How to convert List of Objects to map using object's fields in Java

Question

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);
 }
 }

• JB Nizet: You should only create a new ArrayList if you need one. And you should not put the list if it's already in the map. And you can just call get() and check if the value is null instead of using containsKey. But unless you have millions of users, you probably won't notice any difference.
• JB Nizet: You could also use Streams and their groupingBy collector to make the code simpler. Which what you should aim for, rather than performance.

Answer 1

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]}

Answer 2

Another possible solution would be to use Guavas Maps.toMap