What are Concurrent collections ?

ConcurrentHashMap

• Concurrent replacement of HashMap (uses hash based Map)
• Avoid synchronizing every method on a common lock.
• Many reading threads can access the map concurrently.
• Readers can access the map concurrently with writers.
• A limited number of writers can modify the map concurrently.
• Iterators dont throw ConcurrentModificationException.
• Cannot be locked for exclusive access ; Cannot use client-side locking to create atomic operations

CopyOnWriteArrayList

• Concurrent replacement of ArrayList
• Create a copy every time it is modified (Cost of memeory), so cannot throw ConcurrentModificationException
• Multiple threads can iterate it