Inheritance - the myth of code reuse | Andrei Raifura | CodeWay 2015
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Watch this presentation if you want to know why inheritance is not always the most appropriate method for code reuse - and what to do instead. Watch the video here: https://www.youtube.com/watch?v=H6m0W-eDyAk The code used for the demo: https://github.com/yopeso/Inheritance
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Inheritance - the myth of code reuse | Andrei Raifura | CodeWay 2015
- 1. Andrei Raifura iOS Department Manager, YOPESO #CodeWăy Inheritance - The myth of code reuse
- 2. • How does Inheritance help us to reuse the code • Why it doesn’t work • The pitfalls of Inheritance • To use or not to use Agenda
- 3. • Encapsulation • Abstraction • Inheritance • Polymorphism OOP principles I will teach you OOP
- 4. Inheritance “…It is a mechanism for code reuse and to allow independent extensions of the original software via public classes and interfaces…” Wikipedia, Inheritance (object-oriented programming)
- 5. Inheritance ‣ Two ears ‣ For legs ‣ Tail Animal
- 6. Case study
- 8. Case study Can you program a Toyota Corolla for me? CustomerYou
- 11. Defining Car class Car { let frontLeft = Wheel() let frontRight = Wheel() let rearLeft = Wheel() let rearRight = Wheel()
- 12. Defining Car class Car { let frontLeft = Wheel() let frontRight = Wheel() let rearLeft = Wheel() let rearRight = Wheel() var manufacturer: String { get { return "Undefined" } }
- 13. Defining Car class Car { . . . func turnLeft(degrees: Double) { frontLeft.turnLeft(degrees) frontRight.turnLeft(degrees) } func turnRight(degrees: Double) { frontLeft.turnRight(degrees) frontRight.turnRight(degrees) }
- 14. Defining Car class Car { . . . func accelerate(kph: Double) { frontLeft.rotate(kph) frontRight.rotate(kph) } }
- 15. Defining Wheel class Wheel { private var angle = 0.0 private var rotationSpeed = 0.0 func turnRight(degrees: Double) { angle += degrees } func turnLeft(degrees: Double) { angle -= degrees } func rotate(kph: Double) { rotationSpeed = kph } }
- 16. Defining Toyota Corolla class ToyotaCorolla: Car { override var manufacturer: String { get { return "Toyota" } } }
- 17. Class Diagram - manufacturer - turnLeft - turnRight - accelerate Car ToyotaCorolla - turnLeft - turnRight - rotate Wheel
- 18. Case study Great! Now, i’d like to have a Toyota Corolla Sport CustomerYou
- 19. Case study Ok! will do! CustomerYou
- 23. Defining Toyota Corolla Sport class ToyotaCorollaSport: ToyotaCorolla { override func accelerate(kph: Double) { rearLeft.rotate(kph) rearRight.rotate(kph) } }
- 24. Class Diagram - manufacturer - turnLeft - turnRight - accelerate Car ToyotaCorolla - turnLeft - turnRight - rotate Wheel
- 25. Class Diagram - manufacturer - turnLeft - turnRight - accelerate Car ToyotaCorollaSport ToyotaCorolla - turnLeft - turnRight - rotate Wheel
- 26. Case study Woww! Now a Honda Civic and a Honda Civic Sport CustomerYou
- 29. Honda Civic Sport Wheels Manufacturer Transmission 4 Rear-wheel drive Honda
- 30. Class Diagram - manufacturer - turnLeft - turnRight - accelerate Car ToyotaCorollaSport ToyotaCorolla - turnLeft - turnRight - rotate Wheel
- 31. Class Diagram - manufacturer - turnLeft - turnRight - accelerate Car ToyotaCorollaSport ToyotaCorolla - turnLeft - turnRight - rotate Wheel HondaCivic HondaCivicSport
- 32. Defining Front-wheel Drive class FrontWheelDriveCar: Car { override func accelerate(kph: Double) { frontLeft.rotate(kph) frontRight.rotate(kph) } }
- 33. Defining Rear-wheel Drive class RearWheelDriveCar: Car { override func accelerate(kph: Double) { rearLeft.rotate(kph) rearRight.rotate(kph) } }
- 34. Class Diagram - manufacturer - turnLeft - turnRight - accelerate Car ToyotaCorollaSport ToyotaCorolla - turnLeft - turnRight - rotate Wheel HondaCivic HondaCivicSport
- 35. Class Diagram - manufacturer - turnLeft - turnRight - accelerate Car FrontWheelDriveCar ToyotaCorollaSport RearWheelDriveCar ToyotaCorolla - turnLeft - turnRight - rotate Wheel HondaCivic HondaCivicSport
- 36. Refactoring Toyota Corolla class ToyotaCorolla: FrontWheelDriveCar { override var manufacturer: String { get { return "Toyota" } } }
- 37. Refactoring Toyota Corolla Sport class ToyotaCorollaSport: RearWheelDriveCar { override var manufacturer: String { get { return "Toyota" } } }
- 38. Defining Honda Civic class HondaCivic: FrontWheelDriveCar { override var manufacturer: String { get { return "Honda" } } }
- 39. Defining Honda Civic Sport class HondaCivicSport: RearWheelDriveCar { override var manufacturer: String { get { return "Honda" } } }
- 41. Class Diagram - manufacturer - turnLeft - turnRight - accelerate Car FrontWheelDriveCar ToyotaCorollaSport RearWheelDriveCar ToyotaCorolla - turnLeft - turnRight - rotate Wheel HondaCivic HondaCivicSport
- 42. Class Diagram - manufacturer - turnLeft - turnRight - accelerate Car FrontWheelDriveCar ToyotaCorollaSport RearWheelDriveCar ToyotaCorolla - turnLeft - turnRight - rotate Wheel HondaCivic HondaCivicSport AllWheelDriveCar LexusGX
- 43. Defining All-wheel Drive class AllWheelDriveCar: Car { override func accelerate(kph: Double) { frontLeft.rotate(kph) frontRight.rotate(kph) rearLeft.rotate(kph) rearRight.rotate(kph) } }
- 44. Defining Lexus GX class LexusGX: AllWheelDriveCar { override var manufacturer: String { get { return "Lexus" } } }
- 45. Case study Amazing! But, can you make me an experimental car? CustomerYou
- 46. Case study It will switch between two-wheel drive and all-wheel drive… CustomerYou
- 47. Case study And will turn with all four wheels. CustomerYou
- 49. Experimental Wheels Manufacturer Transmission 4 Front-wheel drive & All-wheel drive Experimental
- 50. Experimental Wheels Manufacturer Transmission 4 Front-wheel drive & All-wheel drive Experimental Steering All-wheel steering
- 52. Class Diagram - manufacturer - turnLeft - turnRight - accelerate Car FrontWheelDriveCar RearWheelDriveCar - turnLeft - turnRight - rotate Wheel AllWheelDriveCar
- 53. Class Diagram - manufacturer - turnLeft - turnRight - accelerate Car FrontWheelDriveCar RearWheelDriveCar - turnLeft - turnRight - rotate Wheel AllWheelDriveCar ExperimentalCar
- 54. Class Diagram - manufacturer - turnLeft - turnRight - accelerate Car FrontWheelDriveCar RearWheelDriveCar - turnLeft - turnRight - rotate Wheel AllWheelDriveCar ExperimentalCar
- 55. Class Diagram - manufacturer - turnLeft - turnRight - accelerate Car FrontWheelDriveCar RearWheelDriveCar - turnLeft - turnRight - rotate Wheel AllWheelDriveCar ExperimentalCar
- 56. Class Diagram - manufacturer - turnLeft - turnRight - accelerate Car FrontWheelDriveCar RearWheelDriveCar - turnLeft - turnRight - rotate Wheel AllWheelDriveCar ExperimentalCar The Diamond of Dread
- 57. Code reuse “…It is a mechanism for code reuse and to allow independent extensions of the original software via public classes and interfaces…” Wikipedia, Inheritance (object-oriented programming)
- 58. Code reuse “…It is a mechanism for code reuse and to allow independent extensions of the original software via public classes and interfaces…” Wikipedia, Inheritance (object-oriented programming)
- 59. Solutions
- 60. Demo
- 61. Inheritance pitfalls • A very tight binding between a superclass and its subclasses • Makes the code fragile • Difficult to debug • Inheritance relationships generally can't be altered at runtime. • Leads to violation of Liskov Substitution Principle • Difficult to test
- 62. Inheritance pitfalls • A very tight binding between a superclass and its subclasses • Makes the code fragile • Difficult to debug • Inheritance relationships generally can't be altered at runtime. • Leads to violation of Liskov Substitution Principle • Difficult to test
- 63. Inheritance pitfalls • A very tight binding between a superclass and its subclasses • Difficult to debug • Inheritance relationships generally can't be altered at runtime. • Leads to violation of Liskov Substitution Principle • Difficult to test • Makes the code fragile
- 64. Inheritance pitfalls • A very tight binding between a superclass and its subclasses • Difficult to debug • Inheritance relationships generally can't be altered at runtime. • Leads to violation of Liskov Substitution Principle • Difficult to test • Makes the code fragile
- 65. Inheritance pitfalls • A very tight binding between a superclass and its subclasses • Difficult to debug • Inheritance relationships generally can't be altered at runtime. • Leads to violation of Liskov Substitution Principle • Difficult to test • Makes the code fragile
- 66. Liskov Substitution Principle Subtypes must be substitutable for their base types. Robert C. Martin (Uncle Bob)
- 67. Violation of LSP class File { var path: String! var data: NSData! func loadData() { // Retrieve data from disk } func saveData() { // Write data to disk } }
- 68. class ReadOnlyFile: File { override func saveData() { print("Cannot write data") } } Violation of LSP
- 69. Inheritance pitfalls • A very tight binding between a superclass and its subclasses • Difficult to debug • Inheritance relationships generally can't be altered at runtime. • Leads to violation of Liskov Substitution Principle • Difficult to test • Makes the code fragile
- 70. Inheritance pitfalls • A very tight binding between a superclass and its subclasses • Makes the code fragile • Difficult to debug • Inheritance relationships generally can't be altered at runtime. • Leads to violation of Liskov Substitution Principle • Difficult to test
- 71. Should we avoid inheritance altogether?
- 72. To use? Use Inheritance when your derived class truly is the type you're extending. And it will always be!
- 73. The Employee - firstName - lastName - age - department Employee
- 74. The Employee and the Student - firstName - lastName - age - year - faculty Student - firstName - lastName - age - department Employee
- 75. The Employee and the Student - firstName - lastName - age Person - department Employee - year - faculty Student
- 76. The Employee and the Student - firstName - lastName - age Person - department Employee - year - faculty Student I am a Student. And an Employee.
- 77. The Employee and the Student - firstName - lastName - age Person - department Employee - year - faculty Student I am a Student. And an Employee. I've just become unemployed.
- 78. The Employee and the Student - firstName - lastName - age Person Occupation - department Employee - year - faculty Student Unemployed
- 79. Or not to use? How about polymorphism? Consider using Protocols* to achieve a polymorphic behaviour. * Also known as Interfaces in other languages
- 80. I need to reuse some code from superclass. No The derived class is almost the extending type No The derived class is the extending type but might change in the future No I need a polymorphic behaviour No The derived class truly is the extending type. And it won't change. I swear! Yes To use or Not to use?
- 81. • How does Inheritance help us to reuse the code • Why it doesn’t work • The pitfalls of Inheritance • To use or not to use Review
- 83. Questions?
- 84. Contacts Raifura Andrei iOS Department Manager, YOPESO andrei.raifura@yopeso.com thelvis4@gmail.com #CodeWăy