How do I create two different compliementary lists using same input

user_6072667 0
Viewed 235

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 = users.stream().filter(u -> u.getAge() > 21).collect(toList());
List<User> userBelowThreshold = users.stream().filter(u -> 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?

java-stream java
2 Answers

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);
user_58620710

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.

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);
user_17461180
Related