What are the differences between HashMap, ConcurrentHashMap, and SynchronizedMap?

HashMap is a non-synchronized implementation of Map, not thread-safe. It is used when access to the map occurs from a single thread or synchronization is provided externally.

SynchronizedMap is a wrapper over any Map that synchronizes all methods, making it thread-safe by locking the entire object. This can lead to bottlenecks under high thread contention.

ConcurrentHashMap is a specialized thread-safe implementation of Map that uses finer-grained locking (segments or other mechanisms) to improve performance during concurrent access. It allows reading and writing simultaneously without full locking.

Example:

Map<String, String> map = new ConcurrentHashMap<>();
map.put("key", "value");
String val = map.get("key");

ConcurrentHashMap is preferable for multi-threaded environments with high contention.