1. How does arraylist work internally in java? if you were to implement array list how would you implement. Collections interface implementations and its differences. Whats the difference between set and list interface. Is Hashlist a thread safe or not. 2. Is there time difference in the array list is accessed, if the index is to the begining vs last. 3. How does HashMAp? Deep discussions about that, as well as HashMaps internal implementation details of O(n).