CSC/ECE 517 Fall 2010/ch6 6f ag

Interface Segregation Principle

Introduction

Design phase is a very important phase in the entire process of [1] object-oriented software development. The need for a good design of software is to accommodate change into the software, a definite attribute of any software. For the benefit of the developers of these software, certain principles are laid out which ensure that the outcome of development phase is an easily manageable software rather than a rigid and fragile software. The principles of object-oriented software development are listed differently in different sources. The most commonly accepted and recognized is the [2]SOLID object oriented principles. In this article we focus on Interface segregation principle (ISP).

Interface Segregation Principle

Interface Segregation Principle emphasizes on

• MANY CLIENT SPECIFIC INTERFACES ARE BETTER THAN ONE GENERAL PURPOSE INTERFACE

Implications of the emphasis

1. The interfaces that are designed should not be fat, i.e the interface should not have too many methods which often do not get used together. The idea here is to avoid loading all the methods into a single interface, instead segregate the methods in the interface such that all the methods that contribute towards offering a single type of service (or single type of clients) are grouped under one single interface. Thereby dealing with a bulky interface that might be difficult to manage is avoided.
2. The interface should be specific. The methods in the interface should be the ones that contribute towards offering only a specific type of service. For example, if there is an interface that offers methods that provide mathematical services to its clients, then the same interface should not contain methods that accomplish totally different types of tasks say, like a database functionality implementation. The main reason behind such interfaces being discouraged is that if a class decides to (or has to) implement that interface, then, it has to provide an implementation even for those methods that it doesn’t intend to use. Even though such implementations may be empty methods, it is not convenient and also distorts the readability of the code.

Results of following ISP

Designing software by adopting ISP will yield many simple interfaces that are more specific in nature which in turn will make the classes more cohesive.

How ISP can be implemented in the design

Segregation through Delegation

The object form of the ADAPTER Pattern can be used to delegate the responsibility of implementing the thin interface to the class that is the client of the interface.

Segregation through multiple inheritance

The methods that are to be included into the interfaces are grouped into interfaces such that all the methods in the interface are directed towards accomplishing a certain type of service. When a class has to use the methods in the interface, it has to implement the interface. When the class is required to implement methods from many interfaces, the class will have to inherit multiple light interfaces and also implement the methods in the interfaces.

Conclusion

Fat Interfaces are the interfaces that are not specific to a single client. Fat interfaces lead to unintended couplings between clients that ought otherwise to be isolated. By making use of the ADAPTER pattern, through multiple inheritance (class form), fat interfaces can be segregated into abstract base classes that resolve the unwanted coupling between clients. Thus maintaining loose coupling and high cohesion, which is an essential attribute of good software design.

See Also

1. Design principles
2. Three Sources of a Solid Object-Oriented Design
3. Principles of OOD
4. Hanselminutes episode on SOLID

External References

1. The Interface Segregation Principle
2. Design Principles and Design Patterns, Robert C Martin
3. 2009 Gotham Ruby Conference, presentation on SOLID Object Oriented Design by Sandi Metz, Duke University.