What does it mean when we say an ArrayList is not synchronized?

Question

What does it mean when we say an ArrayList is not synchronized?

Does it mean that if we declare an ArrayList in object scope, multiple threads accessing the objects hav

Answer 1

Being synchronized means that every operation is thread safe - if you use the same vector from two threads at the same time, they can't corrupt the state. However, this makes it slower.

If you are working in a single threaded environment (or the list is limited to a thread and never shared), use ArrayList. If you are working with multiple threads that share the same collection, either use Vector, or use ArrayList but synchronize in some other way (e.g., manually or via a wrapper).