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Initializing HashSet in Java
  • Difficulty Level : Easy
  • Last Updated : 11 Dec, 2018

Set in Java is an interface which extends Collection. It is an unordered collection of objects in which duplicate values cannot be stored.
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.

Method 1: Using Constructor:
In this method first we create an array then convert it to a list and then pass it to the HashSet constructor that accepts another collection.
Integer elements of the set are printed in sorted order.




// Java code for initializing a Set
import java.util.*;
public class Set_example {
    public static void main(String[] args)
    {
        // creating and initializing an array (of non
        // primitive type)
        Integer arr[] = { 5, 6, 7, 8, 1, 2, 3, 4, 3 };
  
        // Set demonstration using HashSet Constructor
        Set<Integer> set = new HashSet<>(Arrays.asList(arr));
  
        // Duplicate elements are not printed in a set.
        System.out.println(set);
    }
}

Method 2 using Collections:
Collections class consists of several methods that operate on collections.
a) Collection.addAll() : adds all the specified elements to the specified collection of the specified type.
b) Collections.unmodifiableSet() : adds the elements and returns an unmodifiable view of the specified set.




// Java code for initializing a Set
import java.util.*;
public class Set_example {
    public static void main(String[] args)
    {
        // creating and initializing an array (of non 
        // primitive type)
        Integer arr[] = { 5, 6, 7, 8, 1, 2, 3, 4, 3 };
  
        // Set deonstration using Collections.addAll
        Set<Integer> set = Collections.<Integer> emptySet();
        Collections.addAll(set =
                    new HashSet<Integer>(Arrays.asList(arr)));
  
        // initializing set using Collections.unmodifiable set
        Set<Integer> set2 = 
             Collections.unmodifiableSet(new HashSet<Integer>
                                         (Arrays.asList(arr)));
  
        // Duplicate elements are not printed in a set.
        System.out.println(set);
    }
}

Method 3: using .add() each time
Create a set and using .add() method we add the elements into the set




// Java code for initializing a Set
import java.util.*;
public class Set_example {
  
    public static void main(String[] args)
    {
        // Create a set
        Set<Integer> set = new HashSet<Integer>();
  
        // Add some elements to the set
        set.add(1);
        set.add(2);
        set.add(3);
        set.add(4);
        set.add(5);
        set.add(6);
        set.add(7);
        set.add(8);
  
        // Adding a duplicate element has no effect
        set.add(3);
        System.out.println(set);
    }
}

Output:



[1, 2, 3, 4, 5, 6, 7, 8]

This article is contributed by Nikita Tiwari. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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