How to iterate over several lists of objects and get summary of objects in Java?

Suppose I have a class as follows.

public class User {
    private String userType;
    private int numOfUsers;
}

And there are more than one list which consists objects of User type.

List<User> list1 = < (userType:clerk, numofUsers:3 ), (userType:painter, numofUsers:4 ), (userType:engineer, numofUsers:10 ) >
        
List<User> list2 = < (userType:clerk, numofUsers:3 ),(userType:electrician, numofUsers:14 ), (userType:welder, numofUsers:5 ), (userType:engineer, numofUsers:10) >
    
List<User> list3 = < (userType:carpenter, numofUsers:4 ),(userType:welder, numofUsers:10 ) >

I need to create single list of users which gives the summation of every types of user objects as follows.

List<User> userCount  = < (userType:clerk, numofUsers:6 ), (userType:painter, numofUsers:4 ), (userType:engineer, numofUsers:20 ),(userType:electrician, numofUsers:14 ), (userType:carpenter, numofUsers:4 ) ,(userType:welder, numofUsers:15 ) >

How can i do that in Java?

Are you looking to use :

List<User> result = Stream.of(list1, list2, list3)
        .flatMap(Collection::stream)
        .collect(Collectors.groupingBy(User::getUserType, Collectors.summingInt(User::getNumOfUsers)))
        .entrySet().stream()
        .map(e -> new User(e.getKey(), e.getValue()))
        .collect(Collectors.toList());

You can use Java Stream API. Use Collectors.toMap and merge NumOfUsers for same type. Then get the values in ArrayList.

List<User> result = new ArrayList<>(
       Stream.of(list1, list2, list3)  // Stream of List of List
        .flatMap(Collection::stream)   // Flatten into a single list
        .collect(Collectors.toMap(User::getUserType, i -> i,   //Map by UserType and  merge user having same type
                 (a,b) -> new User(a.getUserType(), a.getNumOfUsers() + b.getNumOfUsers())))
        .values());   // Take only values of the map