如何使用java流对作为HashMap值的ArrayList的元素求和？

Question

I have a HashMap with integer and ArrayList of some Element objects. Element objects are described with price and amount. I want to go through all those elements in each ArrayList, sum them up by calling on each element element.price() , and create a new HashMap which contains old keys from and new values representing summ of each arraylist. Keys for new hash map should remain the same. Trying to do this using streams.

public static HashMap<Integer, Double> findIncomes(HashMap<Integer, ArrayList<Element>> mapa){


    Map<String, Double> m = mapa.entrySet().stream().flatMap()

    return m;
}

Answer 1

The first solution that came to my mind was to use mapToDouble and sum .

That would have looked like this:

public static HashMap<Integer, Double> findIncomes(HashMap<Integer, List<Element>> mapa) {
    HashMap<Integer, Double> sumsByKey = new HashMap<>();
    mapa.entrySet().stream().forEach(entry -> sumsByKey.put(entry.getKey(), entry.getValue().stream().mapToDouble(element -> element.getPrice()).sum()));
    return sumsByKey;
}

But when summing up 1.5d, 5.4d and 6.7d you get 13.600000000000001 as a result.

Therefore I had to remember: When performing calculations with doubles you usually better use BigDecimal .

So a more accurate solution could look like this:

public static HashMap<Integer, Double> findIncomes(HashMap<Integer, ArrayList<Element>> mapa){
    HashMap<Integer, Double> sumsByKey = new HashMap<>();
    mapa.entrySet().stream().forEach(entry -> sumsByKey.put(entry.getKey(),
        entry.getValue().stream().map(element -> BigDecimal.valueOf(element.getPrice())).reduce(BigDecimal.ZERO, BigDecimal::add).doubleValue()));
    return sumsByKey;
}

Since stream in stream is not really readable it might make sense to refactor it further.

Answer 2

You need a new map always. You cannot alter the same map with different types. Something like this would do the job,

public static HashMap<Integer, Double> findIncomes(HashMap<Integer, ArrayList<Element>> mapa) {

        final HashMap<Integer, Double> m = new HashMap<>();
        mapa.entrySet().stream().forEach(entry -> m.put(entry.getKey(), Double.valueOf(entry.getValue().stream().mapToDouble(Element::price).sum())));
        return m;
    }