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Chapter 1: Introduction to Design Patterns

Chapter 1: Introduction to Design Patterns. SimUDuck Example. System Update. Revise the current system to cater for ducks that fly. How can this be done?. A Possible Solution. Suppose that a RubberDuck class needs to be added?. Revision: Use Inheritance?. Suppose that you have to add

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Chapter 1: Introduction to Design Patterns

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  1. Chapter 1: Introduction to Design Patterns

  2. SimUDuck Example

  3. System Update • Revise the current system to cater for ducks that fly. How can this be done?

  4. A Possible Solution Suppose that a RubberDuck class needs to be added?

  5. Revision: Use Inheritance? Suppose that you have to add a decoy duck that does not fly or quack…

  6. Another Solution: Interfaces • The aspects that change for each type of duck are the methods fly() and quack(). • Take these methods out of the Duck class. • Create interfaces for these methods: • Flyable – has a method fly() • Quackable – has a method quack()

  7. Using Interfaces Problems with this?

  8. Disadvantages of the Approach • All methods in Java interfaces are abstract. • Code has to be duplicated for classes. • Modification will have to be made to more than one class. • This will introduce bugs.

  9. Design Principle • Identify the aspects of the application that vary and separate them from what stays the same. • Encapsulate the parts that vary. • Future changes can be made without affecting parts that do not vary. • Results in fewer unexpected consequences from code change.

  10. Solution • Design principle: Program to interface and not to implementation. • An interface FlyBehaviorwill be implemented by subclasses for different types of flight. • An interface Quackable will be implemented for different types of “quack”.

  11. Example Abstract class OR Interface

  12. FlyBehavior Interface

  13. QuackBehavior

  14. New Design

  15. On Last Problem • Still programming to implementation in the subclasses. • Solution – include set methods in the Duck class • To set quack behavior • To set fly behavior • The set methods allow for behavior to be dynamic at runtime

  16. Exercise • Add a new type of duck to the simulator, namely, model duck. A model duck does not fly and quacks. • Add a new fly behavior to the simulator, namely, FlyRocketPower, which represents flight via a rocket. • Create an instance of a model duck and change its behaviour at runtime to be flight via a rocket.

  17. Has-A Can be Better than Is-A • Inheritance promotes reuse but not extensibility. • Composition facilitates more flexibility. • Composition allows for behavior to change at runtime (rather than editing code). • Design principle: Favor composition over inheritance.

  18. The Strategy Pattern Defines a family of algorithms, encapsulates each one, and makes them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

  19. Lessons Learned from the Strategy Pattern • Encapsulate what varies. • Favor composition over inheritance. • Program to interfaces, not implementations

  20. Advantages of Sharing a Pattern Vocabulary • Shared pattern vocabularies are powerful. • Patterns allow you to say more with less. • Talking at the pattern level allows to stay “in design” longer. • Shared vocabularies can turbo charge your development. • Shared vocabularies encourage more junior developers to get up to speed.

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