5

I have a list of dto with following element. userSeqId have duplicate values,

private int userSeqId;
private String firstName;
private String lastName;
private String acctAgencyNumber;

I am trying to use Java 8 Lambda to group by 'userSeqId' to a Map.

I want Map<Integer, List<String>> where Key should be userSeqId and Value is List of acctAgencyNumber.

When I use 

Map<Integer, List<UserBasicInfoDto>> superUserAcctMap = customerSuperUserList.stream()
    .collect(Collectors.groupingBy(UserBasicInfoDto::getUserSeqId));

I get Map<Integer, List<UserBasicInfoDto>> where key is userSeqId but value is list of whole object.

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2 Answers 2

Reset to default
5

There is a dedicated version of groupingBy() for your use case:

Map<Integer, List<String>> result = customerSuperUserList.stream()
      .collect(Collectors.groupingBy(
        UserBasicInfoDto::getUserSeqId,
        Collectors.mapping(UserBasicInfoDto::getAcctAgencyNumber, toList())));

The key point of this is to use the helper mapping collector, using which you can override the default groupingBy behaviour.

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1 Comment

Collectors.mapping(obj -> obj, toList()) didn't work for me.
2

You can try this:

customerSuperUserList.stream().collect(Collectors.groupingBy(UserBasicInfoDto::getUserSeqId,Collectors.mapping(UserBasicInfoDto::getAcctAgencyNumber, Collectors.toList())));
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