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?

Answer 1

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

Answer 2

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

Answer 3

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

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