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    How to Convert Set to List in Java

    Introduction to Convert Set to List

    Lists in Java are ordered collections of data, whereas sets are unordered collections of data. A list can have duplicate entries, but a set cannot. Both data structures are useful in different scenarios.

    Knowing how to convert a set into a list is useful as it transforms unordered data into ordered data.

    Initializing a Set

    Let’s initialize a set and add some elements to it.

    Copy Code 
    

    import java.util.*;

public class Main {

    public static void main(String[] args)
    {
        Set<Integer> a = new HashSet<>();
        a.add(1);
        a.add(2);
        a.add(3);
        a.add(1);
        System.out.println(a);
    }
}

    The add() method of HashSet adds elements to the set. Note that the elements are distinct. Sets are unordered, so there’s no way to get elements by their insertion order.

    Convert Set to List in Java

    Let’s convert the set into a list. There are multiple ways of doing so, each with subtle differences.

    1. List Constructor with Set Argument

    The most straightforward way is by passing the set as an argument while creating the list.

    Copy Code 
    

    import java.util.*;

public class Main {

    public static void main(String[] args)
    {
        Set<Integer> a = new HashSet<>();
        a.add(1);
        a.add(2);
        a.add(3);
        a.add(1);

        List<Integer> arr = new ArrayList<>(a);
        System.out.println(arr);
        System.out.println(arr.get(1));
    }
}

    Since the set has been converted to a list, the elements are now ordered. This means we can use the get() method to access elements by index.

    2. Using Conventional For Loop

    We can use a conventional for loop to explicitly copy the elements from the set to the list.

    Copy Code 
    

    import java.util.*;

public class Main {

    public static void main(String[] args)
    {
        Set<Integer> a = new HashSet<>();
        a.add(1);
        a.add(2);
        a.add(3);
        a.add(1);

        List<Integer> arr = new ArrayList<>();
        for (int i : a)
            arr.add(i);
        System.out.println(arr);
        System.out.println(arr.get(1));
    }
}

    The for loop iterates over the set and adds each element to the list.

    3. List addAll() Method

    The addAll() method of List adds multiple values to the list at once. It also works for adding elements of a set to a list.

    Copy Code 
    

    import java.util.*;

public class Main {

    public static void main(String[] args)
    {
        Set<Integer> a = new HashSet<>();
        a.add(1);
        a.add(2);
        a.add(3);
        a.add(1);

        List<Integer> arr = new ArrayList<>();
        arr.addAll(a);
        System.out.println(arr);
        System.out.println(arr.get(1));
    }
}

    4. Stream API collect() Method

    The Stream.collect() method, available from Java 8 onwards, collects all Stream elements into a List.

    Copy Code 
    

    import java.util.*;
import java.util.stream.Collectors;

public class Main {

    public static void main(String[] args)
    {
        Set<Integer> a = new HashSet<>();
        a.add(1);
        a.add(2);
        a.add(3);
        a.add(1);

        List<Integer> arr = a.stream().collect(Collectors.toList());
        System.out.println(arr);
        System.out.println(arr.get(1));
    }
}

    Note that Stream.collect() does not guarantee the type or thread-safety of the returned List.

    5. List.copyOf() Method

    From Java 10 onwards, the copyOf() method creates an unmodifiable list from a collection.

    Copy Code 
    

    import java.util.*;

public class Main {

    public static void main(String[] args)
    {
        Set<Integer> a = new HashSet<>();
        a.add(1);
        a.add(2);
        a.add(3);
        a.add(1);

        List<Integer> arr = List.copyOf(a);
        System.out.println(arr);
        System.out.println(arr.get(1));
    }
}

    Conclusion to Convert Set to List

    We explored multiple methods to convert a set into a list in Java. Each method has its advantages and limitations. For example, copyOf() creates an immutable list that doesn’t support null values, while addAll() and constructors are reliable for general use cases. Choose the method based on your specific requirements.