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Need of Concurrent Collections in Java

Last Updated : 09 May, 2025

In Java, multiple threads can work at the same time, but when the applications become complex, it is very important to handle multiple threads at the same time. With the help of concurrency, different tasks can run in parallel, which can improve the performance of the application. Managing shared data or resources in a multi-threaded environment can cause issues like data inconsistency or crashes. This is where concurrent collections become useful.

In this article, we are going to understand why concurrent collections are important and how they help with multi-threaded tasks, and also going to understand the need for concurrent collections.

Concurrent collections are designed to work with multiple threads at the same time and also avoid basic problems like data conflicts.

1. Thread Safety: When multiple threads try accessing a collection at the same time it can cause errors. Concurrent collections handles thread automatically.

Example:

ConcurrentHashMap<String, String> map = new ConcurrentHashMap<>();

map.put("1", "Java");

map.put("2", "C++");

map.put("3", "Python"");

System.out.println(map);

2. Performance Improvement: In traditional collections like HashMap and ArrayList, the entire collection is locked when one thread is making changes, this slows down the working. Concurrent collections like ConcurrentHashMap lock only the specific part which need to be modified, so that other threads can keep working without waiting.

3. Avoiding Deadlocks: A deadlock happens when threads get stuck and waits for each other to release resources. Concurrent collections automatically handles the locks, which reduces the chances of deadlock. For example, CopyOnWriteArrayList creates a new copy of the list each time it is updated, so that other threads can keep reading the old list without waiting.

4.Handling the Producer-Consumer Problem: In producer-consumer problem, the one thread creates data and the other thread uses it. Java BlockingQueue helps manage this by making the producer wait if the queue is full and the consumer wait if the queue is empty, until there’s data to consume.

Example:

BlockingQueue<String> q = new ArrayBlockingQueue<>(10);

// Producer thread
new Thread(() -> {
    try {
        q.put("Item1");
    } catch (InterruptedException e) {
        Thread.currentThread().interrupt();
    }
}).start();

// Consumer thread
new Thread(() -> {
    try {
        String item = q.take();
        System.out.println(item);
    } catch (InterruptedException e) {
        Thread.currentThread().interrupt();
    }
}).start();

Some of the most commonly used concurrent collections are listed below:

• ConcurrentHashMap: It is a thread-safe version of HashMap. It allows mutliple threads to work at the same time without causing any error.
• CopyOnWriteArrayList: It is a thread-safe version of ArrayList. A new copy is creared evertime a list is modified. This allows other threads to keep reading the old list without waiting.
• BlockingQueue: It is usefull in scenories where one thread produces the data and other thread consume it. It blocks the producer when the queue is full and blocks the consumer when it is empty.
• ConcurrentSkipListMap: It is a thread-safe version of TreeMap. It keeps the keys in sorted order while allowing multiple threads to access and modify the map concurrently.

Example: This example demonstrate the use of concurrent collections in a multi-threaded application.

// Demonstrating how to use ConcurrentHashMap and 
// BlockingQueue in a simple multithreaded application
import java.util.concurrent.*;

public class Geeks {
    
    public static void main(String[] args) 
    throws InterruptedException {
        
        // ConcurrentHashMap
        ConcurrentHashMap<String, String> m = new ConcurrentHashMap<>();
        m.put("1", "Java");
        m.put("2", "C++");
        m.put("3", "Python");

        System.out.println("languages: " + m);

        // BlockingQueue for producer-consumer scenario
        BlockingQueue<String> q = new ArrayBlockingQueue<>(3);

        // Producer thread
        new Thread(() -> {
            try {
                q.put("C#");
                System.out.println("Add: C#");
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        }).start();

        // Consumer thread
        new Thread(() -> {
            try {
                String i = q.take();
                System.out.println("Consumed: " + i);
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        }).start();
    }
}

languages: {1=Java, 2=C++, 3=Python}
Add: C#
Consumed: C#

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