CODEWITHSUNDEEP
filterlambdajava-8java-streampredicate
gianglaodai
gianglaodai

Reputation: 321

How can I create multiple List with lambda expression?

I have a User with age property. And in my method I have List. How can I split it to multiple List for another user like:

List<User> lt6Users = new ArrayList<User>();
List<User> gt6Users = new ArrayList<User>();
for(User user:users){
   if(user.getAge()<6){
      lt6Users.add(user);
   }
   if(user.getAge()>6){
      gt6Users.add(user);
   }
   // more condition
}

I just known 2 way with lambda expression:

lt6Users = users.stream().filter(user->user.getAge()<6).collect(Collectors.toList());
gt6Users = users.stream().filter(user->user.getAge()>6).collect(Collectors.toList());

The code above is very poor for performance because it will loop through the list many time 

users.stream().foreach(user->{
  if(user.getAge()<6){
     lt6Users.add(user);
  }
  if(user.getAge()>6{
     gt6Users.add(user);
  }
});

the code above is look like the code from start code without lambda expression. Is there another way to write code using lambda expression feature like filter and Predicate?

Upvotes: 3

Views: 2210

Answers (2)

gianglaodai
gianglaodai

Reputation: 321

I've found a way to use lambda expression for this problem: Write a method with Predicate:

public void addUser(List<User> users,User user,Predicate<User> p){
  if(p.test(user)){
    users.add(user);
  }
}

So the loop can be write like this:

users.foreach(user->{
  addUser(lt6Users,user,(User u)->u.getAge()<6);
  addUser(gt6Users,user,(User u)->u.getAge()>6);
  // more condition
});

Upvotes: -1

Eran
Eran

Reputation: 393781

You can use Collectors.partitioningBy(Predicate<? super T> predicate) :

Map<Boolean, List<User>> partition = users.stream()
                                          .collect(Collectors.partitioningBy(user->user.getAge()<6));

partition.get(true) will give you the list of Users with ages < 6, and partition.get(false) will give you the list of the Users with ages >= 6.

Upvotes: 6

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