Java 8 Lambda: Comparator

java lambda java-8 comparator
Nunyet de Can Calçada picture Nunyet de Can Calçada · May 28, 2017 · Viewed 39.1k times · Source

I want to sort a list with Lambda:

List<Message> messagesByDeviceType = new ArrayList<Message>();      
messagesByDeviceType.sort((Message o1, Message o2)->o1.getTime()-o2.getTime());

But I got this compilation error:

 Multiple markers at this line
    - Type mismatch: cannot convert from long to int
    - The method sort(Comparator<? super Message>) in the type List<Message> is not applicable for the arguments ((Message o1, Message o2) 
     -> {})

Answer

Eugene picture Eugene · May 28, 2017

Comparator#compareTo returns an int; while getTime is obviously long.

It would be nicer written like this:

 .sort(Comparator.comparingLong(Message::getTime))

Related questions

Very confused by Java 8 Comparator type inference

I've been looking at the difference between Collections.sort and list.sort, specifically regarding using the Comparator static methods and whether param types are required in the lambda expressions. Before we start, I know I could use method references, e.…

Read More
Comparator.reversed() does not compile using lambda

I have a list with some User objects and i'm trying to sort the list, but only works using method reference, with lambda expression the compiler gives an error: List<User> userList = Arrays.asList(u1, u2, u3); userList.…

Read More
Comparator.comparing(...) of a nested field

Suppose I have a domain model like this: class Lecture { Course course; ... // getters } class Course { Teacher teacher; int studentSize; ... // getters } class Teacher { int age; ... // getters } Now I can create a Teacher Comparator like this: return Comparator .comparing(Teacher::getAge); But …

Read More