How to convert a Java list to a set?

A Java List can be converted to a Set to eliminate duplicate entries. The resulting Set will contain only unique values. There are three common ways to perform this conversion −

Method 1: Using Set Constructor

Pass the list directly to the HashSet constructor −

Set<String> set = new HashSet<>(list);

Method 2: Using addAll()

Create an empty set and use addAll() to add all elements from the list −

Set<String> set = new HashSet<>();
set.addAll(list);

Method 3: Using Streams (Java 8+)

Use the Stream API to collect list elements into a set −

Set<String> set = list.stream().collect(Collectors.toSet());

Example

The following example demonstrates all three approaches ?

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;

public class CollectionsDemo {
    public static void main(String[] args) {
        List<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3, 3, 3, 4, 5));
        System.out.println("List: " + list);

        // Method 1: Constructor
        Set<Integer> set1 = new HashSet<>(list);
        System.out.println("Set (constructor): " + set1);

        // Method 2: addAll()
        Set<Integer> set2 = new HashSet<>();
        set2.addAll(list);
        System.out.println("Set (addAll): " + set2);

        // Method 3: Streams
        Set<Integer> set3 = list.stream().collect(Collectors.toSet());
        System.out.println("Set (stream): " + set3);
    }
}

The output of the above code is ?

List: [1, 2, 3, 3, 3, 4, 5]
Set (constructor): [1, 2, 3, 4, 5]
Set (addAll): [1, 2, 3, 4, 5]
Set (stream): [1, 2, 3, 4, 5]

Conclusion

All three methods produce the same result − duplicates are removed and only unique values remain. The constructor approach is the most concise, while the Stream approach offers more flexibility for additional transformations before collecting into a Set.