Instantiation and Encapsulation

Question

Hi
Can any one explain me about instantiation and Encasulation with example, i am new to ABAP Objects.
Please dont send any links.
Arun Joseph

Answer 1

Hi,
First let me give small introduction to OO ABAP programing.
Object-oriented programming is a method of implementation in which programs are organized      as cooperative collections of objects, each of which represents  an instance of some class...?
A class is a set of objects that share a common
structure and a common behavior
A class will have attributes ( i.e data definition) and methods ( nothing but functions )
We can also call  object as Instance of the class.
Lets see a simple demo class to calcualte SUM of 2 numbers.
define 2 variables a and b of type i. (Attributes)
SUM is the method name. this will have the logic to do the sum.
2 importing paramers and an exporting parameter res for the method.
Note: you can either create a class either globally or locally..
Goto SE24 to create Global class, this class will be avaialable for use in all the programs, this is like class library with all global classes.
Local class is the one which you create in SE38 i.e; your own program,
by this it is understood that it's scope is only upto that program
SO let us discuss the example with SE24(global class)
give a class name, description,
In the attributes tab, define a and b of type i, level as instance attribute and
visibility as public That means these attributes are available outside this class also. we will come to this later.
in the methods tab give name, level as instance, visibilty as pubilc again.
now double click on the method name, it will take to you to write the code.
(method implementation)
just wirte between the method ---endmethod,
res = num1 + num2.
come back to methods tab, click on parameters button, to define exporting, importing etc.. paramters for your method,
define 'res' as an exporting parameter, 2 variables num1 and num2 as importing parameters.Activate the class.
now you have defined a class.
whenever you want the logic to sum 2 variables, in all those cases you can make use of the SUM method in this class. This is what Reusability is.A very imp. feature of OO ABAP.
So now comes Instantiation.
you can't directly access a class.
You have to create a reference variable of the type of the class, then create an object with the reference variable. (memory allocation).
This step is called Instantiation or Instantiating the class
For this go to se38, in which ever program you want to instatantiate the above class.
do as below.
data: obj type ref to zsow_cl1.
start-of-selection.
create object obj.
Now object of the above class is ready for you.
Now all the contents i.e either attributes or methods etc of the class can be accessed with this object
now you have to call the method 'sum' of the above class using the above object in your program for the sum functionalty.
for that click on Pattern button select 'Abap Object Patterns' press enter
select 'Call method' radio button, give your object name ('OBJ')
class name  ( ZSOW_CL1)
give the method name (SUM)
class and method names can be selected from the F4
press enter.
you will get the below line in your program
CALL METHOD OBJ->SUM
  EXPORTING
    NUM1   =
    NUM2   =
*  IMPORTING
*    RES    =
now define 2 variables of type i to pass the inputs to the method and one more vairable to hold the output coming from the FM
on the whole code in your se38 is as follows.
DATA: OBJ TYPE REF TO ZSOW_CL1,
      X TYPE I,
      Y TYPE I,
      Z TYPE I.
START-OF-SELECTION.
  CREATE OBJECT OBJ.
  X = 10.
  Y = 10.
  CALL METHOD OBJ->SUM
    EXPORTING
      NUM1 = X
      NUM2 = Y
    IMPORTING
      RES  = Z.
  WRITE: Z.
so Z will give the output.
Next is Encapsulation
while defining the method i asked you to give the visibility as Public.
Visibility can be of 3 types:
              Public
              Protected
              Private
public means as i said above the method or varaibles can be used outside the class also.as you did above outside the class you have created an object to the class and
accessed the method.
private means the attributes/methods that you define in the class can only be accessed in/by the method of the same class, they cannot be accessed anywhere else and outside.
change your 'a' parameter visibilty to private and try the below code in se38.
obj->b = 10.
obj->a = 10.
1st line doesn't give error, since 'b' is public attribte,
2nd line will give error since 'a' is now private.
you can use 'a' in the 'SUM' method if you want.
Protected means it can't be accessed outside of the class like with the help of object, but can be accessed in  the child class if defined to the above class.
just post if you have any other doubts after trying this example.
Do reward points if it helps you.
Regards,
Sowjanya