Topic 13436 min readStructured Tutorial

Understanding HashSet in Java

Quick Answer

HashSet explains hashSet is a part of Java's Collection Framework and implements the Set interface.

Learning Objectives

Explain the purpose of HashSet in a practical learning context.
Identify the main ideas, terms, and decisions involved in HashSet.
Apply HashSet in a simple real-world scenario or practice task.

Introduction to HashSet

HashSet is a part of Java's Collection Framework and implements the Set interface.

It stores unique elements and does not maintain any order.

HashSet is widely used when you want to prevent duplicates and do not care about element order.

A Set is a Collection that contains no duplicate elements.

What is HashSet?

HashSet is a collection class that implements the Set interface backed by a hash table.

It allows null elements but only one null because duplicates are not allowed.

HashSet does not guarantee any specific iteration order of the elements.

Implements Set interface
Stores unique elements only
Allows one null element
No guaranteed order of elements
Not synchronized

Creating and Using a HashSet

You can create a HashSet by instantiating it with the default constructor or by passing a collection.

Common operations include adding, removing, and checking for elements.

HashSet uses the hashCode() and equals() methods to determine uniqueness.

HashSet<String> set = new HashSet<>();
set.add("Java");
set.contains("Java");
set.remove("Java");

Key Methods of HashSet

HashSet provides several useful methods inherited from the Set interface and Collection interface.

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

How HashSet Works Internally

HashSet internally uses a HashMap to store its elements.

Each element is stored as a key in the underlying HashMap with a constant dummy value.

The hashCode() of the element determines the bucket location in the hash table.

Collisions are handled using linked lists or balanced trees depending on Java version.

Uses HashMap internally
Elements are keys in the HashMap
hashCode() and equals() determine uniqueness
Handles collisions with chaining or trees

When to Use HashSet

Use HashSet when you need a collection of unique elements without duplicates.

It is ideal when order does not matter and fast lookup, insertion, and deletion are required.

Avoid HashSet if you need ordered elements; consider LinkedHashSet or TreeSet instead.

Fast operations: O(1) average time for add, remove, contains
No duplicates allowed
Unordered collection
Allows null element

Example: Basic HashSet Usage

Here is a simple example demonstrating common HashSet operations.

Practical Example

This example creates a HashSet, adds elements including a duplicate and null, checks for an element, and removes an element.

Examples

Basic HashSet Example

import java.util.HashSet;

public class HashSetExample {
    public static void main(String[] args) {
        HashSet<String> set = new HashSet<>();
        set.add("Java");
        set.add("Python");
        set.add("Java"); // Duplicate, will not be added
        set.add(null); // Allowed

        System.out.println("HashSet elements: " + set);
        System.out.println("Contains Python? " + set.contains("Python"));
        set.remove("Java");
        System.out.println("After removing Java: " + set);
    }
}

This example creates a HashSet, adds elements including a duplicate and null, checks for an element, and removes an element.

Best Practices

Always override hashCode() and equals() methods for custom objects stored in HashSet.
Use HashSet when you need fast lookup and no duplicates.
Avoid modifying objects stored in HashSet in a way that affects their hashCode or equals.
Use LinkedHashSet if you need predictable iteration order.
Use TreeSet if you need sorted elements.

Common Mistakes

Assuming HashSet maintains insertion order.
Storing mutable objects without proper hashCode and equals implementations.
Modifying objects in the set that affect their hashCode or equals after insertion.
Using HashSet when order or sorting is required.
Not handling null elements properly if your logic depends on them.

Hands-on Exercise

Create a HashSet of Integers

Write a Java program to create a HashSet of integers, add numbers 1 to 5, remove number 3, and print the set.

Expected output: [1, 2, 4, 5]

Hint: Use add(), remove(), and print the set using System.out.println().

Check for Duplicates Using HashSet

Write a method that takes an array of strings and returns true if there are duplicates using HashSet.

Expected output: true if duplicates exist, false otherwise

Hint: Add elements to HashSet and compare sizes.

Interview Questions

What is the difference between HashSet and TreeSet?

Interview

HashSet stores elements without any order and offers constant time performance for basic operations. TreeSet stores elements in sorted order and has logarithmic time performance.

How does HashSet ensure uniqueness of elements?

Interview

HashSet uses the hashCode() and equals() methods of objects to ensure that no duplicate elements are stored.

Can HashSet contain null elements?

Interview

Yes, HashSet allows one null element.