What is web container?
Web Container is special JVM which manages the life cycle of server side pages.
If a method in superclass throws NullPointerException , can we override it with a method which throws RuntimeException?
Yes. you can throw RuntimeException of superclass in overridden method, but you can not do same if its checked Exception.
What data structure is used to store the primary key index ?
Most database(like Oracle, Mysql, Solar) uses B+ tree data structure to store the primary key index.