Convert List to Set in Java

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. Few important features of Java Set interface are as follows:

The set interface is an unordered collection of objects in which duplicate values cannot be stored.
The Java Set does not provide control over the position of insertion or deletion of elements.
Basically, Set is implemented by HashSet, LinkedHashSet or TreeSet (sorted representation).
Set has various methods to add, remove clear, size, etc to enhance the usage of this interface.

In this post different methods to convert a List interface to Set in Java is discussed:

Naive Approach: The naive solution is to create an empty set and add every element of the specified list to the newly created set.

Below is the implementation of the naive approach:

// Java Program to convert 
// List to Set in Java 8 
  
import java.util.*; 
import java.util.stream.*; 
import java.util.function.Function; 
  
class GFG { 
  
    // Generic function to convert list to set 
    public static <T> Set<T> convertListToSet(List<T> list) 
    { 
        // create an empty set 
        Set<T> set = new HashSet<>(); 
  
        // Add each element of list into the set 
        for (T t : list) 
            set.add(t); 
  
        // return the set 
        return set; 
    } 
  
    public static void main(String args[]) 
    { 
  
        // Create a stream of integers 
        List<String> list = Arrays.asList("GeeksForGeeks", 
                                          "Geeks", 
                                          "forGeeks", 
                                          "A computer portal", 
                                          "for", 
                                          "Geeks"); 
  
        // Print the List 
        System.out.println("List: " + list); 
  
        // Convert List to stream 
        Set<String> set = convertListToSet(list); 
  
        // Print the Set 
        System.out.println("Set from List: " + set); 
    } 
} 

Output:

List: [GeeksForGeeks, Geeks, forGeeks, A computer portal, for, Geeks]
Set from List: [Geeks, for, GeeksForGeeks, A computer portal, forGeeks]

Using Constructor: HashSet constructor can take another collection object to construct a new set containing the elements of the specied list.

Below is the implementation of the above approach:

// Java Program to convert 
// List to Set in Java 8 
  
import java.util.*; 
import java.util.stream.*; 
  
class GFG { 
  
    // Generic function to convert list to set 
    public static <T> Set<T> convertListToSet(List<T> list) 
    { 
        // create a set from the List 
        return new HashSet<>(list); 
    } 
  
    public static void main(String args[]) 
    { 
  
        // Create a stream of integers 
        List<String> list = Arrays.asList("GeeksForGeeks", 
                                          "Geeks", 
                                          "forGeeks", 
                                          "A computer portal", 
                                          "for", 
                                          "Geeks"); 
  
        // Print the List 
        System.out.println("List: " + list); 
  
        // Convert List to stream 
        Set<String> set = convertListToSet(list); 
  
        // Print the Set 
        System.out.println("Set from List: " + set); 
    } 
} 

Output:

List: [GeeksForGeeks, Geeks, forGeeks, A computer portal, for, Geeks]
Set from List: [Geeks, for, GeeksForGeeks, A computer portal, forGeeks]

Using Java 8 Stream API: HashSet constructor can take another collection object to construct a new set containing the elements of the specied list.

Below is the implementation of the above approach:

// Java Program to convert 
// List to Set in Java 8 
  
import java.util.*; 
import java.util.stream.*; 
  
class GFG { 
  
    // Generic function to convert list to set 
    public static <T> Set<T> convertListToSet(List<T> list) 
    { 
        // create a set from the List 
        return list.stream().collect(Collectors.toSet()); 
    } 
  
    public static void main(String args[]) 
    { 
  
        // Create a stream of integers 
        List<String> list = Arrays.asList("GeeksForGeeks", 
                                          "Geeks", 
                                          "forGeeks", 
                                          "A computer portal", 
                                          "for", 
                                          "Geeks"); 
  
        // Print the List 
        System.out.println("List: " + list); 
  
        // Convert List to stream 
        Set<String> set = convertListToSet(list); 
  
        // Print the Set 
        System.out.println("Set from List: " + set); 
    } 
} 

Output:

List: [GeeksForGeeks, Geeks, forGeeks, A computer portal, for, Geeks]
Set from List: [Geeks, for, GeeksForGeeks, A computer portal, forGeeks]

Using Guava Sets.newHashSet(): Sets.newHashSet() creates a mutable HashSet instance containing the elements of the specified list.

Below is the implementation of the above approach:

// Java Program to convert 
// List to Set in Java 8 
  
import java.util.*; 
import java.util.stream.*; 
  
class GFG { 
  
    // Generic function to convert list to set 
    public static <T> Set<T> convertListToSet(List<T> list) 
    { 
        // create a set from the List 
        return return Sets.newHashSet(list); 
    } 
  
    public static void main(String args[]) 
    { 
  
        // Create a stream of integers 
        List<String> list = Arrays.asList("GeeksForGeeks", 
                                          "Geeks", 
                                          "forGeeks", 
                                          "A computer portal", 
                                          "for", 
                                          "Geeks"); 
  
        // Print the List 
        System.out.println("List: " + list); 
  
        // Convert List to stream 
        Set<String> set = convertListToSet(list); 
  
        // Print the Set 
        System.out.println("Set from List: " + set); 
    } 
} 

Output:

List: [GeeksForGeeks, Geeks, forGeeks, A computer portal, for, Geeks]
Set from List: [Geeks, for, GeeksForGeeks, A computer portal, forGeeks]

My Personal Notes

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