What is a local, member and a class variable?

Variables declared within a method are “local” variables.
Variables declared within the class i.e not within any methods are “member” variables (global variables).
Variables declared within the class i.e not within any methods and are defined as “static” are class variables

What is the need for Hibernate xml mapping file? .

. hbm.xml file is required for configuring the Hibernate configuration for the app. Configuration parameters like SQL Dialect to be used are defined here. SQL dialect differs for every DB.

What are the Core interfaces are of Hibernate framework?

Core Interfaces are- SessionFactory, Session, Query, Transaction.

What is the general flow of Hibernate communication with RDBMS?

For any communication with DB, hibernate converts each HQL query to the equivalent SQL query with the configuration based in hbm.xml file to map the database. All HQL queries are parsed and converted to SQL using JDBC and then sent to the DB.

What is Hibernate Query Language (HQL)?

HQL is nothing but object oriented query language which is very similar to SQL. HQL is used for communication with DB. It supports almost all the features of native SQL query

What is the diffence between Constructor and Method ?

constructor  :-

A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator.

method :-
 A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator. 

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