How to reverse comparator order using stream API?

Question

String obtained = transactions.stream()
    .map(Transaction::getTrader)
    .map(Trader::getName)
    .sorted(String::compareTo)
    .collect(Collectors.joining(","));

What should I do if I want to reverse the natural alphabetical ordering in the returned string without writing a specific Comparator<String>, so that the concatenated string result starts with the names 'z..', 'y..', 'x..' and so on?

colin aygalinc · Answer 1 · 2016-11-28T11:39:27.470

2

In java, something like this can do the trick :

String obtained = transactions.stream()
                  .map(Transaction::getTrader).map(Trader::getName)
                  .sorted(Comparator.reverseOrder())
                  .collect(Collectors.joining(","));

Note that it works because String implements Comparable Interface.

edited Nov 28 '16 at 11:39

answered Nov 25 '16 at 14:31

colin aygalinc

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That is what i was expecting! Thx, TDRS – Nov 25 '16 at 16:46
I think you meant `Comparator.reverseOrder()` instead of `Collection` – 4castle Nov 26 '16 at 02:02

gr7 · Answer 2 · 2016-11-28T14:59:21.753

Only one operation is required to sort collection in reversed order.

String implements Comparable interface. The javadoc for java.lang.Comparable#compareTo() says that it returns:

a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object

To reverse order of collection you can:

[recommended] use Comparator.reverseOrder (javadoc) which

Returns a comparator that imposes the reverse ordering of the specified comparator. (...)

List<String> strings = Arrays.asList("a","b","d", "c", "e");
reversedOrderedStrings = strings.stream()
                            .sorted(Comparator.reverseOrder())
                            .collect(Collectors.joining(","));

invert order of compared parameters

reversedOrderedStrings = strings.stream()
                            .sorted((s1, s2) ->  s2.compareTo(s1))
                            .collect(Collectors.joining(","));

[not recommended] change the sign of compareTo() method. This is not recommended as it can cause numeric overflow for Integer.MIN_VALUE

reversedOrderedStrings = strings.stream()
                            .sorted((s1, s2) -> -s1.compareTo(s2))
                            .collect(Collectors.joining(","));

Output for all is the same:

System.out.println(reversedOrderedStrings);
e,d,c,b,a

You are right but the question says to not write your own comparator. I acknowledge that your method is faster and must be used. — colin aygalinc, Nov 25 '16 at 16:14
Yes, i know i can do that, i was looking Collection.reverseOrder() method. — , Nov 25 '16 at 16:47
I should have used "sorted(Comparator.reverseOrder())" in my answer as described in [Reverse a comparator in Java 8](http://stackoverflow.com/questions/32995559/reverse-a-comparator-in-java-8). — gr7, Nov 25 '16 at 17:12

How to reverse comparator order using stream API?

2 Answers2