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Convert List to Set in Java

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  • Last Updated : 09 Jun, 2022
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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 the 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 are discussed:

Way 1: 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




// 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]

Way 2: 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




// 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& lt;
    T& gt;
    Set& lt;
    T& gt;
    convertListToSet(List& lt; T & gt; list)
    {
        // create a set from the List
        return new HashSet& lt;
        >
        (list);
    }
 
    public static void main(String args[])
    {
 
        // Create a stream of integers
        List& lt;
        String& gt; list = Arrays.asList("GeeksForGeeks",
                                          "Geeks",
                                          "forGeeks",
                                          "A computer portal",
                                          "for",
                                          "Geeks");
 
        // Print the List
        System.out.println(" List : " + list);
 
        // Convert List to stream
        Set& lt;
        String& gt;
        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]

Way 3: 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




// 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]

Way 4: 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




// 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]

Way 5: Using addAll() method in Collection package

In this, we can convert the list items to set by using addAll() method. For this, we have to import the package java.util.Collection

This addAll(Collection c) method is a boolean datatype. It is used to add the collection of one object to the collection of another object 

Java




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 using addAll() method
        HashSet<T> set = new HashSet<>();
        set.addAll(list);
        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]

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