Java ConcurrentHashMap putIfAbsent() Method

🎓 Top 15 Udemy Courses (80-90% Discount): My Udemy Courses - Ramesh Fadatare — All my Udemy courses are real-time and project oriented courses.

▶️ Subscribe to My YouTube Channel (178K+ subscribers): Java Guides on YouTube

▶️ For AI, ChatGPT, Web, Tech, and Generative AI, subscribe to another channel: Ramesh Fadatare on YouTube

The ConcurrentHashMap.putIfAbsent() method in Java is used to insert a key-value pair into a ConcurrentHashMap only if the key is not already present in the map.

Table of Contents

1. Introduction
2. putIfAbsent Method Syntax
3. Examples
   • Adding Entries to a ConcurrentHashMap
   • Handling Existing Keys
4. Real-World Use Case
   • Example: Managing a Student Registration System
5. Conclusion

Introduction

The ConcurrentHashMap.putIfAbsent() method is a member of the ConcurrentHashMap class in Java. It allows you to insert a key-value pair into the map only if the key is not already associated with a value. The ConcurrentHashMap class is part of the java.util.concurrent package, designed for high concurrency and scalability.

putIfAbsent() Method Syntax

The syntax for the putIfAbsent method is as follows:

public V putIfAbsent(K key, V value) 

• The method takes two parameters:
  • key of type K, which represents the key to be inserted.
  • value of type V, which represents the value to be associated with the key.
• The method returns the current value associated with the specified key, or null if there was no mapping for the key.

Examples

Adding Entries to a ConcurrentHashMap

The putIfAbsent method can be used to add key-value pairs to a ConcurrentHashMap only if the key is not already present.

Example

import java.util.concurrent.ConcurrentHashMap; public class PutIfAbsentExample { public static void main(String[] args) { // Creating a ConcurrentHashMap with String keys and Integer values ConcurrentHashMap<String, Integer> people = new ConcurrentHashMap<>(); // Adding entries to the ConcurrentHashMap people.putIfAbsent("Ravi", 25); people.putIfAbsent("Priya", 30); people.putIfAbsent("Vijay", 35); // Printing the ConcurrentHashMap System.out.println("ConcurrentHashMap: " + people); } } 

Output:

ConcurrentHashMap: {Ravi=25, Priya=30, Vijay=35} 

Handling Existing Keys

The putIfAbsent method returns the current value associated with the specified key if the key is already present in the ConcurrentHashMap.

Example

import java.util.concurrent.ConcurrentHashMap; public class ExistingKeyExample { public static void main(String[] args) { // Creating a ConcurrentHashMap with String keys and Integer values ConcurrentHashMap<String, Integer> people = new ConcurrentHashMap<>(); // Adding entries to the ConcurrentHashMap people.put("Ravi", 25); people.put("Priya", 30); // Trying to add a new value for an existing key Integer existingValue = people.putIfAbsent("Ravi", 40); // Printing the results System.out.println("Existing value for 'Ravi': " + existingValue); System.out.println("ConcurrentHashMap: " + people); } } 

Output:

Existing value for 'Ravi': 25 ConcurrentHashMap: {Ravi=25, Priya=30} 

Real-World Use Case

Example: Managing a Student Registration System

A common real-world use case for ConcurrentHashMap is managing a student registration system where each student can register only once.

Example

import java.util.concurrent.ConcurrentHashMap; public class StudentRegistration { public static void main(String[] args) { // Creating a ConcurrentHashMap to manage student registrations ConcurrentHashMap<String, String> studentRegistrations = new ConcurrentHashMap<>(); // Adding student registrations to the ConcurrentHashMap studentRegistrations.putIfAbsent("Ravi", "Registered"); studentRegistrations.putIfAbsent("Priya", "Registered"); studentRegistrations.putIfAbsent("Vijay", "Registered"); // Attempting to register a student who is already registered String registrationStatus = studentRegistrations.putIfAbsent("Ravi", "Already Registered"); // Printing the results System.out.println("Registration status for 'Ravi': " + registrationStatus); System.out.println("Student Registrations: " + studentRegistrations); } } 

Output:

Registration status for 'Ravi': Registered Student Registrations: {Ravi=Registered, Priya=Registered, Vijay=Registered} 

In this example, ConcurrentHashMap is used to manage student registrations, ensuring that each student can register only once using the putIfAbsent method.

Conclusion

The ConcurrentHashMap.putIfAbsent() method in Java provides a way to insert key-value pairs into a ConcurrentHashMap only if the key is not already present, in a thread-safe manner. By understanding how to use this method, you can efficiently manage collections of key-value pairs in your Java applications, especially in concurrent environments. The method allows you to handle conditional insertions, making it a versatile tool for data management in multi-threaded scenarios.