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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.
220
With Java 8, you can convert a List to Map in one line using the stream() and Collectors. toMap() utility methods. The Collectors. toMap() method collects a stream as a Map and uses its arguments to decide what key/value to use.
Since the map contains a key, value pair, we need two lists to store each of them, namely keyList for keys and valueList for values. We used map's keySet() method to get all the keys and created an ArrayList keyList from them.
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.
153
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