I applied online and received a call for a phone interview. The phone interview was a technical round, focusing heavily on data structures and API questions. Although it's a senior-level role, the questions were framed for college-level students.

1. How does ArrayList work internally in Java? If you were to implement an ArrayList, how would you do it? Discuss the Collections interface implementations and their differences. What is the difference between the Set and List interfaces? Is HashSet thread-safe or not?

2. Is there a time difference when accessing an ArrayList at the beginning versus the end?

3. How does HashMap work? Provide deep discussion about that, as well as HashMap's internal implementation details, including its O(n) complexity.