Java Program to Convert List to HashSet - GeeksforGeeks

15 Nov, 2021
The List interface provides a way to store the ordered 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.
The HashSet class permits the null element. The class also offers constant time performance for the basic operations like add, remove, contains, and size assuming the hash function disperses the elements properly among the buckets, which we shall see further in the article.  
Note: The set doesn’t allow to store duplicate values. Therefore, any duplicate value in list will be ignored. 
The ways to convert List to HashSet :
Passing List Object as parameter in HashSet.Adding each element of List into HashSet using loop.Using addAll() Method of Set class.Using stream in Java
Passing List Object as parameter in HashSet.
Adding each element of List into HashSet using loop.
Using addAll() Method of Set class.
Using stream in Java
Method 1: Passing List Object as parameter in HashSet
We use the HashSet constructor for converting it to List.
Java
// Java program to demonstrate conversion of// list to set using constructor import java.util.*;class GFG {    public static void main(String[] args)    {        // Create a List        List<String> L = new ArrayList<String>();               // Add values to the List        L.add("Aragorn");        L.add("Gandalf");        L.add("Legolas");        L.add("Frodo");               // Create a Set and pass List object as parameter        HashSet<String> S = new HashSet<String>(L);               // Print values of Set        System.out.println("HashSet Elements are : ");               // since the set is of string type, create a String        // object to iterate through set        for (String ob : S)        {            System.out.println(ob);        }    }}
HashSet Elements are : 
Aragorn
Frodo
Gandalf
Legolas
Method 2: Adding each element of List into HashSet using loop 
We simply create a List. We traverse the given List and one by one add elements to the Set.
Java
// Java program to demonstrate conversion of// list to set using simple traversal import java.util.*;class GFG {    public static void main(String[] args)    {        // Create a List        List<Integer> L = new ArrayList<Integer>();               // Add values to the List        L.add(1);        L.add(4);        L.add(30);        L.add(100);        L.add(15);        L.add(30);               // Create a Set and pass List object as parameter        HashSet<Integer> S = new HashSet<Integer>();               // add each element of list into set        for (Integer ob : L)        {            S.add(ob);        }               // Print values of Set        System.out.println("HashSet Elements are : ");               // Create an Object ob that will automatically        // identify the type of object of HashSet to iterate        // through set        for (Object ob : S)        {            System.out.println(ob);        }    }}
HashSet Elements are : 
1
4
100
30
15
Method 3: Using addAll() Method of Set class
The java.util.Set.addAll(Collection C) method is used to append all of the elements from the mentioned collection to the existing set. The elements are added randomly without following any specific order.
Syntax:
boolean addAll(Collection C)
Parameters: The parameter C is a collection of any type that is to be added to the set.
Return Value: The method returns true if it successfully appends the elements of the collection C to this Set otherwise it returns False.
Java
// Java program to demonstrate conversion of// Set to array using addAll() method. import java.util.*;class GFG {    public static void main(String[] args)    {        // Create a List        List<Integer> L = new ArrayList<Integer>();               // Add values to the List        L.add(1);        L.add(4);        L.add(30);        L.add(100);        L.add(15);        L.add(30);               Set<Integer> S = new HashSet<Integer>();               // Use addAll() method        S.addAll(L);               // Print values of Set        System.out.println("HashSet Elements are : ");               // Create an Object ob that will automatically        // identify the type of object of HashSet to iterate        // through set        for (Object ob : S)        {            System.out.println(ob);        }    }}
HashSet Elements are : 
1
4
100
30
15
Method 4: Using stream in Java
Note: Stream only works in Java8 or versions above it.
We use stream in java to convert the given list to stream, then stream to set. This works only in Java 8 or versions after that.
Java
// Java program to demonstrate conversion of// Set to list using stream import java.util.*;class GFG {    public static void main(String[] args)    {        // Create a List        List<String> L = new ArrayList<String>();               // Add values to the List        L.add("Rohan");        L.add("Ritik");        L.add("Yogesh");        L.add("Sangeeta");        L.add("Palak");        L.add("Laxmi");               // create a stream from List and convert it into a        // Set        Set<String> S = L.stream().collect(Collectors.toSet());               // Print values of Set        System.out.println("HashSet Elements are : ");               // Create an Object ob that will automatically        // identify the type of object of HashSet to iterate        // through set        for (String ob : S)        {            System.out.println(ob);        }    }}
Output:
HashSet Elements are :
1
4
100
30
15
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