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

You're after the partitioningBy collector:

Map> result = 
             users.stream().collect(partitioningBy(u -> u.getAge() > 21));

Then use it as follows:

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