Java HashSet add() Method

The HashSet.add() method in Java is used to add elements to a HashSet. This guide will cover the method's usage, explain how it works, and provide real-world use cases to demonstrate its functionality.

Table of Contents

1. Introduction
2. add Method Syntax
3. Examples
   • Basic Example
   • Handling Duplicates
   • Real-World Use Cases
     • Ensuring Unique Usernames
     • Tracking Unique IP Addresses
     • Collecting Unique Error Codes
4. Conclusion

Introduction

The HashSet class in Java is part of the Java Collections Framework and implements the Set interface. A HashSet is used to store unique elements and provides constant-time performance for basic operations like add, remove, contains, and size. The HashSet.add() method is used to add elements to the set, ensuring that no duplicates are added.

add() Method Syntax

The syntax for the add method is as follows:

public boolean add(E e)

• e: The element to be added to the set.
• Returns: true if the element was added to the set (it was not already present); false otherwise.

Examples

Basic Example

In this example, we'll use the add method to add elements to a HashSet.

Example

import java.util.HashSet;

public class HashSetAddExample {
    public static void main(String[] args) {
        HashSet<String> set = new HashSet<>();
        set.add("Java");
        set.add("Python");
        set.add("C");
        set.add("JavaScript");

        System.out.println("HashSet after adding elements: " + set);
    }
}

Output:

HashSet after adding elements: [Java, JavaScript, Python, C]

Handling Duplicates

The HashSet ensures that no duplicates are added. If an element is already present in the set, the add method returns false.

Example

import java.util.HashSet;

public class HashSetDuplicateExample {
    public static void main(String[] args) {
        HashSet<String> set = new HashSet<>();
        set.add("Java");
        set.add("Python");
        set.add("C");

        // Attempt to add a duplicate element
        boolean added = set.add("Java");

        System.out.println("Was 'Java' added again? " + added);
        System.out.println("HashSet after attempting to add duplicate: " + set);
    }
}

Output:

Was 'Java' added again? false
HashSet after attempting to add duplicate: [Java, C, Python]

Real-World Use Cases

Ensuring Unique Usernames

In a web application, you might want to ensure that usernames are unique. You can use a HashSet to store usernames and check for duplicates when a new user registers.

Example

import java.util.HashSet;

public class UniqueUsernames {
    public static void main(String[] args) {
        HashSet<String> usernames = new HashSet<>();

        // Simulating user registration
        addUser(usernames, "john_doe");
        addUser(usernames, "jane_smith");
        addUser(usernames, "john_doe"); // Duplicate

        System.out.println("Usernames: " + usernames);
    }

    private static void addUser(HashSet<String> usernames, String username) {
        if (usernames.add(username)) {
            System.out.println("Username '" + username + "' added.");
        } else {
            System.out.println("Username '" + username + "' is already taken.");
        }
    }
}

Output:

Username 'john_doe' added.
Username 'jane_smith' added.
Username 'john_doe' is already taken.
Usernames: [john_doe, jane_smith]

Tracking Unique IP Addresses

In a network monitoring application, you might want to track unique IP addresses that have accessed a server.

Example

import java.util.HashSet;

public class UniqueIPAddresses {
    public static void main(String[] args) {
        HashSet<String> ipAddresses = new HashSet<>();

        // Simulating server access
        logAccess(ipAddresses, "192.168.1.1");
        logAccess(ipAddresses, "192.168.1.2");
        logAccess(ipAddresses, "192.168.1.1"); // Duplicate

        System.out.println("Unique IP addresses: " + ipAddresses);
    }

    private static void logAccess(HashSet<String> ipAddresses, String ipAddress) {
        if (ipAddresses.add(ipAddress)) {
            System.out.println("IP address '" + ipAddress + "' logged.");
        } else {
            System.out.println("IP address '" + ipAddress + "' is already logged.");
        }
    }
}

Output:

IP address '192.168.1.1' logged.
IP address '192.168.1.2' logged.
IP address '192.168.1.1' is already logged.
Unique IP addresses: [192.168.1.1, 192.168.1.2]

Collecting Unique Error Codes

In a logging system, you might want to collect unique error codes to avoid duplicate error entries.

Example

import java.util.HashSet;

public class UniqueErrorCodes {
    public static void main(String[] args) {
        HashSet<Integer> errorCodes = new HashSet<>();

        // Simulating error logging
        logError(errorCodes, 404);
        logError(errorCodes, 500);
        logError(errorCodes, 404); // Duplicate

        System.out.println("Unique error codes: " + errorCodes);
    }

    private static void logError(HashSet<Integer> errorCodes, int errorCode) {
        if (errorCodes.add(errorCode)) {
            System.out.println("Error code '" + errorCode + "' logged.");
        } else {
            System.out.println("Error code '" + errorCode + "' is already logged.");
        }
    }
}

Output:

Error code '404' logged.
Error code '500' logged.
Error code '404' is already logged.
Unique error codes: [404, 500]

Conclusion

The HashSet.add() method in Java provides a way to add elements to a HashSet while ensuring that no duplicates are present. This method is particularly useful in scenarios where uniqueness is required, such as ensuring unique usernames, tracking unique IP addresses, and collecting unique error codes. By understanding how to use this method, you can efficiently manage collections of unique elements in your Java applications.