How do I create two different compliementary lists using same input

  • A+

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<User> users. Now I am trying to find the different User groups from ages according to the threshold. I tried this

List<User> userAboveThreshold = -> u.getAge() > 21).collect(toList()); List<User> userBelowThreshold = -> u.getAge() <= 21).collect(toList()); 

This time it works I can see using

userAboveThreshold.forEach(u -> System.out.println(u.getName() + " " + u.getAge())); userBelowThreshold.forEach(u -> System.out.println(u.getName() + " " + u.getAge())); 

But I have to access the users list again to find the complimentary list. Can this not be done simpler?



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.


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


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