Reverse a comparator in Java 8

Question

I have an ArrayList and want sort it in descending order. I use for it java.util.stream.Stream.sorted(Comparator) method. Here is a description according Java API:

Returns a stream consisting of the elements of this stream, sorted according to the provided Comparator.

this methods return me a sort with ascending order. Which parameter should I change, just to have the descending order?

Change the Comparator behavior, of course. – duffymo Oct 07 '15 at 14:56 — duffymo, Oct 07 '15 at 14:56

score 217 · Accepted Answer · edited Mar 19 '19 at 04:21

You can use Comparator.reverseOrder() to have a comparator giving the reverse of the natural ordering.

If you want to reverse the ordering of an existing comparator, you can use Comparator.reversed().

Sample code:

Stream.of(1, 4, 2, 5)
    .sorted(Comparator.reverseOrder()); 
    // stream is now [5, 4, 2, 1]

Stream.of("foo", "test", "a")
    .sorted(Comparator.comparingInt(String::length).reversed()); 
    // stream is now [test, foo, a], sorted by descending length

score 46 · Answer 2 · edited Jan 24 '22 at 07:15

46

You can also use Comparator.comparing(Function, Comparator)
It is convenient to chain comparators when necessary, e.g.:

Comparator<SomeEntity> ENTITY_COMPARATOR = 
        Comparator.comparing(SomeEntity::getProperty1, Comparator.reverseOrder())
        .thenComparingInt(SomeEntity::getProperty2)
        .thenComparing(SomeEntity::getProperty3, Comparator.reverseOrder());

edited Jan 24 '22 at 07:15

Capn Sparrow

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answered Feb 04 '19 at 22:19

dim42

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Comparator ENTITY_COMPARATOR = Comparator.comparing(SomeEntity::getProperty1, reverseOrder()) – Kyung Hwan Min Jun 24 '20 at 19:30
1

This example is extremely helpful if you're sorting on multiple properties, and only want to reverse some of them! – SouthShoreAK Jun 15 '21 at 21:56

score 10 · Answer 3 · answered Oct 07 '15 at 14:57

10

Java 8 Comparator interface has a reversed method : https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#reversed--

answered Oct 07 '15 at 14:57

Dici

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score 3 · Answer 4 · answered Aug 26 '21 at 05:46

I found this question serching for reversing order when you have a custom comparator and the reverse is conditional (you can do it or not).

The idea proposed by Pinocchio helped me.

So here is the code:

int order = requestedOrder.equals("asc") ? 1 : -1;

Collections.sort(List<CustomObj>, new Comparator<CustomObj>() {
    public int compare(CustomObj first, CustomObj scnd) {
        return first.getComparableParam().compareTo(scnd.getComparableParam()) * order;
    }
});

Hope it helps somebody else in the future

score -1 · Answer 5 · answered May 08 '20 at 20:14

Why not to extend the existing comperator and overwrite super and nor the result. The implementation the Comperator Interface is not nessesery but it makes it more clear what happens.

In result you get a easy reusable Class File, testable unit step and clear javadoc.

public class NorCoperator extends ExistingComperator implements Comparator<MyClass> {
    @Override
    public int compare(MyClass a, MyClass b) throws Exception {
        return super.compare(a, b)*-1;
    }
}

Reverse a comparator in Java 8

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