Java ConcurrentHashMap keySet() Method

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The ConcurrentHashMap.keySet() method in Java is used to obtain a set view of the keys contained in the ConcurrentHashMap.

Table of Contents

1. Introduction
2. keySet Method Syntax
3. Examples
   • Retrieving Keys from a ConcurrentHashMap
   • Iterating Over Keys
4. Real-World Use Case
   • Example: Listing Active User Sessions
5. Conclusion

Introduction

The ConcurrentHashMap.keySet() method is a member of the ConcurrentHashMap class in Java. It provides a set view of the keys contained in the map. The ConcurrentHashMap class is part of the java.util.concurrent package, designed for high concurrency and scalability.

keySet() Method Syntax

The syntax for the keySet method is as follows:

public Set<K> keySet()

• The method takes no parameters.
• The method returns a Set view of the keys contained in the map.

Examples

Retrieving Keys from a ConcurrentHashMap

The keySet method can be used to retrieve the keys from a ConcurrentHashMap.

Example

import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;

public class KeySetExample {
    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);
        people.put("Vijay", 35);

        // Retrieving the set of keys
        Set<String> keys = people.keySet();

        // Printing the set of keys
        System.out.println("Keys in ConcurrentHashMap: " + keys);
    }
}

Output:

Keys in ConcurrentHashMap: [Ravi, Priya, Vijay]

Iterating Over Keys

You can iterate over the keys obtained from the keySet method.

Example

import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;

public class IterateKeysExample {
    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);
        people.put("Vijay", 35);

        // Retrieving the set of keys
        Set<String> keys = people.keySet();

        // Iterating over the keys
        System.out.println("Iterating over keys:");
        for (String key : keys) {
            System.out.println(key);
        }
    }
}

Output:

Iterating over keys:
Ravi
Priya
Vijay

Real-World Use Case

Example: Listing Active User Sessions

A common real-world use case for ConcurrentHashMap is managing user session data and listing active sessions.

Example

import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;

public class UserSessionStore {
    public static void main(String[] args) {
        // Creating a ConcurrentHashMap to manage user sessions
        ConcurrentHashMap<String, String> userSessions = new ConcurrentHashMap<>();

        // Adding user sessions to the ConcurrentHashMap
        userSessions.put("Ravi", "Active");
        userSessions.put("Priya", "Inactive");
        userSessions.put("Vijay", "Active");

        // Retrieving the set of user session keys
        Set<String> sessionKeys = userSessions.keySet();

        // Printing the active user sessions
        System.out.println("Active User Sessions:");
        for (String key : sessionKeys) {
            if ("Active".equals(userSessions.get(key))) {
                System.out.println(key);
            }
        }
    }
}

Output:

Active User Sessions:
Ravi
Vijay

In this example, ConcurrentHashMap is used to manage user session data, and the keySet method is employed to list active user sessions in a thread-safe manner.

Conclusion

The ConcurrentHashMap.keySet() method in Java provides a way to obtain a set view of the keys contained in a ConcurrentHashMap 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 retrieve and iterate over keys, making it a versatile tool for data management in multi-threaded scenarios.