List to Set in Java

Last Updated : 26 Dec, 2025

In Java, converting a List to a Set is commonly done to remove duplicate elements, since a Set does not allow duplicates. This can be achieved using simple iteration, collection constructors, the addAll() method, or the Java 8 Stream API. Different Set implementations such as HashSet and TreeSet also determine whether the elements remain unordered or sorted.

Method 1: Using Simple Traversal

Example: This code demonstrates how to convert a List into a HashSet using a simple loop. A HashSet removes duplicate elements and does not maintain any specific order.

Java 
import java.util.*;
class GFG {
    public static void main(String[] args)
    {
        List<String> aList = Arrays.asList("Geeks", "for",
                     "Geeks", "GeeksforGeeks", "GFG");

        Set<String> hSet = new HashSet<String>();
        for (String x : aList)  hSet.add(x);

        System.out.println("Created HashSet is :");
        for (String x : hSet)
            System.out.println(x);

    }
}

Output
Created HashSet is :
GeeksforGeeks
Geeks
for
GFG

Explanation:

  • Each element from the list is added to the set using a for-each loop.
  • Since a HashSet does not allow duplicates, all elements are stored uniquely and then printed.

Method 2: Using Set Constructor

Example: This code demonstrates how to convert a List into a HashSet and a TreeSet using their constructors in Java.

Java 
import java.util.*;
class GFG {
    public static void main(String[] args) {

        List<String> aList = Arrays.asList(
            "Geeks", "for", "Geeks", "GeeksforGeeks", "for");

        Set<String> hSet = new HashSet<>(aList);
        Set<String> tSet = new TreeSet<>(aList);

        System.out.println("Created HashSet: ");
        hSet.forEach(System.out::println);

        System.out.println("Created TreeSet: ");
        tSet.forEach(System.out::println);
    }
}

Output
Created HashSet: 
GeeksforGeeks
Geeks
for
Created TreeSet: 
Geeks
GeeksforGeeks
for

Explanation:

  • The HashSet constructor removes duplicates and stores elements in no specific order.
  • The TreeSet constructor removes duplicates and stores elements in sorted order.
  • Both sets are printed to show the difference in ordering.

Method 3: Using addAll method

Example:This code shows how to convert a List into a HashSet using addAll(), which removes duplicates and does not preserve order.

Java 
import java.util.*;
class GFG {
    public static void main(String[] args) {
        List<String> aList = Arrays.asList("Geeks", "for", "Geeks", "GeeksforGeeks", "GFG");

        Set<String> hSet = new HashSet<>();
        hSet.addAll(aList);

        System.out.println("Created HashSet is:");
        hSet.forEach(System.out::println);
    }
}

Output
Created HashSet is:
GeeksforGeeks
Geeks
for
GFG

Explanation: addAll() copies elements from the list into the set, removing duplicates.

Method 4: Using Java 8 Stream API

Example: This code demonstrates how to convert a List into a Set using Java 8 Streams. The distinct behavior of Set removes duplicates automatically.

Java 
import java.util.*;
import java.util.stream.Collectors;
class GFG {
    public static void main(String[] args) {
        List<String> aList = Arrays.asList("Geeks", "for", "Geeks", "GeeksforGeeks", "GFG");
        
        Set<String> set = aList.stream().collect(Collectors.toSet());
        set.forEach(System.out::println);
    }
}

Output
GeeksforGeeks
Geeks
for
GFG

Explanation: stream() converts the list into a stream, and Collectors.toSet() collects unique elements into a set.

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