Question

You need to store elements in a collection that guarantees that no duplicates are stored, and all elements can be access in nature order, which interace provides that capability?

Answer 1

the collection class allows you to access its elements by Association it with an element's value and provides synchronisation is Java.util.Hashtable because it is the only class listed that provides synchronised methods if you need and synchronisation great; otherwise use hashmap it's faster.
I hope its help you.