TSYS|Total System Services interview question

Technical Questions : 1. Difference between comparator and comparable ? when would we use comparator ? 2. what is a functional interface? 3. what are different type of data structures you use ? 4. using java 8 convert an array of integers to comma seprated string . 5. difference between array list and linked list ? and which is faster for retrieval? 6. can we use classes as keys in has map ? 7. Singleton prototype in thread. 8. Scope in spring boot ? 9 Do you know patch in spring boot ? 10. when will the dependencies be loaded ? at run time or static time ? 11. If I have two classes with same name what happens ? 12. if I use a concurrent hash map how is it thread safe ? 13. what happens if I try to put a same key in hash map twice ? 14.if i have a custom object as key in hash map how is hash value computed ? 15. what is a method and function level locking ? 16. what are functional interfaces ? 17 . does spring boot use eager or lazy loading ? 18. what is Optional class in spring boot ? 19. what is circular dependency in spring boot.