How do I create two different compliementary lists using same input

Question

In my previous question - How to filter the age while grouping in map with list I was able to find the name to age groups using List users. Now I am

Answer 1

List.removeAll

You can use removeAll to obtain the complimentary list.

List<User> userBelowThreshold = new ArrayList<>(users); // initiated with 'users'
userBelowThreshold.removeAll(userAboveThreshold);

Note: This would require overridden equals and hashCode implementation for User.

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Collectors.partitioningBy

On the other hand, if you further want to iterate over the complete users list just once, you can use Collectors.partitioningBy as:

Map<Boolean, List<User>> userAgeMap = users.stream()
        .collect(Collectors.partitioningBy(user -> user.getAge() > 21, Collectors.toList()));
List<User> userAboveThreshold = userAgeMap.get(Boolean.TRUE);
List<User> userBelowThreshold = userAgeMap.get(Boolean.FALSE);

Answer 2

You're after the partitioningBy collector:

Map<Boolean, List<User>> result = 
             users.stream().collect(partitioningBy(u -> u.getAge() > 21));

Then use it as follows:

List<User> userAboveThreshold = result.get(true);
List<User> userBelowThreshold = result.get(false);