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How to sort HashSet in Java

  • Difficulty Level : Easy
  • Last Updated : 11 Dec, 2018

Given a HashSet in Java, the task is to sort this HashSet.

Examples:

Input: HashSet: [Geeks, For, ForGeeks, GeeksforGeeks]
Output: [For, ForGeeks, Geeks, GeeksforGeeks]

Input: HashSet: [2, 5, 3, 1, 4]
Output: [1, 2, 3, 4, 5] 

The HashSet class implements the Set interface, backed by a hash table which is actually a HashMap instance. No guarantee is made as to the iteration order of the set which means that the class does not guarantee the constant order of elements over time.

It means that HashSet does not maintains the order of its elements. Hence sorting of HashSet is not possible.

However, the elements of the HashSet can be sorted indirectly by converting into List or TreeSet, but this will keep the elements in the target type instead of HashSet type.



Below is the implementation of the above approach:

Program 1: By Converting HashSet to List.




// Java program to sort a HashSet
  
import java.util.*;
  
public class GFG {
    public static void main(String args[])
    {
        // Creating a HashSet
        HashSet<String> set = new HashSet<String>();
  
        // Adding elements into HashSet using add()
        set.add("geeks");
        set.add("practice");
        set.add("contribute");
        set.add("ide");
  
        System.out.println("Original HashSet: "
                           + set);
  
        // Sorting HashSet using List
        List<String> list = new ArrayList<String>(set);
        Collections.sort(list);
  
        // Print the sorted elements of the HashSet
        System.out.println("HashSet elements "
                           + "in sorted order "
                           + "using List: "
                           + list);
    }
}
Output:
Original HashSet: [practice, geeks, contribute, ide]
HashSet elements in sorted order using List: [contribute, geeks, ide, practice]

Program 2: By Converting HashSet to TreeSet.




// Java program to sort a HashSet
  
import java.util.*;
  
public class GFG {
    public static void main(String args[])
    {
  
        // Creating a HashSet
        HashSet<String> set = new HashSet<String>();
  
        // Adding elements into HashSet using add()
        set.add("geeks");
        set.add("practice");
        set.add("contribute");
        set.add("ide");
  
        System.out.println("Original HashSet: "
                           + set);
  
        // Sorting HashSet using TreeSet
        TreeSet<String> treeSet = new TreeSet<String>(set);
  
        // Print the sorted elements of the HashSet
        System.out.println("HashSet elements "
                           + "in sorted order "
                           + "using TreeSet: "
                           + treeSet);
    }
}
Output:
Original HashSet: [practice, geeks, contribute, ide]
HashSet elements in sorted order using TreeSet: [contribute, geeks, ide, practice]

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