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   -1  as a hack  Arrays asList files  stream        filter file - gt  isNameLikeBaseLine file  baseLineFile getName          sorted new Comparator lt File gt              public int compare File o1  File o2                int answer              if  o1 lastModified      o2 lastModified                      answer   0                else if  o1 lastModified    gt  o2 lastModified                      answer   1                else                   answer   -1                            return -1   answer                        skip numOfNewestToLeave       forEach item - gt  item delete

User · Accepted Answer

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.

User · Answer

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 lt File gt  filesList   Arrays asList files            Comparator lt File gt  comparator   Comparator comparingLong File  lastModified           Comparator lt File gt  reverseComparator   comparator reversed             List lt File gt  forwardOrder   filesList stream   sorted comparator  collect Collectors toList             List lt File gt  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

User · Answer

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 lt File gt  fileList   new ArrayList lt  gt             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 - gt  System out println x getName              Collections sort fileList  Comparator comparing File  getName  reversed             System out println                                                         fileList forEach x - gt  System out println x getName

User · Answer

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 - gt  isNameLikeBaseLine file  baseLineFile getName      sorted Comparator reverseOrder    skip numOfNewestToLeave   forEach item - gt  item delete

User · Answer

If your stream elements implements Comparable then the solution becomes simpler       stream     sorted Comparator reverseOrder

User · Answer

In simple  using Comparator and Collection you can sort like below in reversal order using JAVA 8  import java util Comparator   import java util stream Collectors   Arrays asList files  stream        sorted Comparator comparing File  getLastModified  reversed         collect Collectors toList

User · Answer

For reverse sorting just change the order of x1  x2 for calling the x1 compareTo x2  method the result will be reverse to one another  Default order   List lt String gt  sortedByName   citiesName stream   sorted  s1 s2 - gt s1 compareTo s2   collect Collectors toList     System out println  Sorted by Name      sortedByName     Reverse Order  List lt String gt  reverseSortedByName   citiesName stream   sorted  s1 s2 - gt s2 compareTo s1   collect Collectors toList     System out println  Reverse Sorted by Name      reverseSortedByName

User · Answer

You can define your Comparator with your own logic like this  private static final Comparator lt UserResource gt  sortByLastLogin    c1  c2  - gt        if  Objects isNull c1 getLastLoggedin               return -1        else if  Objects isNull c2 getLastLoggedin               return 1            return c1 getLastLoggedin   compareTo c2 getLastLoggedin            And use it inside foreach as  list stream         sorted sortCredentialsByLastLogin reversed          collect Collectors toList

User · Answer

Alternative way sharing   ASC  List lt Animal gt  animals   this service findAll    animals   animals stream   sorted Comparator comparing Animal  getName   collect Collectors toList       DESC  List lt Animal gt  animals   this service findAll    animals   animals stream   sorted Comparator comparing Animal  getName  reversed    collect Collectors toList

User · Answer

You can use a method reference   import static java util Comparator    import static java util stream Collectors     Arrays asList files  stream        filter file - gt  isNameLikeBaseLine file  baseLineFile getName          sorted comparing File  lastModified  reversed         skip numOfNewestToLeave       forEach item - gt  item delete       In alternative of method reference you can use a lambda expression  so the argument of comparing become    sorted comparing file - gt  file lastModified    reversed

User · Answer

Use  Comparator lt File gt  comparator   Comparator comparing File  lastModified    Collections sort list  comparator reversed       Then    forEach item - gt  item delete

User · Answer

If you want to sort by Object s date type property then public class Visit implements Serializable  Comparable lt Visit gt   private static final long serialVersionUID   4976278839883192037L   private Date dos   public Date getDos         return dos     public void setDos Date dos        this dos   dos      Override public int compareTo Visit visit        return this getDos   compareTo visit getDos          List lt Visit gt  visits   getResults     Method making the list Collections sort visits  Collections reverseOrder      Reverser order

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

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