Hashmap vs Synchronized Hashmap

In Java, a hashmap is a data structure that allows for efficient storage and retrieval of key-value pairs. It is widely used in many applications because of its speed and simplicity. However, in multi-threaded environments, concurrent modifications to a hashmap can cause inconsistencies and errors. To address this, Java provides a synchronized version of the hashmap called “Synchronized Hashmap”. In this blog, we will explore the differences between these two data structures and when to use each of them.

Hashmap

A hashmap is an implementation of the Map interface that uses a hash table to store key-value pairs. It is an efficient data structure for storing and retrieving data, with an average constant-time complexity for operations like put, get, and remove. It works by hashing the keys to generate an index in an array where the corresponding value is stored.

In a single-threaded environment, a hashmap is a great choice because it provides fast access to values with minimal overhead. However, in a multi-threaded environment, problems can arise when multiple threads try to access or modify the hashmap simultaneously. When two threads try to modify the same key-value pair at the same time, one thread can overwrite the changes of another thread. This can lead to unpredictable and incorrect results.

Synchronized Hashmap

To avoid these problems, Java provides a synchronized version of hashmap called “Synchronized Hashmap”. This is a thread-safe version of the hashmap, which means that it is designed to be used in a multi-threaded environment without causing race conditions or other synchronization issues.

The synchronized hashmap works by synchronizing access to the hashmap’s internal data structures. This ensures that only one thread can access or modify the hashmap at a time, preventing race conditions and other concurrency problems. In other words, when a thread wants to access or modify the synchronized hashmap, it must first acquire a lock. Once the lock is acquired, the thread can perform the necessary operations and then release the lock.

Performance

While the synchronized hashmap provides safety in multi-threaded environments, it comes at the cost of performance. Because access to the synchronized hashmap is serialized, only one thread can access or modify it at a time. This can lead to contention when multiple threads are trying to access or modify the hashmap simultaneously. As a result, the synchronized hashmap is slower than the unsynchronized hashmap in single-threaded environments.

When to use which?

The choice between hashmap and synchronized hashmap depends on the specific use case. If you are working in a single-threaded environment, then a hashmap is the obvious choice. It provides fast and efficient access to data without any additional overhead. However, if you are working in a multi-threaded environment, then you should use a synchronized hashmap. While it may be slower than the unsynchronized hashmap in a single-threaded environment, it provides safety and consistency in a multi-threaded environment.

Conclusion

In conclusion, the hashmap and synchronized hashmap are two data structures that are used for storing and retrieving key-value pairs in Java. The hashmap is a fast and efficient data structure that is ideal for single-threaded environments. The synchronized hashmap, on the other hand, is a thread-safe version of the hashmap that provides safety and consistency in multi-threaded environments, albeit at the cost of performance. Therefore, it is important to choose the appropriate data structure based on the specific requirements of the application.