HashSet in Java

A HashSet in Java is a part of the Java Collections Framework and is used to store a collection of unique elements. It is backed by a hash table (actually a HashMap instance), and it does not guarantee any specific order of its elements.

Key Features of HashSet:

• No duplicates: It does not allow duplicate elements.
• Null allowed: It allows the null element.
• Unordered: The order of elements is not guaranteed.
• Fast operations: Basic operations like add, remove, contains, and size have constant time complexity O(1) on average.
• Not synchronised: It is not thread-safe by default.

Thread-safe means that a piece of code or a data structure can be safely used by multiple threads at the same time, without causing data corruption or unexpected behavior.

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Thread: In Java (and many other languages), a thread is a separate path of execution. Multiple threads can run in parallel in a program.

Usage Example:

import java.util.HashSet;

public class HashSetExample {
    public static void main(String[] args) {
        // Create a HashSet of Strings
        HashSet<String> set = new HashSet<>();

        // Add elements
        set.add("Apple");
        set.add("Banana");
        set.add("Orange");
        set.add("Apple"); // Duplicate, will not be added

        // Check if an element exists
        System.out.println(set.contains("Banana")); // true

        // Remove an element
        set.remove("Orange");

        // Iterate over elements
        for (String fruit : set) {
            System.out.println(fruit);
        }

        // Size of the set
        System.out.println("Set size: " + set.size());
    }
}

Common Methods

• add(E e): Adds the specified element if it is not already present.
• remove(Object o): Removes the specified element if present.
• contains(Object o): Returns true if the set contains the specified element.
• size(): Returns the number of elements in the set.
• clear(): Removes all elements from the set.
• isEmpty(): Checks if the set is empty.

When to Use HashSet

• When you need a collection of unique elements.
• When you do not care about the order of elements.
• When you need fast lookup, insertion, and deletion.