Java 8 Lambda: Comparator

Question

I want to sort a list with Lambda:

List messagesByDeviceType = new ArrayList();      
messagesByDeviceType.sort((Message o1, Messag         


        

Answer 1

You should change

 messagesByDeviceType.sort(
     (Message o1, Message o2) -> o1.getTime() - o2.getTime()
 );

to

messagesByDeviceType.sort(
    Comparator.comparing((Message m) -> m.getTime())
);

That assumes that the value is Comparable, which provides a natural sort order.

If you want to add more fields then you can can chain them to the Comparator. e.g. to sort first by time, and then by sender:

messagesByDeviceType.sort(
    Comparator
        .comparing((Message m) -> m.getTime())
        .thenComparing((m)     -> m.getSender())
);

To reverse the order of any Comparator, chain the reveresed() method to it, e.g. to sort first by time descending, and then by sender:

messagesByDeviceType.sort(
    Comparator
        .comparing((Message m) -> m.getTime())
        .reversed()
        .thenComparing((m)     -> m.getSender())
);

See also https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html