"If we pass by value..Inside the subroutine if you change the value it will not have effect (means it will not change the value) in the calling place..
If we pass by reference...Inside the subroutine if you change the value it will change in the calling place..Means it will share the same memory.."

Static methods cannot access instance variables and methods however instance methods can access static methods.

By concept of OOPS it is required however in SAP its not required. We have tried that.

"This variant of the statement CLASS is used to make the class class known, regardless of the location of the actual definition of the class in the program. It does not introduce a declaration part and must not be ended using ENDCLASS.

Example: CLASS c1 DEFINITION DEFERRED.
One example of using the addition DEFERRED PUBLIC would be a type group in which a reference type is declared with a reference to a global class, which itself contains components with references to this reference type. In this situation, the entire class cannot be loaded before the type group, since the types are not yet known. After the statement DEFERRED PUBLIC, however, the class name can be specified after REF TO without the class having been loaded previously."