To convert a HashSet to a TreeSet, simply create a TreeSet with a HashSet as a parameter. This is possible because HashSet is a collection of data. You can also create an empty TreeSet, then copy all the elements with the method addAll(Collection c).

Example:

We have a HashSet of String and we want to create a TreeSet of String by copying the elements of HashSet into TreeSet:

import java.util.HashSet; 
import java.util.TreeSet; 
import java.util.Set; 
public class HashSettoTreeSet{ 
 public static void main(String[] args) {

 HashSet hashset = new HashSet(); 

 hashset.add("E1"); 
 hashset.add("E2"); 
 hashset.add("E3"); 
 hashset.add("E4"); 
 hashset.add("E5"); 
 
 System.out.println("HashSet: "+ hashset); 
 
 // create a TreeSet with the elements of HashSet
 Set treeset = new TreeSet(hashset); 
 
 System.out.println("TreeSet: "); 
 for(String e : treeset){
 System.out.println(e); 
 }
 }
}

Output:

HashSet: [E1, E2, E3, E4, E5]
TreeSet: [E1, E2, E3, E4, E5]