java stream pipeline to create map of lists - stream

Question

Its not real code, but an analogy to my exact scenario. Main concern here is the util stream pipeline.
Lets assume, I've following object model
#Data
class ItemRequest {
String action;
List<ItemId> itemIds;
}
#Data
class ItemId{
ItemType itemType;
long itemKey;
}
#Getter
#AllArgsConstructor
enum ItemType{
String backingService;
String description;
ITEM_TYPE_1("backingService1","type 1 description"),
ITEM_TYPE_2("backingService1","type 2 description"),
ITEM_TYPE_3("backingService2","type 3 description"),
ITEM_TYPE_4("backingService2","type 4 description"),
ITEM_TYPE_5("backingService2","type 5 description"),
ITEM_TYPE_6("backingService3","type 6 description"),
ITEM_TYPE_7("backingService3","type 7 description"),
// so on
}
Now each ItemType has a different backend microservice.
My ItemType enum has getters to return backing service.
So I want to break down my ItemRequest by backing service.
I can easily do it in imperative way or running 2 stream pipeline.
But I want to use it in one stream pipeline
For Example, in simple terms, my question is
How to combine following 2 steps I wrote below into one pipeline
.
Map<String,ItemRequest> breakItemRequestAsPerBackingService
(ItemRequest originalItemRequest ){
Map<String, List<ItemId>> collect
= originalItemRequest
.getItemIds()
.stream()
.collect(Collectors.groupingBy(
e -> e.getItemType().getBackingService()));
return collect
.entrySet()
.stream()
.collect(toMap(
Map.Entry::getKey,
e -> new ItemRequest(
originalItemRequest.getAction(),
e.getValue())));
}

Answer 1

Your second operation
collect.entrySet().stream()
.collect(toMap(
Map.Entry::getKey,
e -> new ItemRequest(originalItemRequest.getAction(), e.getValue())));
is keeping the result key of the previous operation and only applying a function to the values. You can apply a function to the result of a previous Collector using collectingAndThen. To use it with groupingBy for the map values, you have to realize that groupingBy(f) is a short-hand for groupingBy(f, toList()), so toList() is the collector to combine with collectingAndThen.
Map<String,ItemRequest>
breakItemRequestAsPerBackingService(ItemRequest originalItemRequest) {
return originalItemRequest.getItemIds().stream()
.collect(Collectors.groupingBy(e -> e.getItemType().getBackingService(),
Collectors.collectingAndThen(Collectors.toList(),
list -> new ItemRequest(originalItemRequest.getAction(), list))
));
}