CODEWITHSUNDEEP
java
Gopal
Gopal

Reputation: 745

HashSet doesn't guarantee sorting?

import java.util.*;
public class DuplicateCheckMain {
 public static void main(String[] gopal){
  Integer[] args = {6,9,2,55,100,1,6,8,9};
  Integer[] args1 = {3,6,2,3,5};
  Set S = new HashSet();
  DuplicateCheck.checkDuplicate(S,args,new String("HashSet"));
  Set S1 = new HashSet();
  DuplicateCheck.checkDuplicate(S1,args1,new String("HashSet"));

  S = new TreeSet();
  DuplicateCheck.checkDuplicate(S,args,new String("TreeSet"));

  S = new LinkedHashSet();
  DuplicateCheck.checkDuplicate(S,args,new String("LinkedHashSet"));

 }
}

public class DuplicateCheck {

 public static void checkDuplicate(Set S, Integer[] args, String setname){
  for(int i = 0;i<args.length;i++){
   if(!S.add(args[i])){System.out.println("Duplicate element "+args[i]);}
  }
  System.out.println(S +" "+ setname);
 }
}

Question: for the HashSet with reference S, the HashSet is not sorted. But for the reference S1, the HashSet is sorted. Why so?

Upvotes: 4

Views: 5506

Answers (5)

gabuzo
gabuzo

Reputation: 7778

Please note that beyond not guarantying sorting, HashSets iteration order may change completely if you insert new elements. For instance:

public class TestHashSet {
    public static void main(String[] foo) {
        Set<Integer> set = new HashSet<Integer>();
        set.addAll(Arrays.asList(new Integer[] {6,9,2,55,100,1,6,8,9}));
        System.out.println(set);
        set.addAll(Arrays.asList(new Integer[] {7,3,13,37,66}));
        System.out.println(set);
        set.add(42);
        System.out.println(set);
    }
}

Gave me the following output:

[1, 100, 2, 55, 6, 8, 9]
[1, 100, 2, 3, 55, 66, 6, 37, 7, 8, 9, 13]
[1, 2, 100, 3, 6, 66, 7, 37, 8, 42, 9, 13, 55]

Notice how the insertion of a single element changed completely the order.

Upvotes: 3

卢声远 Shengyuan Lu
卢声远 Shengyuan Lu

Reputation: 32004

java.util.Set guarantees "no duplicate elements".

java.util.SortedSet guarantees ordering and "no duplicate elements".

Upvotes: 0

Jon Skeet
Jon Skeet

Reputation: 1500495

HashSet is absolutely not guaranteed to be sorted. The ordering isn't guaranteed at all.

From the documentation of the iterator() method:

Returns an iterator over the elements in this set. The elements are returned in no particular order.

HashSet is designed to insert and check for the presence of elements very quickly, by equality. That's all.

If you need sorting, you should use an implementation of SortedSet, such as TreeSet or ConcurrentSkipListSet.

Upvotes: 7

The Scrum Meister
The Scrum Meister

Reputation: 30111

HashSet used a mod to store the number into a bucket entry

In the args1 array all the number are less then 16 - the default HashSet size. that is why it ends up being sorted.

Upvotes: 3

fastcodejava
fastcodejava

Reputation: 41097

HashSet does not guarantee sorting. To have sorting feature, use TreeSet or something similar.

Upvotes: 0

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