Widening Casting / Down Casting

[Former Member]

Hi Folks,

     I am learning ABAP OO and not able to catch the concept of widening casting. I also followed the below links, but still not able to understand. Kindly help.

is it mandatory to use interfaces when going for the widening casting(as I saw a couple of examples of widenin casting and everywhere they used the interface )

[former_member188458]

Hi Santosh ,

Wide Casting : When you move the child class object reference to parent class object reference

Narrow/Down Casting : Opposite i.e. parent class object ref to child class obj ref

e.g.  Lets Say

Parent class : Car  , Object ref : OBJ_CAR

Child Class : Sedan ( inherits Parent class Car) , object ref : OBJ_SEDAN

Wide casting :

obj_car = obj_sedan

Narrow/Down Casting :

obj_sedan ?= obj_car

Advantage of narrow casting is you can use the methods of the subclass and also that of parent class.

Hope it helps you to understand .

Best Regards,

Rini

P.S : Pls mark the answer as correct/helpful if so.

[Former Member]

Hi Rini,

    Thanks for the quick reply. I understood the concept theoretically. but the problem I am having is implementing it practically in a program. I am also slightly confused in understanding if Interfaces are mandatory to have the widening casting implemented.

One more thing, I read somewhere that the casting operator "?=" is only used for widening casting ( Assigning the instance of a superclass to the sub-class ). However in the above explanation of yours I can see you have used the casting operator under the header "Narrow Casting" .

Regards,

  Santosh

[former_member188458]

Hi Santosh,

I just realized I mixed up the terminology ,my bad ( probably late night effect 🙂 ) .

It is the vice-a-versa . Here is the corrected one

Wide Casting : When you move the child class object reference to parent class object reference

Narrow/Down Casting : Opposite i.e. child class object ref to parent class obj ref

e.g.  Lets Say

Parent class : Car  , Object ref : OBJ_CAR

Methods : get_mileage

Child Class : Sedan ( inherits Parent class Car) , object ref : OBJ_SEDAN

Methods : get_mileage (inherited from parent but redefined)

                   get_back_space ( new method of child class)

Wide casting :

obj_sedan ?= obj_car

So when u do wide casting you can access parent class methods as well as methods belonging to child class e.g. in above case

you can access

OBJ_SEDAN->get_back_space

Narrow/Down Casting :

obj_car = obj_sedan

When you do Narrow casting  you can only access the methods belonging to parent class however if the method is re-defined in the child class , the parent class object reference calls the child class method implementation .

e.g. in above scenrio u cannot call get_back_space method using the obj_car ref , but u can call get_mileage method and because it is re-defined in the child class so it will call the child class implementation of method get_mileage .

OBJ_SEDAN->get_mileage

Regarding your question related to Interface : Implementing Interface in classes is another kind of polymorphism(providing multiple forms)  , which you can think as simlar to inheritance i.e.  parent class & child class relation .

class . Only difference is Interface methods have no implementation but the implementing class has the implementation.

e.g. if we take the above case again , lets say

Interface : IF_CAR

methods : get_mileage

Class which Implements IF_IAR : CL_SEDAN

methods : get_milage ( with method code / implementation)

                  get_back_space ( new method of child class)

and you have the same narrow and wide casting beahivor as explained above .

Hope it helps you to understand better.

Thanks,

Rini