Java: HashMap internals, custom implementation of HashMap, HashCode and Equals: Contract, its custom implementation, problems if not implemented correctly, Inheritance Data Structures: Middle of LinkedList in single traversal, String matching, multiple of 3 till given number n in O(1): the complexity simply directed me to think of a mathematical solution :), Check if a binary tree is BST Database: Indexs, Self Joins, How Indexing work, measures to speed up a select query, etc. How microservices work, what are the techniques for service discovery, etc. (mentioned in my resume)