How to use a Java8 lambda to sort a stream in reverse order?

Question

I\'m using java lambda to sort a list.

how can I sort it in a reverse way?

I saw this post, but I want to use java 8 lambda.

Here is my code (I used

Answer 1

If your stream elements implements Comparable then the solution becomes simpler:

 ...stream()
 .sorted(Comparator.reverseOrder())

Answer 2

Instead of all these complications, this simple step should do the trick for reverse sorting using Lambda .sorted(Comparator.reverseOrder())

Arrays.asList(files).stream()
.filter(file -> isNameLikeBaseLine(file, baseLineFile.getName()))
.sorted(Comparator.reverseOrder()).skip(numOfNewestToLeave)
.forEach(item -> item.delete());

Answer 3

Sort file list with java 8 Collections

Example how to use Collections and Comparator Java 8 to sort a File list.

import java.io.File;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class ShortFile {

    public static void main(String[] args) {
        List<File> fileList = new ArrayList<>();
        fileList.add(new File("infoSE-201904270100.txt"));
        fileList.add(new File("infoSE-201904280301.txt"));
        fileList.add(new File("infoSE-201904280101.txt"));
        fileList.add(new File("infoSE-201904270101.txt"));

        fileList.forEach(x -> System.out.println(x.getName()));
        Collections.sort(fileList, Comparator.comparing(File::getName).reversed());
        System.out.println("===========================================");
        fileList.forEach(x -> System.out.println(x.getName()));
    }
}

Answer 4

This can easily be done using Java 8 and the use of a reversed Comparator.

I have created a list of files from a directory, which I display unsorted, sorted and reverse sorted using a simple Comparator for the sort and then calling reversed() on it to get the reversed version of that Comparator.

See code below:

package test;

import java.io.File;
import java.util.Arrays;
import java.util.Comparator;
import java.util.Date;
import java.util.List;
import java.util.stream.Collectors;

public class SortTest {
    public static void main(String... args) {
        File directory = new File("C:/Media");
        File[] files = directory.listFiles();
        List<File> filesList = Arrays.asList(files);

        Comparator<File> comparator = Comparator.comparingLong(File::lastModified);
        Comparator<File> reverseComparator = comparator.reversed();

        List<File> forwardOrder = filesList.stream().sorted(comparator).collect(Collectors.toList());
        List<File> reverseOrder = filesList.stream().sorted(reverseComparator).collect(Collectors.toList());

        System.out.println("*** Unsorted ***");
        filesList.forEach(SortTest::processFile);

        System.out.println("*** Sort ***");
        forwardOrder.forEach(SortTest::processFile);

        System.out.println("*** Reverse Sort ***");
        reverseOrder.forEach(SortTest::processFile);
    }

    private static void processFile(File file) {
        try {
            if (file.isFile()) {
                System.out.println(file.getCanonicalPath() + " - " + new Date(file.lastModified()));
            }
        } catch (Exception e) {
            System.out.println(e.getMessage());
        }
    }
}

Answer 5

Alternative way sharing:

ASC

List<Animal> animals = this.service.findAll();
animals = animals.stream().sorted(Comparator.comparing(Animal::getName)).collect(Collectors.toList());

DESC

List<Animal> animals = this.service.findAll();
animals = animals.stream().sorted(Comparator.comparing(Animal::getName).reversed()).collect(Collectors.toList());

Answer 6

You can adapt the solution you linked in How to sort ArrayList<Long> in Java in decreasing order? by wrapping it in a lambda:

.sorted((f1, f2) -> Long.compare(f2.lastModified(), f1.lastModified())

note that f2 is the first argument of Long.compare, not the second, so the result will be reversed.