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

There are three groups of Collections.

• Java 1.0 collections which mostly legacy classes. This includes Hashtable, Vector, Stack. These are synchronized but I don't recommend you use them. Properties is perhaps one exception, but I wouldn't use it in a multi-threaded context.
• Java 1.2 collections added in 1998 which largely replaced these collection are not synchronized, but can be synchronized using Collections.synchronizedXxx() methods
• Java 5.0 concurrency collections added in 2004 support lock free, thread safe collections.

In short, none of the collections I would recommend you use are synchronized.

Answer 2

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