SOLID Principles - IILSP – Liskov Substitution PrincipleISP – Interface Segregation Principle

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Liskov Substitution Principle

Subtypes must be substitutable for their base types.

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Is LSP Violated Here ?

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How To Spot LSP Violation ?

Derivative that tries to do less than base class Instance of checks Hiding or stubbing parent methods

Void SomeMethod(){} Void SomeMethod(){ Throw new ShouldNotBecalledException(“…”);}

Polymorphic if Statements

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Resolution

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Interface Segregation Principle

Client should not be forced to depend on methods they do not use

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Why ISP Important ?

Each member of an interface needs to be implemented in its entirety Properties Events Methods

Unless every client of an interface requires every member It does not make sense to force client to implement such a large contract

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Example

Software

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Is ISP Violated Here?

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Resolution

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Re-Visit the solution

Software

IPrint IScan IFaxIPrintIScan

IFax IPrintIScan

IStaple

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Summary

Design cohesive interfaces and avoid “fat” interfaces The dependency of one class to another should depend on the smallest possible

interface The interface of the class can be broken up into groups of methods

Each group serves a different set of clients Check Classes with a high number of Public methods

Group clients according to their calls to the public methods Check for methods that frequently change together

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End of Presentation