Welcome to Concurrent Collections in Java, an important step forward in your journey through Java's concurrent utilities. Previously, we covered synchronized collections, which ensure thread safety but can sometimes introduce bottlenecks. In this lesson, we explore concurrent collections, specifically designed for high-concurrency environments where performance and scalability are critical.

In this lesson, you will:

• Understand the limitations of synchronized collections and how concurrent collections address them.
• Learn about ConcurrentHashMap, a high-performance alternative to synchronized maps.
• Implement and manipulate concurrent collections in practical examples to grasp their benefits.

By the end of this lesson, you’ll be able to confidently use concurrent collections to build thread-safe applications with better performance and scalability.

In earlier lessons, we focused on synchronized collections, which guarantee thread safety by using locks to control access. While effective for ensuring safe access, synchronized collections can slow down performance under heavy load due to their reliance on locking the entire collection.

Concurrent collections, like ConcurrentHashMap, solve this issue by enabling concurrent reads and writes without locking the entire collection. This results in much better performance under high contention and offers a more scalable solution for multi-threaded applications.

Let’s dive into an example using ConcurrentHashMap, a widely-used concurrent collection designed to handle multiple threads accessing and modifying a shared map simultaneously.

Java
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1import java.util.concurrent.ConcurrentHashMap;
2
3public class ConcurrentMapExample {
4    private ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
5
6    public void incrementCount(String key) {
7        map.put(key, map.getOrDefault(key, 0) + 1);
8    }
9
10    public void displayMap() {
11        map.forEach((key, count) -> System.out.println(key + ": " + count));
12    }
13}

The ConcurrentHashMap is initialized to provide a high-performance map for thread-safe operations. Unlike traditional synchronized maps, ConcurrentHashMap allows concurrent reads and updates without locking the entire map, improving scalability. The incrementCount method safely increments the count for a given key by retrieving the current value (or defaulting to 0) and updating it. The displayMap method uses forEach to safely iterate over the map and print its contents, with no need for additional synchronization.

Thread-safe reads and writes are guaranteed in ConcurrentHashMap. It allows multiple threads to read and write simultaneously without blocking, unlike traditional synchronized maps. Updates to individual keys do not require locking the entire map, which ensures better performance and scalability, especially in highly concurrent environments.

To see ConcurrentHashMap in action, we simulate concurrent updates by multiple threads in the following Main class:

Java
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1public class Main {
2    public static void main(String[] args) throws InterruptedException {
3        ConcurrentMapExample example = new ConcurrentMapExample();
4
5        // Thread 1: Increment the count for "apple"
6        Thread t1 = new Thread(() -> {
7            for (int i = 0; i < 1000; i++) {
8                example.incrementCount("apple");
9            }
10        });
11
12        // Thread 2: Increment the count for "banana"
13        Thread t2 = new Thread(() -> {
14            for (int i = 0; i < 1000; i++) {
15                example.incrementCount("banana");
16            }
17        });
18
19        // Start both threads and wait for them to finish
20        t1.start();
21        t2.start();
22        t1.join();
23        t2.join();
24
25        // Display the updated map
26        example.displayMap();
27    }
28}

In this example, two threads (t1 and t2) are created to concurrently increment the count for "apple" and "banana" 1000 times each. ConcurrentHashMap ensures that these operations happen without conflicts. After both threads have finished executing, the map is displayed to show the final counts. This demonstrates how efficiently ConcurrentHashMap can handle concurrent updates.

Even under concurrent modifications, ConcurrentHashMap provides a thread-safe environment without the need for additional synchronization code. It optimizes performance in multi-threaded applications by allowing multiple threads to update the map concurrently, without locking the entire map, making it ideal for read-heavy or highly concurrent applications.

Transitioning to concurrent collections is significant for several reasons:

• Performance Efficiency: By reducing the need for locks and enabling multiple access points, concurrent collections minimize performance bottlenecks.

• Enhanced Scalability: These collections scale better under high concurrency, making them ideal for real-world applications involving numerous threads.

• Simplified Code: With built-in thread safety, concurrent collections reduce the complexity of code by eliminating the need for explicit synchronization in many scenarios.

Concurrent collections are invaluable tools for building robust software systems, ensuring that applications remain efficient even as workloads and concurrency levels increase. They empower you to write performant, scalable multi-threaded programs without the need for extensive synchronization, streamlining the development process in modern software engineering.

Let's now move to the Practice section and see this in action!