ooad 2 interfaces and the facade pattern

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OOAD 2Interfaces and the Facade pattern

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A bit about interfaces

• “Always Program to an Interface”

• We don’t necessarily mean a Java-style Interface language construct

• I guess you could simplify it to:– Always program to a supertype

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“Always Program to an Interface”

To an implementation:

Dog d = new Dog();

d.bark();

To an interface:

Animal a = getAnimal(“dog”);

a.makeSound();

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Advantages?

• We’re not worrying about implementations

• Can deal with things generically (more abstraction), reducing code

• Greater resilience to change

• We’ll see more advantages as we cover more patterns in the coming weeks

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Facade pattern

• Takes a complex subsystem and makes it a simpler one

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Subsystem

Class 1Class 1

Class 5Class 5

Class 4Class 4

Class 3Class 3

Class 6Class 6

Class 2Class 2

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Subsystem

Class 1Class 1

Class 5Class 5

Class 4Class 4

Class 3Class 3

Class 6Class 6

Class 2Class 2

Client classClient class

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Subsystem

Class 1Class 1

Class 5Class 5

Class 4Class 4

Class 3Class 3

Class 6Class 6

Class 2Class 2

FacadeFacadeClient classClient class

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Subsystem

Class 1Class 1

Class 5Class 5

Class 4Class 4

Class 3Class 3

Class 6Class 6

Class 2Class 2

FacadeFacadeClient classClient class Client class 2Client class 2

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Facade pattern summary

• Takes a complex subsystem and makes it a simpler one

• You can still directly access the underlying classes

• Example: home theatre controller– Simple, unified interface– Most common functions in one place– You can still use the individual controllers

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When to use it?

• To provide an easy interface to lots of subsystems

• Principle of Least Knowledge – reduce interactions between objects– Reduces the amount of “spaghetti”