My Favorite Code

Hello.java

  public Result execute(Input in) {
    checkNotNull(in, "In is null.");
    try {
      Integer productId = in.getSelected();
      checkProductId(productId);

      List<Integer> inputCoins = in.getCoins();
      checkInputCoins(inputCoins);

      Product product = dao.findById(productId);
      checkArgument(product != null, "Product doesn't exist.");

      return buyProduct(product, inputCoins);
    } catch (Exception e) {
      LOG.error(e);
      return resultOf(in.getCoins());
    }
  }

  void checkProductId(Integer aProductId) {
    checkNotNull(aProductId, "ProductId must not be null.");

    int productId = aProductId.intValue();
    checkArgument(1 <= productId && productId <= 10,
        "ProductId must be in the range 1-10.");
  }

  void checkInputCoins(List<Integer> aInputCoins) {
    checkArgument(CollectionUtils.isNotEmpty(aInputCoins),
        "InputCoins must not be null nor empty.");
    checkArgument(aInputCoins.size() <= 100,
        "InputCoins size must be up to 100.");
  }

  Result buyProduct(Product aProduct, List<Integer> aInputCoins) {
    int sum = sumUpValidCoins(aInputCoins);
    if (isShortOfMoney(sum, aProduct)) {
      return resultOf(aInputCoins);
    }

    List<Integer> invalidCoins = getInvalidCoins(aInputCoins);

    int change = sum - aProduct.getPrice();
    List<Integer> changeCoins = getChangeCoins(change);

    List<Integer> returnCoins = ListUtils.union(invalidCoins, changeCoins);
    return resultOf(aProduct, returnCoins);
  }

  static final ImmutableSet<Integer> VALID_COINS = ImmutableSet.of(10, 50, 100, 500);

  int sumUpValidCoins(List<Integer> aInputCoins) {
    return aInputCoins.stream()
        .filter(coin -> VALID_COINS.contains(coin))
        .mapToInt(i -> i)
        .sum();
  }

  boolean isShortOfMoney(int aSum, Product aProduct) {
    return aSum < aProduct.getPrice();
  }

  List<Integer> getInvalidCoins(List<Integer> aInputCoins) {
    return aInputCoins.stream()
        .filter(coin -> not(VALID_COINS.contains(coin)))
        .collect(Collectors.toList());
  }

  // お釣り計算のため、大きい順に並んでいる必要がある
  static final ImmutableList<Integer> COINS_FOR_CHANGE = ImmutableList.of(500, 100, 50, 10);

  List<Integer> getChangeCoins(int aChange) {
    List<Integer> changeCoins = new ArrayList<>();

    int rest = aChange;
    for (Integer coin : COINS_FOR_CHANGE) {
      while (rest > coin) {
        changeCoins.add(coin);
        rest -= coin;
      }
    }

    return changeCoins;
  }

  Result resultOf(Product aProduct, List<Integer> aReturnCoins) {
    Result result = new Result();
    result.setProduct(aProduct);
    result.setCoins(aReturnCoins);
    return result;
  }

  Result resultOf(List<Integer> aInputCoins) {
    Result result = new Result();
    result.setProduct(null);
    result.setCoins(aInputCoins);
    return result;
  }