here are some basic concepts of java which is not written in books generally-
1-java is not purely object oreinted language
                                                                          in many books it is written that java is purely onbject oriented which means every thing is object but the exception is there java uses primitive data type such
as integer, float etc which is not an object

2 difference between object and instance:-
                                                               Objects are things in memory while instances are things that reference to them.

3 it is imposible for override main method :-                                                                                
                                                                 we cannot override a method which is static

4:- _jspservice() method cannot be override:-
                                                                   ( _ ) u nderscore sign shows us that this is container method or we can say that it is already defined by the jsp container it is not defined by author and the underscore sign
shows that is special method because we never start any method with special caracter in java