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?
itemList.stream()
.collect(Collectors.toMap(item -> item.id, item -> item));
您可以使用: Map<Integer, Item> itemsMap = items.stream().collect(Collectors.toMap(Item::getId, Function.<Item>identity()));
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 .
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