Which Java Collections are synchronized(thread safe), which are not?

Question

Which Java Collections are synchronized, which are not?

Example: HashSet is not synchronized

Answer 1

Thread safe Collections -

1. ConcurrentHashMap

Thread safe without having to synchronize the whole map Very fast reads while write is done with a lock No locking at the object level Uses multitude of locks.

2. SynchronizedHashMap

Object level synchronization Both read and writes acquire a lock Locking the collection has a performance drawback May cause contention

3. Vector

4. HashTable

5. CopyOnWriteArrayList

6. CopyOnWriteArraySet

7. Stack

Rest all are not thread safe