

			return createSucceededResult(product, changeCoins);
		} catch(IllegalArgumentException | DaoException e){
			LOG.error(e);
			return createCanceledResult(in.getCoins());
		}
	}
	
	private static void assertNotNull(Object value, String message)
	throws IllegalArgumentException{
		if(value == null){
			throw new IllegalArgumentException(message);
		}
	}

	private static void assertTrue(boolean value, String message)
	throws IllegalArgumentException{
		if(!value){
			throw new IllegalArgumentException(message);
		}
	}

	private static void assertInRange(int value, int min, int max, String message)
	throws IllegalArgumentException{
		if(value < min || max < value){
			throw new IllegalArgumentException(message);
		}
	}

	private static Set<Integer> unsupportedCoins = new HashSet<Integer>(){{
		add(1);
		add(5);
	}};
	private static void filterUnsupportedCoins(
			Collection<Integer> coins,
			Collection<Integer> supported,
			Collection<Integer> unsupported) {
		for(Integer c : coins){
			if(unsupportedCoins.contains(c)){
				unsupported.add(c);
			} else{
				supported.add(c);
			}
		}
	}

	private static int sumUpCoins(Collection<Integer> coins) {
		return coins.stream().mapToInt(Integer::intValue).sum();
	}

	private static int[] coinKinds = {500, 100, 50, 10};
	private static void divideToCoins(int money, Collection<Integer> coins) {
		for(int k : coinKinds){
			while(money >= k){
				coins.add(k);
			}
		}
		if(money > 0){
			throw new RuntimeException("Invalid coin combination: " + money);
		}
	}

	private static Result createSucceededResult(Product product, List<Integer> change){
		Result r = new Result();
		r.setProduct(product);
		r.setCoins(change);
		return r;
	}

	private static Result createCanceledResult(List<Integer> coins){
		Result r = new Result();
		r.setProduct(null);
		r.setCoins(coins);
		return r;
	}

方針的なもの

• 非チェック例外も明示する．
• メソッドの責務は他のメソッドに委譲しない．引数チェック内容はexecuteメソッド内に記述する．
• コレクション内に種類の異なる値が入ったままにしない．対応できるコインとできないコインは明示的に分ける．
• 戻り値の意味が複数ある場合(今回は成功と失敗(処理のキャンセル))，どちらを作成するのか明示的にわかるようにする．