Can we convert a List to Set and back in Java?

Yes, Java allows easy conversion between List and Set using their constructors. Converting a List to a Set eliminates duplicate entries, and converting a Set back to a List gives a list with only unique values.

List to Set

Pass the list to the HashSet constructor. Duplicates are automatically removed −

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

Set to List

Pass the set to the ArrayList constructor to get a modifiable list −

List<Integer> list = new ArrayList<>(set);

Example

The following example demonstrates converting a List to a Set and back ?

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

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

        // List to Set (duplicates removed)
        Set<Integer> set = new HashSet<>(list);
        System.out.println("Set: " + set);

        // Set back to List (unique values only)
        List<Integer> uniqueList = new ArrayList<>(set);
        System.out.println("List from Set: " + uniqueList);
    }
}

The output of the above code is ?

Original List: [1, 2, 3, 3, 3, 4, 5]
Set: [1, 2, 3, 4, 5]
List from Set: [1, 2, 3, 4, 5]

The original list had three copies of the value 3. After converting to a Set and back, the resulting list contains only unique values.

Conclusion

Use new HashSet<>(list) for List-to-Set conversion and new ArrayList<>(set) for Set-to-List conversion. This is a common technique for removing duplicates from a list. Note that HashSet does not preserve insertion order; use LinkedHashSet if order needs to be maintained.