Why need ConcurrentHashMap and CopyOnWriteArrayList

Why need ConcurrentHashMap and CopyOnWriteArrayList

1. The synchronized collections classes, Hashtable, and Vector, and the synchronized wrapper classes, Collections.synchronizedMap() and Collections.synchronizedList(), provide a basic conditionally thread-safe implementation of Map and List. 
2. However, several factors make them unsuitable for use in highly concurrent applications, for example, their single collection-wide lock is an impediment to scalability and it often becomes necessary to lock a collection for a considerable time during iteration to prevent ConcurrentModificationException.
3. ConcurrentHashMap and CopyOnWriteArrayList implementations provide much higher concurrency while preserving thread safety, with some minor compromises in their promises to callers.
4. ConcurrentHashMap and CopyOnWriteArrayList are not necessarily useful everywhere you might use HashMap or ArrayList, but are designed to optimize specific common situations.
5.  Many concurrent applications will benefit from their use. 

Read more: https://javarevisited.blogspot.com/2011/04/difference-between-concurrenthashmap.html#ixzz5dYr8gjq7