What are the differences between using HashMap with synchronized methods and using ConcurrentHashMap in a multithreaded environment?

HashMap is not thread-safe. If used in a multithreaded environment, access must be synchronized manually, for example, using Collections.synchronizedMap() or synchronized blocks. This can lead to blocking and reduced performance, as each operation requires acquiring a common monitor.

ConcurrentHashMap is specifically designed for concurrent environments without full locking. It uses segmentation or other mechanisms to minimize locking, allowing multiple threads to read and write to the map simultaneously with high efficiency.

Main differences:

• Synchronized HashMap locks the entire map on each operation, reducing concurrency.
• ConcurrentHashMap allows finer-grained locking or no locking for reads, improving performance.

Example of using ConcurrentHashMap:

ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
map.put("key", 1);
int value = map.get("key");

Therefore, for multithreaded access, it is preferable to use ConcurrentHashMap.