Calling inteface methods - which implementation to consider??

Hi Experts
I am new to ABAP Objects, but I know Java. So I am trying to learn ABAP comparing with Java but it creating lot of confusions.
in a demo program I found the below code :
DATA tab_root TYPE REF TO cl_wd_uielement_container.
    DATA structure_root TYPE REF TO cl_wd_uielement_container.
    DATA new_tab TYPE REF TO cl_wd_table.
    IF wd_this->structure_changed = abap_true.
      structure_root ?= view->get_element( id = 'FIELD_GROUP' ).
      tab_root       ?= view->get_element( id = 'TABLE_GROUP' ).
In the above code snippet,
structure_root ?= view->get_element( id = 'FIELD_GROUP' ).
is a downcasting and view is of type ref to IF_WD_VIEW and class cl_wd_uielement_container implements this interface.
My question is, when we do not know which is the implemenation class for this interface, how can we call a method of this interface (because methods are not implemented in the interface itself but in the implementation class).??
In the above assignment statement, how does system finds out which implementation to point to??
Question may be very basic, but i couldnt find the answer for this.
Please help me
Thanks
Dhananjay Hegde

Hi,
First create the instance(object) of class in you were implement interface method then call the interface method using the object.
for example
Report Zinterface,
interface IF_view.
methods: view.
endinterface.
class c1 definition.
public section.
interfaces: IF_view.
methods:m1,m2. " here some more methods.
endclass.
class c1 implementation.
method IF_view~view.
write:/ 'this is an interface method'.
endmethod.
method m1.
write:/ 'this is M1 method'.
endmethod.
method m2.
write:/ 'this is M2 method'.
endmethod.
endclass.
data: obj type ref to c1.
START-OF-SELECTION.
create object obj.
call method obj->IF_view~view." call the interface method using the insatance of where you were implemeted the interface
The above procedure apply for your requirement.. i hope you may get idea about interface method calling.
Best Regards
sreenivas.

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