Recently I am planning an internal training regarding Software engineering concept to my colleagues and one topic is “Interface Segregation”.

The following guideline is quoted from OODesign:

“When we design an application we should take care how we are going to make abstract a module which contains several submodules. Considering the module implemented by a class, we can have an abstraction of the system done in an interface. But if we want to extend our application adding another module that contains only some of the submodules of the original system, we are forced to implement the full interface and to write some dummy methods. Such an interface is named fat interface or polluted interface.

The Interface Segregation Principle states that clients should not be forced to implement interfaces they don’t use. Instead of one fat interface many small interfaces are preferred based on groups of methods, each one serving one submodule.”

I am very curious whether SAP standard code contains such fat interface or not. So I wrote a small CDS view:

And get all interfaces which contains more than 10 methods:

DATA: lt_int TYPE STANDARD TABLE OF Zfat_Interface.

SELECT * INTO TABLE @lt_int FROM ZFAT_INTERFACE where method_count > 10 ORDER BY method_count DESCENDING.

The result is there are totally 3139 such interfaces in my CRM development system.

I quickly go through the list, most of them are used to build system functionalities and NO application will implement them, so in my opinion the fact is acceptable.

On the other hand there is also another category of interfaces which has NO methods or attributes defined. Refer to this blog Tag(Marker) Interface in ABAP and Java for more detail.

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