Hashmap vs ConcurrentHashMap in java | collections framework | java tutorials

@mahaprabu2581
what is the difference between hashmap and concurrenthashmap:
HashMap and ConcurrentHashMap are two popular data structures in Java used to store and retrieve key-value pairs. While they may seem similar at first glance, there are some key differences between them.

HashMap is a non-synchronized class that is not thread-safe. It means that if multiple threads try to access or modify the same HashMap concurrently, it can lead to inconsistencies and unpredictable results. To make it thread-safe, you can use the synchronized keyword, but this can significantly reduce performance when used in a multi-threaded environment.

On the other hand, ConcurrentHashMap is a thread-safe version of HashMap. It is designed to handle concurrent read and write operations without causing any inconsistencies or performance issues. ConcurrentHashMap uses a technique called lock striping, which involves dividing the internal data structure into smaller segments and applying a lock to each segment, allowing multiple threads to read and write to different segments concurrently.

In terms of performance, ConcurrentHashMap is generally slower than HashMap when used in a single-threaded environment because of the additional overhead required to ensure thread-safety. However, in a multi-threaded environment, ConcurrentHashMap outperforms HashMap as it can handle concurrent access more efficiently.

Another key difference between the two is that ConcurrentHashMap allows null keys and values, whereas HashMap does not. This is because ConcurrentHashMap uses a special marker value to represent null keys and values, while HashMap does not have this feature.

In summary, the main difference between HashMap and ConcurrentHashMap is that ConcurrentHashMap is thread-safe and can handle concurrent read and write operations efficiently, while HashMap is not thread-safe and requires synchronization to be used in a multi-threaded environment. If you need to use a data structure in a multi-threaded environment, ConcurrentHashMap is the preferred choice, but if you are working in a single-threaded environment, HashMap may be more suitable.
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