HashSet in Java

• HashSet class uses a hash table for storing the elements.
• It implements the set interface.
• Duplicate values are not allowed.
• Before storing any object, the hashset uses the hashCode() and equals() method to check any duplicate entry in the hash table.
• Allows null value.
• Best suited for search operations.

Constructors Of HashSet :

1. HashSet(): This constructor is used to create a new empty HashSet that can store 16 elements and have a load factor of 0.75.
2. HashSet(int initialCapacity): This constructor is used to create a new empty HashSet which has the capacity to store the specified number of elements and having a load factor of 0.75.
3. HashSet(int initialCapacity, float loadFactor): This constructor is used to create a new empty HashSet with the capacity & load factor equal to specified integer and float value.
4. HashSet(Collection<? extends E> c): This constructor is used to create a HashSet using the elements of collection c.

Performing Various Operations On HashSet :

1. Inserting elements :

   • add() method is used to add elements in HashSet.
   • The insertion order of the elements does not remains preserved in HashSet.
   • All the duplicate elements are ignored because the set contains only unique values.

     Example :

     import java.util.*;
     public class CWH extends Thread{
         public static void main(String[] args) {
             HashSet<Integer> myHashSet = new HashSet<>(6, 0.5f);
             myHashSet.add(6);
             myHashSet.add(8);
             myHashSet.add(3);
             myHashSet.add(11);
             myHashSet.add(11); // This element will be ignored
     
             System.out.println(myHashSet);
     
         }
     }
     

Output :

[8, 3, 11, 6]

Removing elements from the HashSet : 

• remove() method is used to delete the specified element from the HashSet.
• This method does not throws any exception if the specified element is not present in the HashSet.

  Example :

  import java.util.*;
  public class CWH extends Thread{
      public static void main(String[] args) {
          HashSet<Integer> myHashSet = new HashSet<>(6, 0.5f);
          myHashSet.add(6);
          myHashSet.add(8);
          myHashSet.add(3);
          myHashSet.add(11);
          myHashSet.add(11); // This element will be ignored
  
          System.out.println("myHashSet before removing any element : " + myHashSet);
          myHashSet.remove(3); //deletes 3 from the hashset
          System.out.println("myHashSet after removing a element : " + myHashSet);
  
      }
  }
  

Output :

myHashSet before removing any element : [8, 3, 11, 6]
myHashSet after removing a element : [8, 11, 6]

Checking if the HashSet is empty or not :  

• • isEmpty() method is used to check if there is any object in the HashSet or not.
  • This method returns a boolean value.

    Example :

    import java.util.*;
    public class CWH extends Thread{
        public static void main(String[] args) {
            HashSet<Integer> myHashSet = new HashSet<>(6, 0.5f);
            myHashSet.add(6);
            myHashSet.add(8);
            myHashSet.add(3);
            myHashSet.add(11);
    
            HashSet<Integer> myHashSet1 = new HashSet<>();
    
            System.out.println(myHashSet.isEmpty());
            System.out.println(myHashSet1.isEmpty());
    
    
        }
    }
    

Output :

false
true

Removing all the elements from the HashSet : 

• clear() method is used to remove all the elements from the HashSet at once.

  Example :

  import java.util.*;
  public class CWH extends Thread{
      public static void main(String[] args) {
          HashSet<Integer> myHashSet = new HashSet<>(6, 0.5f);
          myHashSet.add(16);
          myHashSet.add(33);
          myHashSet.add(78);
          myHashSet.add(19);
          myHashSet.add(29);
          myHashSet.add(10); 
  
          System.out.println("myHashSet before : " + myHashSet);
          myHashSet.clear(); //deletes all the elements from the hashset
          System.out.println("myHashSet after  : " + myHashSet);
          
      }
  }
  

Output :

myHashSet before : [16, 33, 19, 10, 29, 78]
myHashSet after  : []

Printing the size of the HashSet :

• size() method is used to get the size of the HashSet.

  Example :

  import java.util.*;
  public class CWH extends Thread{
      public static void main(String[] args) {
          HashSet<Integer> myHashSet = new HashSet<>(6, 0.5f);
          myHashSet.add(16);
          myHashSet.add(33);
          myHashSet.add(78);
          myHashSet.add(19);
          myHashSet.add(29);
          myHashSet.add(10); 
  
          System.out.println("The size of myHashSet is : " + myHashSet.size());
  
      }
  }
  

Output :

The size of myHashSet is : 6

Code as described/written in the video :

package com.company;

import java.util.HashSet;

public class cwh_95_set {
    public static void main(String[] args) {
        HashSet<Integer> myHashSet = new HashSet<>(6, 0.5f);
        myHashSet.add(6);
        myHashSet.add(8);
        myHashSet.add(3);
        myHashSet.add(11);
        myHashSet.add(11);
        System.out.println(myHashSet);
    }
}