How to Group a List of objects by Two fields and obtain a List of objects of resulting type

I have a list of objects shown below:

public class AssignDTO {    
    private String assignmentIbanId;    
    private String agreementFileId;    
    private BankData bankData;

   // getter and setter
}

[
   {
      "assignmentIbanId":"6146dd87-2344-4149-9396-8e8a88493dd6",
      "agreementFileId":"36de8604-ae56-49b3-b972-8071b459bf82",
      "bankData": { "useType" : "a" }
   },
   {
      "assignmentIbanId":"44cb8cc6-fe28-4e44-a78c-48a908795f38",
      "agreementFileId":"854f0c37-b9d8-4533-9c02-0c1b64a909bd",
      "bankData": { "useType" : "b" }
   },
   {
      "assignmentIbanId":"44cb8cc6-fe28-4e44-a78c-48a908795f38",
      "agreementFileId":"854f0c37-b9d8-4533-9c02-0c1b64a909bd",
      "bankData": { "useType" : "c" }
   }
]

How can I group the list by agreementFileId and agreementFileId fields like below

public class ResultDTO {    
    private String assignmentIbanId;    
    private String agreementFileId;    
    private Set<BankData> bankAccounts;

   // getter and setter
}

[
   {
      "assignmentIbanId":"6146dd87-2344-4149-9396-8e8a88493dd6",
      "agreementFileId":"36de8604-ae56-49b3-b972-8071b459bf82",
      "bankAccounts": [{ "useType" : "a" }]
   }
   {
      "assignmentIbanId":"44cb8cc6-fe28-4e44-a78c-48a908795f38",
      "agreementFileId":"854f0c37-b9d8-4533-9c02-0c1b64a909bd",
      "bankAccounts": [{ "useType" : "b" }, { "useType" : "c" }]
   }
]

Solution

As the first step, you might either create a nested map applying the Collector.groupingBy() twice, or group the data using an object that is capable to carry two values, like a Map.Entry, as a key.

In both cases, a combination of mapping() and toSet() would be handy as a downstream collector:

Collectors.mapping(AssignDTO::getBankData, Collectors.toSet()

As a result, it'll produce an intermediate map having Set<BankData> as its values.

Then create a stream over the entry set and apply map() to turn each entry into a ResultDTO and collect the result into a list either with toList() (Java 16+) or by utilizing Collectors.toList().

That's how it might look like (example with map entry for Java-8):

public static void main(String[] args) {
    List<AssignDTO> source =
        List.of(new AssignDTO("6146dd87-2344-4149-9396-8e8a88493dd6",
                    "36de8604-ae56-49b3-b972-8071b459bf82", new BankData("a")),
                new AssignDTO("44cb8cc6-fe28-4e44-a78c-48a908795f38",
                    "854f0c37-b9d8-4533-9c02-0c1b64a909bd", new BankData("b")),
                new AssignDTO("44cb8cc6-fe28-4e44-a78c-48a908795f38",
                    "854f0c37-b9d8-4533-9c02-0c1b64a909bd", new BankData("c")));
    
    List<ResultDTO> result =
        source.stream()
            .collect(Collectors.groupingBy(assignDTO -> new AbstractMap.SimpleEntry<>(assignDTO.getAgreementFileId(), assignDTO.getAgreementFileId()),
                        Collectors.mapping(AssignDTO::getBankData, Collectors.toSet()))) // creating an intermediate map `Map<Map.Entry<String, String>>, Set<BankData>>
            .entrySet().stream()
            .map(entry -> new ResultDTO(entry.getKey().getKey(),
                                        entry.getKey().getValue(),
                                        entry.getValue()))
            .collect(Collectors.toList());

    result.forEach(System.out::println);
}

Output

ResultDTO{
    assignmentIbanId : '854f0c37-b9d8-4533-9c02-0c1b64a909bd',
    agreementFileId : '854f0c37-b9d8-4533-9c02-0c1b64a909bd',
    bankAccounts : [{useType : 'b'}, {useType : 'c'}]}
ResultDTO{
    assignmentIbanId : '36de8604-ae56-49b3-b972-8071b459bf82',
    agreementFileId : '36de8604-ae56-49b3-b972-8071b459bf82',
    bankAccounts : [{useType : 'a'}]}