Java CopyOnWriteArrayList clear() Method

The CopyOnWriteArrayList.clear() method in Java is used to remove all elements from a CopyOnWriteArrayList.

Table of Contents

1. Introduction
2. clear Method Syntax
3. Examples
   • Clearing a CopyOnWriteArrayList
   • Verifying List is Empty After Clear
4. Real-World Use Case
   • Example: Resetting a User List 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 clear method allows you to remove all elements from the list, effectively making it empty. The CopyOnWriteArrayList achieves thread safety by creating a new copy of the array whenever it is modified.

clear() Method Syntax

The syntax for the clear method is as follows:

public void clear()

• The method takes no parameters.
• The method does not return a value.

Examples

Clearing a CopyOnWriteArrayList

The clear method can be used to remove all elements from a CopyOnWriteArrayList.

Example

import java.util.concurrent.CopyOnWriteArrayList;

public class ClearExample {
    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 CopyOnWriteArrayList before clearing
        System.out.println("Before clear: " + names);

        // Clearing the CopyOnWriteArrayList
        names.clear();

        // Printing the CopyOnWriteArrayList after clearing
        System.out.println("After clear: " + names);
    }
}

Output:

Before clear: [Ravi, Priya, Vijay]
After clear: []

Verifying List is Empty After Clear

After using the clear method, you can verify that the list is empty using the isEmpty method.

Example

import java.util.concurrent.CopyOnWriteArrayList;

public class VerifyClearExample {
    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");

        // Clearing the CopyOnWriteArrayList
        names.clear();

        // Verifying the list is empty
        boolean isEmpty = names.isEmpty();
        System.out.println("Is the list empty after clear? " + isEmpty);
    }
}

Output:

Is the list empty after clear? true

Real-World Use Case

Example: Resetting a User List in a Concurrent Application

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

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 operations
        Thread writerThread = new Thread(() -> {
            // Clearing the user list
            userList.clear();
            System.out.println("User list cleared.");
        });

        Thread readerThread = new Thread(() -> {
            // Verifying the list is empty
            boolean isEmpty = userList.isEmpty();
            System.out.println("Is the user list empty? " + isEmpty);
        });

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

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

        // Checking if the user list is empty after all operations
        System.out.println("Final state of the user list: " + userList);
    }
}

Output:

User list cleared.
Is the user list empty? true
Final state of the user list: []

In this example, CopyOnWriteArrayList is used to manage a thread-safe list of user names, allowing concurrent operations while resetting the list when needed.

Conclusion

The CopyOnWriteArrayList.clear() method in Java provides a way to remove all elements from 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 reset the list, making it a versatile tool for data management in multi-threaded scenarios.