[SOLVED] Using a stream instead of a for loop to create a Map

Using a stream instead of a for loop to create a Map

I am working on a Java project where I have to iterate a list and put values in a new map based on computations in that list.

Here's whats working using a basic for loop:

   Map<Long, PickingDetails> pickingDetailsDTOMap = new HashMap<>();
   for(int i=0;i<pickingJobModels.size();i++)
    {
pickingDetailsDTOMap.put(pickingJobModels.get(i).getPickListDTO().getReferenceId(),getPickingDetails(pickingJobModels.get(i)));
     }

But I want to do it with streams so I did this:

pickingJobModels.stream().map(pickingJobModel -> pickingDetailsDTOMap.put(pickingJobModel.getPickListDTO().getReferenceId(),getPickingDetails(pickingJobModel)));

but when I return that pickingDetailsDTOMap I get empty map.

Any suggestions how to use a stream in this case?

Solution

That's not the correct way. map is an intermediate operation (i.e. it is evaluated lazily) and you have no terminal operation, so it's never evaluated. You should collect the data into a Map:

Map<Long, PickingDetails> pickingDetailsDTOMap = 
    pickingJobModels.stream()
                    .collect(Collectors.toMap(pm -> pm.getPickListDTO().getReferenceId(),
                                              pm -> getPickingDetails(pm)));