Java CopyOnWriteArrayList size() Method

The CopyOnWriteArrayList.size() method in Java is used to get the number of elements in a CopyOnWriteArrayList.

Table of Contents

1. Introduction
2. size Method Syntax
3. Examples
   • Getting the Size of a CopyOnWriteArrayList
   • Size After Adding and Removing Elements
4. Real-World Use Case
   • Example: Monitoring the Size of a Thread-Safe User List
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 size method allows you to determine the number of elements currently stored in the list. 

The CopyOnWriteArrayList achieves thread safety by creating a new copy of the array whenever it is modified.

size() Method Syntax

The syntax for the size method is as follows:

public int size()

• The method takes no parameters.
• The method returns an integer representing the number of elements in the list.

Examples

Getting the Size of a CopyOnWriteArrayList

The size method can be used to get the number of elements in a CopyOnWriteArrayList.

Example

import java.util.concurrent.CopyOnWriteArrayList;

public class SizeExample {
    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");
        names.add("Vijay");

        // Getting the size of the CopyOnWriteArrayList
        int size = names.size();

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

Output:

Size of CopyOnWriteArrayList: 3

Size After Adding and Removing Elements

The size method reflects changes in the list after adding or removing elements.

Example

import java.util.concurrent.CopyOnWriteArrayList;

public class AddRemoveSizeExample {
    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");
        names.add("Vijay");

        // Printing the initial size
        System.out.println("Initial size: " + names.size());

        // Removing an element
        names.remove("Priya");

        // Printing the size after removal
        System.out.println("Size after removal: " + names.size());

        // Adding a new element
        names.add("Anita");

        // Printing the final size
        System.out.println("Final size: " + names.size());
    }
}

Output:

Initial size: 3
Size after removal: 2
Final size: 3

Real-World Use Case

Example: Monitoring the Size of a Thread-Safe User List

A common real-world use case for CopyOnWriteArrayList is managing a thread-safe list of users and monitoring the number of users.

Example

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");
        userList.add("Vijay");

        // Simulating concurrent read and write operations
        Thread readerThread = new Thread(() -> {
            int size = userList.size();
            System.out.println("Number of users: " + size);
        });

        Thread writerThread = new Thread(() -> {
            userList.add("Anita");
            System.out.println("Added user: Anita");
        });

        // Starting the threads
        readerThread.start();
        writerThread.start();

        // Waiting for the threads to finish
        try {
            readerThread.join();
            writerThread.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        // Printing the final user list size
        System.out.println("Final number of users: " + userList.size());
    }
}

Output:

Number of users: 3
Added user: Anita
Final number of users: 4

In this example, CopyOnWriteArrayList is used to manage a thread-safe list of user names, allowing concurrent read and write operations while monitoring the number of users.

Conclusion

The CopyOnWriteArrayList.size() method in Java provides a way to get the number of elements in 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 monitor the size of the list, making it a versatile tool for data management in multi-threaded scenarios.