HashSet :

HashSet is a class in the Java Collections Framework that implements the Set interface. 

It is based on a hash table data structure, which provides several features and performance characteristics that make it a powerful choice for handling collections of unique elements.

 Let's explore its features, performance, and use cases with an example:


Features of HashSet:

Uniqueness: HashSet ensures that each element in the set is unique. It does not allow duplicate entries, and if you attempt to add a duplicate element, it will be ignored.


Unordered: Elements in a HashSet are not stored in any specific order. The set does not maintain the insertion order, so iteration through a HashSet does not guarantee a specific sequence.


Fast Operations: HashSet offers fast performance for basic operations like add, remove, and contains. These operations have constant time complexity on average (O(1)) due to its hash table implementation.


Backed by HashMap: Internally, HashSet is backed by a HashMap, where elements serve as keys, and a constant dummy value is used as values. This mapping is why HashSet ensures uniqueness by leveraging the hash table's property of not allowing duplicate keys.


Performance of HashSet:

Add: Adding an element to a HashSet has an average time complexity of O(1) due to its hash table implementation. In some rare cases, when hash collisions occur frequently, the time complexity might approach O(n), where n is the number of elements in the set.


Remove: Removing an element from a HashSet has an average time complexity of O(1).


Contains (Lookup): Checking whether an element exists in a HashSet has an average time complexity of O(1).


Use Cases of HashSet:

Removing duplicates: HashSet is commonly used to remove duplicates from a collection. By adding elements to a HashSet, you can easily eliminate duplicates, as it automatically ensures uniqueness.


Fast Membership checks: HashSet's efficient lookup performance makes it suitable for scenarios where you frequently need to check if an element exists in a collection.


Caching: HashSet can be used in caching mechanisms, where it stores unique items to speed up retrieval and minimize redundant computations.


Implementing custom data structures: HashSet can be used as a building block to create custom data structures or algorithms that require a set of unique elements.


Checking for intersections or unions: HashSet's set-based operations are useful when you need to find common elements between multiple sets or combine sets.

Example of HashSet:


import java.util.HashSet;
import java.util.Set; 
public class HashSetExample {
public static void main(String[] args) {
// Create a HashSet of Strings 
 Set<String> uniqueWords = new HashSet<>();
// Add elements to the HashSet

        uniqueWords.add("apple");
 uniqueWords.add("banana");
 uniqueWords.add("apple"); 
// Attempt to add a duplicate element 
// Print the HashSet

        System.out.println("Unique Words: " + uniqueWords);
 }
 }

Output:

unique Words: [banana, apple]

In this example, we create a HashSet called "uniqueWords" to store unique words. We add three strings: "apple," "banana," and "apple" (which is a duplicate). As HashSet ensures uniqueness, it stores only one occurrence of "apple." When we print the HashSet, we see that it contains two elements: "banana" and "apple." The order of elements in the HashSet is not guaranteed to match the insertion order.