I have Following use case:
User Object is as followed:
UserId payload
123 abc1
123 abc2
456 pqr1
456 pqr2
678 xyz1
And after iterating above list<User Object>, Result should be like:
{123=[abc1, abc2], 456=[pqr1,pqr2], 678=[xyz1]}
so that I can use value(list of other objects) for key(123) for further processing. What will be best way to do it?
I have tried below but not sure how performance efficient if object size increases.
Map<String,List<String>> userMap = new HashMap<String, List<String>>();
for(User user: users) {
String userId = user.getUserId();
List<String> payloads = new ArrayList<String>();
if(userMap.containsKey(userId)) {
payload = userMap.get(userId);
payloads.add(user.getPayload());
userMap.put(userId, payloads);
}
else {
payloads.add(user.getPayload());
userMap.put(user.getUserId(), payloads);
}
}
The easiest way would be to stream the List and use Collectors#groupingBy and Collectors#mapping:
Map<String, List<String>> userMap = users.stream()
.collect(Collectors.groupingBy(User::getUserId,
Collectors.mapping(User::getPayload, Collectors.toList())));
Output:
{123=[abc1, abc2], 456=[pqr1, pqr2], 678=[xyz1]}
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