Part 4: The Interface Segregation Principle

Notes / Software Architecture / Principles of OOD / Part 4: The Interface Segregation Principle

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The Interface Segregation Principle (ISP). This principle deals with the disadvantages of “fat” interfaces. Classes that have “fat” interfaces are classes whose interfaces are not cohesive. In other words, the interfaces of the class can be broken up into groups of member functions. Each group serves a different set of clients. Thus some clients use one group of member functions, and other clients use the other groups.

The ISP acknowledges that there are objects that require non-cohesive interfaces; however it suggests that clients should not know about them as a single class. Instead, clients should know about abstract base classes that have cohesive interfaces. Some languages refer to these abstract base classes as “interfaces”, “protocols” or “signatures”.

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Separate Clients mean Separate Interfaces.

Door and TimerClient represent interfaces that are used by complely different clients. Timer uses TimerClient, and classes that manipulate doors use Door. Since the clients are separate, the interfaces should remain separate too. Why? Because … clients exert forces upon their server interfaces.

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When a change in one part of the program affects other completely unerlated parts of the program, the cost and repercussions of changes become unpredictable; and the risk of fallout from the change increases dramatically.

But it’s just a recompile.

True. But recompiles can be very expensive for a number of reasons. First of all, they take time. When recompiles take too much time, developers begin to take shortcuts. They may hack a change in the “wrong” place, rather than engineer a change in the “right” place; because the “right” place will force a huge recompilation. Secondly, a recompilation means a new object module. In this day and age of dynamically linked libraries and incremental loaders, generating more object modules than necessary can be a significant disadvantage. The more DLLs that are affected by a change, the greater the problem of distributing and managing the change.

The Interface Segregation Principle (ISP)


When clients are forced to depend upon interfaces that they don’t use, then those clients are subject to changes to those interfaces. This results in an inadvertent coupling between all the clients. Said another way, when a client depends upon a class that contains interfaces that the client does not use, but that other clients do use, then that client will be affected by the changes that those other clients force upon the class. We would like to avoid such couplings where possible, and so we want to separate the interfaces where possible.

Class Interfaces vs Object Interfaces

Consider the TimedDoor again. Here is an object which has two separate interfaces used by two separate clients; Timer, and the users of Door. These two interfaces must be implemented in the same object since the implementation of both interfaces manipulates the same data. So how can we conform to the ISP? How can we separate the interfaces when they must remain together?

The answer to this lies in the fact that clients of an object do not need to access it through the interface of the object. Rather, they can access it through delegation, or through a base class of the object.

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Separation through Delegation

We can employ the object form of the ADAPTER pattern to the TimedDoor problem. The solution is to create an adapter object that derives from TimerClient and delegates to the TimedDoor.

When the TimedDoor wants to register a timeout request with the Timer, it creates a DoorTimerAdapter and registers it with the Timer. When the Timer sends the TimeOut message to the DoorTimerAdapter, the DoorTimerAdapter delegates the message back to the TimedDoor.

This solution conforms to the ISP and prevents the coupling of Door clients to Timer. Even if the change to Timer shown in Listing 3 were to be made, none of the users of Door would be affected. Moreover, TimedDoor does not have to have the exact same interface as TimerClient. The DoorTimerAdapter can translate the TimerClient interface into the TimedDoor interface. Thus, this is a very general purpose solution.

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However, this solution is also somewhat inelegant. It involves the creation of a new object every time we wish to register a timeout. Moreover the delegation requires a very small, but still non-zero, amount of runtime and memory. There are application domains, such as embedded real time control systems, in which runtime and memory are scarce enough to make this a concern.

Separation through Multiple Inheritance

Figure 3 and Listing 5 show how Multiple Inheritance can be used, in the class form of the ADAPTER pattern, to achieve the ISP. In this model, TimedDoor inherits from both Door and TimerClient. Although clients of both base classes can make use of TimedDoor, neither actually depend upon the TimedDoor class. Thus, they use the same object through separate interfaces.

This solution is my normal preference. Multiple Inheritance does not frighten me. Indeed, I find it quite useful in cases such as this. The only time I would choose the solution in Figure 2 over Figure 3 is if the translation performed by the DoorTimerAdapter object were necessary, or if different translations were needed at different times.

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In this article we have discussed the disadvantages of “fat interfaces”; i.e. interfaces that are not specific to a single client. Fat interfaces lead to inadvertent couplings beween clients that ought otherwise to be isolated. By making use of the ADAPTER pattern, either through delegation (object form) or multiple inheritance (class form), fat interfaces can be segregated into abstract base classes that break the unwanted coupling between clients.