HashMap vs ConcurrentHashMap in Java: Understanding the Differences and When to Use Them

HashMap and ConcurrentHashMap are two popular data structures in Java used to store and retrieve key-value pairs. While both of them serve the same purpose, there are significant differences between the two. In this article, we will explore these differences and understand why ConcurrentHashMap is preferred in certain scenarios.

HashMap is a non-synchronized data structure, meaning that it is not thread-safe. Multiple threads can access a HashMap object simultaneously, which can lead to race conditions and inconsistencies in the data. On the other hand, ConcurrentHashMap is designed to be thread-safe, and it can be accessed by multiple threads concurrently without any issues.

The reason ConcurrentHashMap is thread-safe is that it uses a different locking mechanism than HashMap. HashMap uses a single lock for the entire data structure, meaning that only one thread can access the HashMap at any given time. In contrast, ConcurrentHashMap uses a segmented locking mechanism where locks are applied only to a specific segment of the data structure. This allows multiple threads to access different segments of the data structure concurrently.

Consider the following example where two threads are trying to access a HashMap object:

HashMap<String, Integer> map = new HashMap<>();

Thread thread1 = new Thread(() -> {
    for (int i = 0; i < 10000; i++) {
        map.put("key" + i, i);
    }
});
Thread thread2 = new Thread(() -> {
    for (int i = 0; i < 10000; i++) {
        map.put("key" + i, i);
    }
});
thread1.start();
thread2.start();

In this example, both threads are trying to insert key-value pairs into the HashMap object concurrently. Since HashMap is not thread-safe, this can lead to race conditions where the data is inconsistent or corrupted.

To avoid this issue, we can use a ConcurrentHashMap instead:

ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
Thread thread1 = new Thread(() -> {
    for (int i = 0; i < 10000; i++) {
        map.put("key" + i, i);
    }
});
Thread thread2 = new Thread(() -> {
    for (int i = 0; i < 10000; i++) {
        map.put("key" + i, i);
    }
});
thread1.start();
thread2.start();

In this example, both threads are still trying to insert key-value pairs into the map concurrently. However, since ConcurrentHashMap is thread-safe, there will be no race conditions or inconsistencies in the data.

In addition to thread safety, ConcurrentHashMap also provides better performance in certain scenarios. For example, if the data structure is accessed by a large number of threads concurrently, using ConcurrentHashMap can improve the overall throughput of the system.

In conclusion, ConcurrentHashMap is a thread-safe alternative to HashMap in Java. It uses a segmented locking mechanism to allow multiple threads to access different segments of the data structure concurrently, which can improve performance in certain scenarios. If you are working with multi-threaded applications, it is recommended to use ConcurrentHashMap instead of HashMap to avoid race conditions and ensure data consistency.