java stream pipeline to create map of lists

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())));
}

score 1 · Answer 1 · answered Nov 08 '21 at 11:50

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))
        ));
}

java stream pipeline to create map of lists

1 Answers1