What is the difference between ConcurrentHashMap and Collections.synchronizedMap?

ConcurrentHashMap and Collections.synchronizedMap both provide thread safety for working with Map in Java, but they do so in different ways.

• Collections.synchronizedMap is a wrapper around a regular Map that synchronizes all methods entirely. This means that any access to the map (reading or writing) uses a single shared monitor, which can lead to blocking and reduced performance under high contention.

• ConcurrentHashMap implements a more complex locking mechanism (for example, segmentation or CAS operations in newer versions), allowing multiple threads to read and write to different parts of the map simultaneously without locking the entire structure. This significantly improves performance in multi-threaded scenarios.

Example of using ConcurrentHashMap:

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

Summary:

• synchronizedMap locks the entire map on each method — simple but less scalable.
• ConcurrentHashMap provides high concurrency and is better suited for multi-threaded environments with intensive access.