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Program to Convert Set to List in Java

  • Difficulty Level : Medium
  • Last Updated : 08 Sep, 2021

Java Set is a part of java.util package and extends java.util.Collection interface. It does not allow the use of duplicate elements and at max can accommodate only one null element.

The List is a child interface of Collection. It is an ordered collection of objects in which duplicate values can be stored. Since List preserves the insertion order, it allows positional access and insertion of elements. List Interface is implemented by ArrayList, LinkedList, Vector and Stack classes.

Below are methods to convert Set to List in Java.

  1. Brute Force or Naive method: This method includes creating an empty list and add every element of the set to it.

    Algorithm:

    1. Get the Set to be converted.
    2. Create an empty list
    3. Push each element in the set into the list
    4. Return the formed List

    Program:






    // Java Program to convert
    // Set to List in Java 8
      
    import java.util.*;
    import java.util.stream.*;
      
    class GFG {
      
        // Generic function to convert set to list
        public static <T> List<T> convertSetToList(Set<T> set)
        {
            // create an empty list
            List<T> list = new ArrayList<>();
      
            // push each element in the set into the list
            for (T t : set)
                list.add(t);
      
            // return the list
            return list;
        }
      
        public static void main(String args[])
        {
      
            // Create a Set using HashSet
            Set<String> hash_Set = new HashSet<String>();
      
            // Add elements to set
            hash_Set.add("Geeks");
            hash_Set.add("For");
            hash_Set.add("Geeks");
            hash_Set.add("Example");
            hash_Set.add("Set");
      
            // Print the Set
            System.out.println("Set: " + hash_Set);
      
            // construct a new List from Set
            List<String> list = convertSetToList(hash_Set);
      
            // Print the List
            System.out.println("List: " + list);
        }
    }
    Output:
    Set: [Set, Example, Geeks, For]
    List: [Set, Example, Geeks, For]
    
  2. With help of Constructor: Collections in Java have a constructor in which the direct set can be passed to create a List from it.

    Algorithm:

    1. Get the Set to be converted.
    2. Create a list by passing the set as parameter to the constructor.
    3. Return the formed List

    Program:




    // Java Program to convert
    // Set to List in Java 8
      
    import java.util.*;
    import java.util.stream.*;
      
    class GFG {
      
        // Generic function to convert set to list
        public static <T> List<T> convertSetToList(Set<T> set)
        {
            // create a list from Set
            List<T> list = new ArrayList<>(set);
      
            // return the list
            return list;
        }
      
        public static void main(String args[])
        {
      
            // Create a Set using HashSet
            Set<String> hash_Set = new HashSet<String>();
      
            // Add elements to set
            hash_Set.add("Geeks");
            hash_Set.add("For");
            hash_Set.add("Geeks");
            hash_Set.add("Example");
            hash_Set.add("Set");
      
            // Print the Set
            System.out.println("Set: " + hash_Set);
      
            // construct a new List from Set
            List<String> list = convertSetToList(hash_Set);
      
            // Print the List
            System.out.println("List: " + list);
        }
    }
    Output:
    Set: [Set, Example, Geeks, For]
    List: [Set, Example, Geeks, For]
    
  3. Using Java 8 Stream: In Java 8, Stream can be used convert a set to a list by converting the set to a sequential Stream using Set.stream() and using a Collector to collect the input elements into a new List.

    Algorithm:

    1. Get the HashMap to be converted.
    2. Create stream from the Set
    3. Convert the set to list and collect it
    4. Return the collected List

    Program:




    // Java Program to convert
    // HashMap to TreeMap in Java 8
      
    import java.util.*;
    import java.util.stream.*;
      
    class GFG {
      
        // Generic function to convert set to list
        public static <T> List<T> convertSetToList(Set<T> set)
        {
            // create a list from Set
            return set
      
                // Create stream from the Set
                .stream()
      
                // Convert the set to list and collect it
                .collect(Collectors.toList());
        }
      
        public static void main(String args[])
        {
      
            // Create a Set using HashSet
            Set<String> hash_Set = new HashSet<String>();
      
            // Add elements to set
            hash_Set.add("Geeks");
            hash_Set.add("For");
            hash_Set.add("Geeks");
            hash_Set.add("Example");
            hash_Set.add("Set");
      
            // Print the Set
            System.out.println("Set: " + hash_Set);
      
            // construct a new List from Set
            List<String> list = convertSetToList(hash_Set);
      
            // Print the List
            System.out.println("List: " + list);
        }
    }
    Output:
    Set: [Set, Example, Geeks, For]
    List: [Set, Example, Geeks, For]
    
  4. Using Guava Library List.newArrayList(set): Lists.newArrayList(set) creates a mutable ArrayList instance containing the elements of the specified set.

    Algorithm:

    1. Get the Set to be converted.
    2. Create a new List using Lists.newArrayList() by passing the set as parameter to this function of Guava library
    3. Return the formed List

    Program:




    // Java Program to convert
    // HashMap to TreeMap in Java 8
      
    import java.util.*;
    import java.util.stream.*;
      
    class GFG {
      
        // Generic function to convert set to list
        public static <T> List<T> convertSetToList(Set<T> set)
        {
            // create a list from Set
            return Lists.newArrayList(set);
        }
      
        public static void main(String args[])
        {
      
            // Create a Set using HashSet
            Set<String> hash_Set = new HashSet<String>();
      
            // Add elements to set
            hash_Set.add("Geeks");
            hash_Set.add("For");
            hash_Set.add("Geeks");
            hash_Set.add("Example");
            hash_Set.add("Set");
      
            // Print the Set
            System.out.println("Set: " + hash_Set);
      
            // construct a new List from Set
            List<String> list = convertSetToList(hash_Set);
      
            // Print the List
            System.out.println("List: " + list);
        }
    }

    Output:

    Set: [Set, Example, Geeks, For]
    List: [Set, Example, Geeks, For]
    

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