http://bufferings.hatenablog.com/entry/2015/05/06/000943 で紹介されていた「手続き型で典型的」かつ、かなりアレなコードを自分なりにリファクタリングしてみました。

## Hello.java
  public Result execute(Input in) {
    try {
      if ( !IsSelectedInRange(in.getSelected(), 1, 10) ||
           !IsCoinCountInRange(in.getCoins(), 1, 100) ) {
        throw new IllegalArgumentException();
      }

      List<Integer> smallCoins = new ArrayList<>();
      int sumOfLargeCoins = 0;
      for (Integer coin : in.getCoins()) {
        if (coin == 1 || coin == 5) {
          smallCoins.add(coin);
        } else
        if (coin == 10 || coin == 50 || coin == 100 || coin == 500 ) {
          sumOfLargeCoins += coin;
        } else {
          // throw new IllegalArgumentException();
        }
      }

      Product product = dao.findById(in.getSelected());
      if (product == null) {
        return createResultWitoutProduct(in.getCoins());
      }

      if (product.getPrice() < sumOfLargeCoins) {
        Result result = new Result();
        result.setProduct(product);
        sumOfLargeCoins -= product.getPrice();

        List<Integer> coins3 = new ArrayList<>();
        coins3.add(500);
        coins3.add(100);
        coins3.add(50);
        coins3.add(10);
        for (Integer coin : coins3) {
          while (sumOfLargeCoins > coin) {
            smallCoins.add(coin);
            sumOfLargeCoins -= coin;
          }
        }
        result.setCoins(smallCoins);
        return result;
      }

      return createResultWitoutProduct(in.getCoins());

    } catch (Exception e) {
      LOG.error(e);
      return createResultWitoutProduct(in.getCoins());
    }

  }

  boolean IsSelectedInRange(Integer selected, int from, int to)
  {
    return selected != null && from <= selected  && selected <= to;
  }

  boolean IsCoinCountInRange(List<Integer> coins, int from, int to)
  {
    return coins != null && from <= coins.size() && coins.size() <= to;
  }

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