Cognizant Programmer/Analyst interview questions

I got this interview after clearing the test assessment. Followed by, 1 round of interview. The interview was technical round and the tech was JAVA. 1 round of document verification. The process was smooth and I got the offer letter after the interview.

Programming & Coding Explain the difference between abstract class and interface in Java (or your preferred language). How would you optimize a SQL query that is running slow? Can you explain time complexity of HashMap get/put operations? Write a program to detect duplicates in a large dataset (without using extra space). What’s the difference between synchronous and asynchronous processing? Give examples. Databases What are indexes in a database and when would you avoid using them? Explain normalization and denormalization with examples. How do you handle a scenario where a query returns inconsistent results across environments? System & Integration How do you design an API contract for a system that needs to interact with multiple clients?