Grouping a List of Items using java 8 Streams and populating resulting Map with FIRST Item rather than List

user_4297749 0
Viewed 2450

I have an item defined by the following class:

public class Item {
    private int id;
    private float price;
}

I've been given a list of items and I need to group them by id. Once grouped, I know the List will only ever contain a single value for each id, so I want the resulting Map to be defined as follows:

Map<Integer, Item>

rather than:

Map<Integer, List<Item>>

The benefit of this, is that my calling code will not need to perform extra dereferencing:

mapName.get(id).get(0)

The following code will group the items by id and create a Map containing a list:

Map<Integer, List<Item>> itemsMap = itemList.stream()
        .collect(Collectors.groupingBy(Item::getId));

Is there a way in Java 8 to achieve my desired result?

java-stream java-8 java
3 Answers

You can use: Map<Integer, Item> itemsMap = items.stream().collect(Collectors.toMap(Item::getId, Function.<Item>identity()));

user_62516650 
itemList.stream()
    .collect(Collectors.toMap(item -> item.id, item -> item));
user_685870

One way to do around such conversions using IDE's such as intelliJ is:

List<Item> itemList = ...
Map<Integer, Item> itemMap = new HashMap<>();
for (Item item : itemList) {
    itemMap.put(item.getId(), item);
}

and then convert/replace the statement as:

Map<Integer, Item> itemMap =
            itemList.stream().collect(Collectors.toMap(Item::getId, item -> item, (a, b) -> b));

which is same as John's answer.

user_17461180
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