How to sort a HashSet in java

HashSet does not preserve the order of the elements, however you can convert HashSet and order the elements in ascending order with two methods: using the method Collections.sort() or by copying the elements into a TreeSet.

1) Sort HashSet with Collections.sort()

The method Collections.sort() allows you to sort collections of objects that implement the interface java.util.List. You should copy the HashSet elements into one of the collections: ArrayList, LinkedList, or Vector. We're going to use the ArrayList class, which is the simplest. Next, we call the method Collections.sort() on our ArrayList.

import java.util.ArrayList; 
import java.util.Collections; 
import java.util.HashSet; 
import java.util.List; 

public class TriHashSet {

 public static void main(String[] args) {
 //create a hashset
 HashSet< String> hset = new HashSet< String> (); 
 
 //add elements
 hset.add("a2"); 
 hset.add("a3"); 
 hset.add("b1"); 
 hset.add("b3"); 
 hset.add("a1"); 
 hset.add("b2"); 
 
 System.out.println("Before Sorting:"); 
 for(String s: hset)
 System.out.println(s); 

 //copy HashSet elements to ArrayList
 List< String> list = new ArrayList< String> (hset); 
 Collections.sort(list); 
 
 System.out.println("After sorting:"); 
 for(String s: hset)
 System.out.println(s); 
 }
}

Output:

Before sorting: 
b1
a1
b3
b2
a2
a3
After sorting:  
b1
a1
b3
b2
a2
a3

1) Sort HashSet using TreeSet

Using TreeSet is very simple. Just create an instance of TreeSet with the HashSet.

import java.util.HashSet as an argument; 
import java.util.TreeSet; 

public class TriHashSetTreeMap {

 public static void main(String[] args) {
 //create a hashset
 HashSet hset = new HashSet(); 
 
 //add elements
 hset.add(5); 
 hset.add(16); 
 hset.add(8); 
 hset.add(22); 
 hset.add(14); 
 hset.add(11); 
 
 System.out.println("Before sorting: "+hset); 
 for(int n: hset)
 System.out.println(n); 
 
 //copy HashSet elements into ArrayList
 TreeSet treeset = new TreeSet< > (hset); 
 
 System.out.println("After sorting: "+treeset); 
 for(int n : treeset)
 System.out.println(n); 
 }
}

Output:

Before sorting: [16, 5, 22, 8, 11, 14]
16
5
22
8
11
14
After IR: [5, 8, 11, 14, 16, 22]
5
8
11
14
16
22