Java CopyOnWriteArrayList addAll() Method

📘 Premium Read: Access my best content on Medium member-only articles — deep dives into Java, Spring Boot, Microservices, backend architecture, interview preparation, career advice, and industry-standard best practices.

🎓 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 (176K+ subscribers): Java Guides on YouTube

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

The CopyOnWriteArrayList.addAll() method in Java is used to add all the elements from a specified collection to the CopyOnWriteArrayList.

Table of Contents

1. Introduction
2. addAll Method Syntax
3. Examples
   • Adding All Elements from Another Collection
   • Adding All Elements at a Specific Position
4. Real-World Use Case
   • Example: Merging User Lists in a Concurrent Application
5. Conclusion

Introduction

The CopyOnWriteArrayList is a thread-safe variant of ArrayList in Java. It is part of the java.util.concurrent package and is designed for scenarios where read operations are more frequent than write operations. The addAll method allows you to add all the elements from another collection to the CopyOnWriteArrayList. The CopyOnWriteArrayList achieves thread safety by creating a new copy of the array whenever it is modified.

addAll() Method Syntax

There are two variations of the addAll method:

Basic Add All

public boolean addAll(Collection<? extends E> c) 

• The method takes one parameter:
  • c of type Collection<? extends E>, which represents the collection containing elements to be added to the list.
• The method returns true if the list changed as a result of the call.

Add All at a Specific Position

public boolean addAll(int index, Collection<? extends E> c) 

• The method takes two parameters:
  • index of type int, which represents the position at which to insert the first element from the specified collection.
  • c of type Collection<? extends E>, which represents the collection containing elements to be added to the list.
• The method returns true if the list changed as a result of the call.

Examples

Adding All Elements from Another Collection

The basic addAll method can be used to add all elements from another collection to a CopyOnWriteArrayList.

Example

import java.util.ArrayList; import java.util.concurrent.CopyOnWriteArrayList; public class AddAllExample { public static void main(String[] args) { // Creating a CopyOnWriteArrayList with String elements CopyOnWriteArrayList<String> names = new CopyOnWriteArrayList<>(); // Adding elements to the CopyOnWriteArrayList names.add("Ravi"); names.add("Priya"); // Creating another collection with String elements ArrayList<String> newNames = new ArrayList<>(); newNames.add("Vijay"); newNames.add("Anita"); // Adding all elements from newNames to names names.addAll(newNames); // Printing the CopyOnWriteArrayList System.out.println("CopyOnWriteArrayList: " + names); } } 

Output:

CopyOnWriteArrayList: [Ravi, Priya, Vijay, Anita] 

Adding All Elements at a Specific Position

The addAll method can also be used to add elements from another collection at a specific position in the CopyOnWriteArrayList.

Example

import java.util.ArrayList; import java.util.concurrent.CopyOnWriteArrayList; public class AddAllAtIndexExample { public static void main(String[] args) { // Creating a CopyOnWriteArrayList with String elements CopyOnWriteArrayList<String> names = new CopyOnWriteArrayList<>(); // Adding elements to the CopyOnWriteArrayList names.add("Ravi"); names.add("Priya"); // Creating another collection with String elements ArrayList<String> newNames = new ArrayList<>(); newNames.add("Vijay"); newNames.add("Anita"); // Adding all elements from newNames to names at index 1 names.addAll(1, newNames); // Printing the CopyOnWriteArrayList System.out.println("CopyOnWriteArrayList: " + names); } } 

Output:

CopyOnWriteArrayList: [Ravi, Vijay, Anita, Priya] 

Real-World Use Case

Example: Merging User Lists in a Concurrent Application

A common real-world use case for CopyOnWriteArrayList is managing thread-safe lists of users and merging multiple user lists.

Example

import java.util.ArrayList; import java.util.concurrent.CopyOnWriteArrayList; public class UserListManager { public static void main(String[] args) { // Creating a CopyOnWriteArrayList to manage user names CopyOnWriteArrayList<String> userList = new CopyOnWriteArrayList<>(); // Adding user names to the CopyOnWriteArrayList userList.add("Ravi"); userList.add("Priya"); // Creating another collection with new user names ArrayList<String> newUserList = new ArrayList<>(); newUserList.add("Vijay"); newUserList.add("Anita"); // Simulating concurrent operations Thread writerThread1 = new Thread(() -> { userList.addAll(newUserList); System.out.println("New users added."); }); Thread writerThread2 = new Thread(() -> { userList.addAll(1, newUserList); System.out.println("New users added at index 1."); }); // Starting the threads writerThread1.start(); writerThread2.start(); // Waiting for the threads to finish try { writerThread1.join(); writerThread2.join(); } catch (InterruptedException e) { e.printStackTrace(); } // Printing the final user list System.out.println("Final user list: " + userList); } } 

Output:

New users added. New users added at index 1. Final user list: [Ravi, Vijay, Anita, Vijay, Anita, Priya] 

In this example, CopyOnWriteArrayList is used to manage a thread-safe list of user names, allowing concurrent operations while merging multiple user lists.

Conclusion

The CopyOnWriteArrayList.addAll() method in Java provides a way to add all elements from a specified collection to a CopyOnWriteArrayList in a thread-safe manner. By understanding how to use this method, you can efficiently manage collections of elements in your Java applications, especially in concurrent environments. The method allows you to merge collections and insert elements at specific positions, making it a versatile tool for data management in multi-threaded scenarios.