[FIXED] Java Stream from List<Object[]> to a List<MyDTO>

Issue

I have a List<Object[]> where the Object[] has the size of 3 and comes from a query. The result of the query is as follows:

| vehicleId                          | modelId | serviceId       |
|------------------------------------|---------|-----------------|
|93bf3e92-7a37-4e23-836d-eed5a104341f|      214|80a7-ce5640b18879|    
|b4066520-e127-44b7-bcc0-1d1187de559c|      214|80a7-ce5640b18879|
|ae6cb0fe-1501-4311-a2b4-cfb8b4f51ca4|      214|80a7-ce5640b18879|
|cf80ff11-6e23-4c19-8b6d-55d34d131566|      214|80a7-ce5640b18879|

It should be mapped in the List below. The second and last columns will be mapped to modelId and serviceId whilst the first column should become a list of vehicleIds.

I need to map it into a List<MyDTO> where MyDTO is as follows:

MyDTO{

// constructor    
MyDTO(String modelId, String serviceId, List<String> vehicleIds){...}

String modelId;
String serviceId;
List<String> vehicleIds;
}

I am trying to figure out how to group by in a stream but nothing seems to come out. That's where I'm blocked...

listOfObjectArrays.stream()
            .map(objects -> new MyDTO((String) objects[0], (String) objects[1], null));

Can't figure out how to apply a reduce operation that does the job, any help really appreciated!

Edit: Sorry I forgot to mention that I'm stuck with Java 8. Thank you all for the great answers.

Solution

You can create a nested intermediate map by grouping your data by modelId and then serviceId using groupingBy() and mapping() collectors.

And then create a stream over entry set. And flatten each inner map creating new MyDTO based on every combination of modelId and serviceId.

Map<String, Map<String, List<String>>> vehicleIdByModelIdAndServiceId =
    listOfObjectArrays.stream()
        .collect(Collectors.groupingBy(objects -> (String) objects[1],
            Collectors.groupingBy(objects -> (String) objects[2],
                Collectors.mapping(objects -> (String) objects[0],
                    Collectors.toList()))));
        
List<MyDTO> result = vehicleIdByModelIdAndServiceId.entrySet().stream()
        .flatMap(baseEntry -> baseEntry.getValue().entrySet().stream()
                    .map(entry -> new MyDTO(baseEntry.getKey(), entry.getKey(), entry.getValue())))
        .collect(Collectors.toList());

Another option is to use a Map.Entry as a key in the intermediate map, and a value will be a list of vehicleId.

List<MyDTO> result = listOfObjectArrays.stream()
        .collect(Collectors.groupingBy(objects -> new AbstractMap.SimpleEntry<>((String) objects[1], (String) objects[2]),
                Collectors.mapping(objects -> (String) objects[0],
                    Collectors.toList())))
        .entrySet().stream()
        .map(entry -> new MyDTO(entry.getKey().getKey(),
                                entry.getKey().getValue(),
                                entry.getValue()))
        .collect(Collectors.toList());

Answered By - Alexander Ivanchenko
Answer Checked By - Gilberto Lyons (JavaFixing Admin)