Explain Java follow-ups after solving coding problem

After you implement an algorithmic solution in Java, the interviewers ask follow-up questions and request modifications. Answer/discuss (and, where relevant, describe how you would change code):

1. Change request / refactor: A new requirement arrives (e.g., different input format, extra validation, new edge case, different return type). How do you adjust the code while keeping it readable and testable?
2. Large data handling: If input size grows significantly, how do you reason about time complexity , memory usage , and potential bottlenecks in Java? What changes might you make?
3. Collections choice: Which Java collections would you choose (e.g., ArrayList , LinkedList , HashMap , TreeMap , Deque / Stack ) and why? What are common pitfalls?
4. Generics: Explain how Java generics work, trade-offs, and common issues (e.g., type erasure, wildcards).
5. Exceptions: Discuss checked vs unchecked exceptions, best practices for throwing/catching, and how you would design error handling for invalid input.
6. Testing: What unit tests would you write (e.g., with JUnit)? How do you cover boundary cases and regression for the new requirement?