Java - Merge objects of list given a condition

Guillermo Nahuel Varelli :

I uttlerly convinced that my question its quite simple but im unable to do it with streams (if there is a way to do it without stream will be helpfull too) Suppose that we have this list of users

public class Users {
   String firstName;
   String lastName;
   double accountBalance;
   String type;
   String extraField;
}

and suppose that we have the following data in my List < Users >

"Users": [{
            "firstName": "Scott",
            "lastName": "Salisbury",
            "accountBalance": "100",
            "type" : "A"
        }, {
            "firstName": "John",
            "lastName": "Richards",
            "accountBalance": "200",
            "type" :"C"

        }, {
            "firstName": "John",
            "lastName": "Richards",
            "accountBalance": "200",
            "type " : "C",
            "ExtraField": "Apply"
        }]

the expected result here its given that firstName, lastName and type appears twice on the list just merge the results that are common without missing any field
Expected output

"Users": [{
            "firstName": "Scott",
            "lastName": "Salisbury",
            "accountBalance": "100",
            "type" : "A"
        }, {
            "firstName": "John",
            "lastName": "Richards",
            "accountBalance": "400",//merged values
            "type " : "C",
            "ExtraField": "Apply" //value that remains in one object of the list
        }]

daniu :

You can create a key class containing the three fields, like

@Data
class UserKey {
    String firstName;
    String lastName;
    String type;

    static UserKey from(User user) { /* TODO (trivial) */ }
}

groupingBy

Those can be used to group your users

Map<UserKey,List<User>> grouped = 
    users.stream().collect(Collectors.groupingBy(UserKey::from));

Each of these lists can then be merged by

Optional<User> summed = userList.stream()
    .collect(Collectors.reducing((u1, u2) -> {
        u1.setAccountBalance(u1.accountBalance() + u2.accountBalance());
    });

This can also be given directly as a downstream collector to the groupingBy:

Map<UserKey,Optional<User>> mergedMap = 
    users.stream().collect(Collectors.groupingBy(UserKey::from,
        Collectors.reducing((u1, u2) -> {
            u1.setAccountBalance(u1.accountBalance() + u2.accountBalance());
            return u1;
        }));

Since those Optionals are guaranteed to be filled, you can just call get() on them; also, you don't need the keys anymore, so

List<User> result = mergedMap.values().stream()
                 .map(Optional::get)
                 .collect(toList());

toMap

As Naman suggested in the comments, you can also shortcut this by toMap.

Map<UserKey,User> mergedMap = users.stream()
    .collect(toMap(UserKey::from, Function.identity(), 
        (u1, u2) -> {
            u1.setAccountBalance(u1.accountBalance() + u2.accountBalance());
            return u1;
        }));
List<User> result = new ArrayList<>(mergedMap.values());

Note that the reducing function has the side effect of manipulating one of the original user objects in the list, so make sure you don't need them again.

Java - Merge objects of list given a condition

Guess you like