What is the default number of segments used in ConcurrentHashMap?

In the classic implementation of ConcurrentHashMap in Java (before Java 8), 16 segments were used by default. Each segment is a separate part of the hash table with its own lock, which allows increased parallelism during access.

Starting from Java 8, the implementation of ConcurrentHashMap was redesigned and segments were removed. Instead, a finer-grained locking mechanism at the level of individual buckets using CAS and other mechanisms is used, which improved performance and scalability.

Thus, if we talk about the classic implementation, there are 16 segments by default.