OOPs Concepts: Inheritance, Polymorphism
Exception handling (try-catch-finally, custom exceptions)
Collections vs Streams API
Multithreading basics (Runnable vs Callable)
✅ Data Structures
Array vs LinkedList
Stack & Queue implementation
HashMap internal working
Binary Search, Recursion logic
Sorting algorithms (Merge/Quick)
✅ SQL
Joins: Inner, Left, Right, Full
Group By, Having, Subqueries
Indexing, normalization
Query optimization tips
