Java CopyOnWriteArraySet retainAll() Method

The CopyOnWriteArraySet.retainAll() method in Java is used to retain only the elements in the CopyOnWriteArraySet that are also contained in a specified collection.

Table of Contents

1. Introduction
2. retainAll Method Syntax
3. Examples
   • Retaining Elements in a Collection
   • Handling Non-Matching Elements
4. Real-World Use Case
   • Example: Retaining Specific Users in a Thread-Safe Set
5. Conclusion

Introduction

The CopyOnWriteArraySet is a thread-safe variant of Set 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 retainAll method allows you to retain only the elements in the set that are also contained in another collection. The CopyOnWriteArraySet achieves thread safety by creating a new copy of the underlying array whenever it is modified.

retainAll() Method Syntax

The syntax for the retainAll method is as follows:

public boolean retainAll(Collection<?> c)

• The method takes one parameter:
  • c of type Collection<?>, which represents the collection containing elements to be retained in the set.
• The method returns true if the set changed as a result of the call, and false otherwise.

Examples

Retaining Elements in a Collection

The retainAll method can be used to retain only the elements in a CopyOnWriteArraySet that are also contained in another collection.

Example

import java.util.ArrayList;
import java.util.concurrent.CopyOnWriteArraySet;

public class RetainAllExample {
    public static void main(String[] args) {
        // Creating a CopyOnWriteArraySet with String elements
        CopyOnWriteArraySet<String> names = new CopyOnWriteArraySet<>();

        // Adding elements to the CopyOnWriteArraySet
        names.add("Ravi");
        names.add("Priya");
        names.add("Vijay");

        // Creating another collection with String elements
        ArrayList<String> retainNames = new ArrayList<>();
        retainNames.add("Ravi");
        retainNames.add("Priya");

        // Retaining only the elements in names that are also in retainNames
        boolean changed = names.retainAll(retainNames);

        // Printing the CopyOnWriteArraySet and the result of the retainAll operation
        System.out.println("CopyOnWriteArraySet: " + names);
        System.out.println("Did the set change? " + changed);
    }
}

Output:

CopyOnWriteArraySet: [Ravi, Priya]
Did the set change? true

Handling Non-Matching Elements

The retainAll method does not change the CopyOnWriteArraySet if none of the elements in the specified collection are contained in the set.

Example

import java.util.ArrayList;
import java.util.concurrent.CopyOnWriteArraySet;

public class NonMatchingElementsExample {
    public static void main(String[] args) {
        // Creating a CopyOnWriteArraySet with String elements
        CopyOnWriteArraySet<String> names = new CopyOnWriteArraySet<>();

        // Adding elements to the CopyOnWriteArraySet
        names.add("Ravi");
        names.add("Priya");
        names.add("Vijay");

        // Creating another collection with String elements
        ArrayList<String> retainNames = new ArrayList<>();
        retainNames.add("Anita");
        retainNames.add("Suresh");

        // Trying to retain elements in names that are in retainNames
        boolean changed = names.retainAll(retainNames);

        // Printing the CopyOnWriteArraySet and the result of the retainAll operation
        System.out.println("CopyOnWriteArraySet: " + names);
        System.out.println("Did the set change? " + changed);
    }
}

Output:

CopyOnWriteArraySet: []
Did the set change? true

Real-World Use Case

Example: Retaining Specific Users in a Thread-Safe Set

A common real-world use case for CopyOnWriteArraySet is managing a thread-safe set of users and retaining specific users based on certain criteria.

Example

import java.util.ArrayList;
import java.util.concurrent.CopyOnWriteArraySet;

public class UserSetManager {
    public static void main(String[] args) {
        // Creating a CopyOnWriteArraySet to manage user names
        CopyOnWriteArraySet<String> userSet = new CopyOnWriteArraySet<>();

        // Adding user names to the CopyOnWriteArraySet
        userSet.add("Ravi");
        userSet.add("Priya");
        userSet.add("Vijay");

        // Creating another collection with user names to retain
        ArrayList<String> retainUserSet = new ArrayList<>();
        retainUserSet.add("Ravi");
        retainUserSet.add("Priya");

        // Simulating concurrent operations
        Thread retainThread = new Thread(() -> {
            boolean changed = userSet.retainAll(retainUserSet);
            System.out.println("Were users retained? " + changed);
        });

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

        // Starting the threads
        retainThread.start();
        addThread.start();

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

        // Printing the final user set
        System.out.println("Final user set: " + userSet);
    }
}

Output:

Were users retained? true
Added user: Anita
Final user set: [Ravi, Priya, Anita]

In this example, CopyOnWriteArraySet is used to manage a thread-safe set of user names, allowing concurrent operations while retaining specific users based on certain criteria.

Conclusion

The CopyOnWriteArraySet.retainAll() method in Java provides a way to retain only the elements in a CopyOnWriteArraySet that are also contained in a specified collection 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 handle bulk retention operations, making it a versatile tool for data management in multi-threaded scenarios.