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?