How to sort a HashSet

Question

For lists  we use the Collections sort List  method  What if we want to sort a HashSet

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If you want want the end Collection to be in the form of Set and if you want to define your own natural order rather than that of TreeSet then -  1  Convert the HashSet into List 2  Custom sort the List using Comparator 3  Convert back the List into LinkedHashSet to maintain order 4  Display the LinkedHashSet  Sample program -  package demo31   import java util Collections  import java util Comparator  import java util HashSet  import java util LinkedHashSet  import java util LinkedList  import java util List  import java util Set   public class App26       public static void main String   args            Set lt String gt  set   new HashSet lt  gt             addElements set           List lt String gt  list   new LinkedList lt  gt             list   convertToList set           Collections sort list  new Comparator lt String gt                   Override             public int compare String s1  String s2                    int flag   s2 length   - s1 length                    if flag    0                        return flag                    else                       return -s1 compareTo s2                                                       Set lt String gt  set2   new LinkedHashSet lt  gt             set2   convertToSet list           displayElements set2             public static void addElements Set lt String gt  set            set add  Hippopotamus            set add  Rhinocerous            set add  Zebra            set add  Tiger            set add  Giraffe            set add  Cheetah            set add  Wolf            set add  Fox            set add  Dog            set add  Cat              public static List lt String gt  convertToList Set lt String gt  set            List lt String gt  list   new LinkedList lt  gt             for String element  set                list add element                     return list            public static Set lt String gt  convertToSet List lt String gt  list            Set lt String gt  set   new LinkedHashSet lt  gt             for String element  list                set add element                     return set            public static void displayElements Set lt String gt  set            System out println set             Output -   Hippopotamus  Rhinocerous  Giraffe  Cheetah  Zebra  Tiger  Wolf  Fox  Dog  Cat    Here the collection has been sorted as -  First - Descending order of String length Second - Descending order of String alphabetical hierarchy

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Elements in HashSet can t be sorted  Whenever you put elements into HashSet  it can mess up the ordering of the whole set  It is deliberately designed like that for performance  When you don t care about the order  HashSet will be the most efficient set for fast insertion and search  TreeSet will sort all the elements automatically every time you insert an element  Perhaps  what you are trying to do is to sort just once  In that case  TreeSet is not the best option because it needs to determine the placing of newly added elements all the time  The most efficient solution is to use ArrayList  Create a new list and add all the elements then sort it once  If you want to retain only unique elements  remove all duplicates like set does  then put the list into a LinkedHashSet  it will retain the order you have already sorted  List lt Integer gt  list   new ArrayList lt  gt     list add 6   list add 4   list add 4   list add 5   Collections sort list   Set lt Integer gt  unique   new LinkedHashSet lt  gt  list      4 5 6    The above line is not copying the objects  It only copies references   Now  you ve gotten a sorted set if you want it in a list form then convert it into list

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Just in-case you don t wanna use a TreeSet you could try this   set   set stream   sorted   collect Collectors toCollection LinkedHashSet  new

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We can not decide that the elements of a HashSet would be sorted automatically  But we can sort them by converting into TreeSet or any List like ArrayList or LinkedList etc      Create a TreeSet object of class E TreeSet lt E gt  ts   new TreeSet lt E gt          Convert your HashSet into TreeSet ts addAll yourHashSet    System out println ts toString       t Sorted Automatically

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This simple command did the trick for me   myHashSet toList sorted   I used this within a print statement  so if you need to actually persist the ordering  you may need to use TreeSets or other structures proposed on this thread

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Java 8 way to sort it would be   fooHashSet stream      sorted Comparator comparing Foo  getSize     comparator - how you want to sort it    collect Collectors toList       collector - what you want to collect it to    Foo  getSize it s an example how to sort the HashSet of YourItem s naturally by size    Collectors toList   is going to collect the result of sorting into a List the you will need to capture it with List lt Foo gt  sortedListOfFoo

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You can wrap it in a TreeSet like this   Set mySet   new HashSet    mySet add 4   mySet add 5   mySet add 3   mySet add 1   System out println  mySet items    mySet       TreeSet treeSet   new TreeSet mySet      System out println  treeSet items    treeSet        output   mySet items  1  3  4  5  treeSet items  1  3  4  5        Set mySet   new HashSet    mySet add  five    mySet add  elf    mySet add  four    mySet add  six    mySet add  two    System out println  mySet items    mySet    TreeSet treeSet   new TreeSet mySet   System out println  treeSet items    treeSet     output  mySet items  six  four  five  two  elf  treeSet items  elf  five  four  six  two      requirement for this method is that the objects of the set list should be comparable  implement the Comparable interface

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You can use a TreeSet instead

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You can use Java 8 collectors and TreeSet  list stream   collect Collectors toCollection TreeSet  new

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A HashSet does not guarantee any order of its elements  If you need this guarantee  consider using a TreeSet to hold your elements   However if you just need your elements sorted for this one occurrence  then just temporarily create a List and sort that   Set lt   gt  yourHashSet   new HashSet lt  gt           List lt   gt  sortedList   new ArrayList lt  gt  yourHashSet   Collections sort sortedList

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You can use TreeSet as mentioned in other answers    Here s a little more elaboration on how to use it   TreeSet lt String gt  ts   new TreeSet lt String gt     ts add  b1    ts add  b3    ts add  b2    ts add  a1    ts add  a2    System out println ts   for  String s  ts      System out println s     Output    a1  a2  a3  a4  a5  a1 a2 b1 b2 b3

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Based on the answer given by  LazerBanana i will put my own example of a Set sorted by the Id of the Object   Set lt Clazz gt  yourSet           yourSet stream   sorted new Comparator lt Clazz gt           Override     public int compare Clazz o1  Clazz o2            return o1 getId   compareTo o2 getId              collect Collectors toList        Returns the sorted List  using toSet   wont work

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SortedSet has been added Since java 7 https   docs oracle com javase 8 docs api java util SortedSet html

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you can do this in the following ways   Method 1    Create a list and store all the hashset values into it sort the list using Collections sort   Store the list back into LinkedHashSet as it preserves the insertion order   Method 2    Create a treeSet and store all the values into it    Method 2 is more preferable because the other method consumes lot of time to transfer data back and forth between hashset and list

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Use java util TreeSet as the actual object  When you iterate over this collection  the values come back in a well-defined order   If you use java util HashSet then the order depends on an internal hash function which is almost certainly not lexicographic  based on content

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You can use guava library for the same  Set lt String gt  sortedSet   FluentIterable from myHashSet  toSortedSet new Comparator lt String gt           Override     public int compare String s1  String s2               descending order of relevance           required code

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In my humble opinion   LazerBanana s answer should be the top rated answer  amp  accepted because all the other answers pointing to java util TreeSet   or first convert to list then call Collections sort      on the converted list   didn t bothered to ask OP as what kind of objects your HashSet has i e  if those elements have a predefined natural ordering or not  amp  that is not optional question but a  mandatory question    You just can t go in  amp  start putting your HashSet elements into a TreeSet if element type doesn t already implement Comparable interface or if you are not explicitly passing Comparator to TreeSet constructor     From TreeSet JavaDoc        Constructs a new  empty tree set  sorted according to the natural   ordering of its elements  All elements inserted into the set must   implement the Comparable interface  Furthermore  all such elements   must be mutually comparable  e1 compareTo e2  must not throw a   ClassCastException for any elements e1 and e2 in the set  If the user   attempts to add an element to the set that violates this constraint    for example  the user attempts to add a string element to a set whose   elements are integers   the add call will throw a ClassCastException    That is why only all Java8 stream based answers - where you define your comparator on the spot -  only make sense because implementing comparable in POJO becomes optional  Programmer defines comparator as and when needed  Trying to collect into TreeSet without asking this fundamental question is also incorrect   Ninja s answer   Assuming object types to be String or Integer is also incorrect    Having said that  other concerns like      Sorting Performance  Memory Foot Print   retaining original set and creating new sorted sets each time sorting is done or wish to sort the set in - place etc etc      should be the other relevant points too  Just pointing to API shouldn t be only intention    Since Original set already contains only unique elements  amp  that constraint is  also maintained by sorted set so original set needs to be cleared from memory since data is duplicated

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1  Add all set element in list - gt  al addAll s   2  Sort all the elements in list using - gt  Collections sort al      public class SortSetProblem    public static void main String   args        ArrayList lt String gt  al   new ArrayList        Set lt String gt  s   new HashSet lt  gt         s add  ved        s add  prakash        s add  sharma        s add  apple        s add  ved        s add  banana        System out println  Before Sorting        for  String s1   s            System out print        s1              System out println  After Sorting        al addAll s       Collections sort al       for  String set   al            System out print       set                     input - ved prakash sharma apple ved banana       Output - apple banana prakash sharma ved

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Add all your objects to the TreeSet  you will get a sorted Set  Below is a raw example   HashSet myHashSet   new HashSet    myHashSet add 1   myHashSet add 23   myHashSet add 45   myHashSet add 12    TreeSet myTreeSet   new TreeSet    myTreeSet addAll myHashSet   System out println myTreeSet      Prints  1  12  23  45

[java] How to sort a HashSet?

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