In Java 1.5 many Concurrent collection classes were added in SDK.

These are ConcurrentHashMap, CopyOnWriteArrayList,

BlockingQueue etc.

Java also provides utility methods to get a synchronized copy of

collection like ArrayList, HashMap etc. by using

Collections.synchronizedList(), Collections.synchronizedMap()

methods.

The main difference is in performance. Concurrent collection

classes have better performance than synchronized collection

classes because they lock only a portion of the class to achieve

concurrency and thread-safety

https://javarevisited.blogspot.com/2016/05/what-is-difference-between-synchronized.html#axzz7sNmYn2qu