Java stream operations

Question

I have an object, "Item" with fields: int: id String: prices

The string prices contains one or more price values separated by comma.

Problem: Using stream operations I want a map of type Map<Integer,Set<Integer>> from List<Item> items

where key of Map is id and Set has value of prices as Integer extracted from the String prices.

There will be repeated ids in the List and can have different price strings.

I have come up with an approach which produces the following result:

Map<Integer, Set<List<Integer>>> itemStoresMapIntermediate = items.stream()
            .collect(Collectors.groupingBy(Item::getItemId,Collectors.mapping(Item::getStoresAsIntList, Collectors.toSet())));

getStoresAsIntList() returns List of price values from String prices in the object.

The above is certainly not what I want.

getStoresAsIntList returns List<Integer> of price values in String prices — rohit
– rohit, Commented Mar 7, 2017 at 11:38
If the IDs are unique, you can use Collectors.toMap(Item::getItemId, i->new HashSet<>( i.getStoresAsIntList())). Otherwise, you need to do either, flatMap to some kind of key-price pair before groupingBy or use a flatMapping collector like at the end of this answer. — Holger
– Holger, Commented Mar 7, 2017 at 11:48

Eugene · Accepted Answer · 2017-03-07 13:38:25Z

5

If I understood correctly...

 Item item1 = new Item(1, "22,23,24");
 Item item2 = new Item(2, "33,34,35");
 Item item3 = new Item(1, "22,57,58");

 Map<Integer, Set<Integer>> map = Stream.of(item1, item2, item3)
            .collect(Collectors.toMap(
                    Item::getId,
                    i -> new HashSet<>(i.getStoresAsIntList()),
                    (left, right) -> {
                        left.addAll(right);
                        return left;
                    }, HashMap::new));

 System.out.println(map); // {1=[22, 23, 24, 57, 58], 2=[33, 34, 35]}

edited Mar 7, 2017 at 13:38

answered Mar 7, 2017 at 12:12

Eugene

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