Chapter 10 INHERITANCE 11/7/16 1

Transcription

1 Chapter 10 INHERITANCE 11/7/16 1

2 Chapter Goals To learn about inheritance To implement subclasses that inherit and override superclass methods To understand the concept of polymorphism In this chapter, you will learn how the no=on of inheritance expresses the rela=onship between specialized and general classes. 11/7/16 2

3 Contents Inheritance Hierarchies Implemen=ng Subclasses Calling the Superclass constructor Overriding Methods Polymorphism Applica=on: A geometric shape hierarchy 11/7/16 3

4 Inheritance Hierarchies In object-oriented programming, inheritance is a rela=onship between: A superclass: a more generalized class A subclass: a more specialized class The subclass inherits data (variables) and behavior (methods) from the superclass 11/7/16 4

5 A Vehicle Class Hierarchy General Vehicle Specialized Motorcycle Car Truck More Specific Sedan SUV 11/7/16 5

6 The Substitution Principle Since the subclass Car is-a Vehicle Car shares common traits with Vehicle You can subs=tute a Car object in an algorithm that expects a Vehicle object mycar = Car(...) processvehicle(mycar) is-a The is-a relationship is represented by an arrow in a class diagram and means that the subclass can behave as an object of the superclass. 11/7/16 6

7 Quiz Question Hierarchy There are different types of quiz ques=ons: 1) Fill-in-the-blank 2) Single answer choice 3) Mul=ple answer choice 4) Numeric answer 5) Free Response The root of the hierarchy is shown at the top. A ques=on can: Display its text Check for correct answer 11/7/16 7

8 Questions.py The class Question is the root of the hierarchy, also known as the superclass Only handles Strings No support for: Numeric answers Mul=ple answer choice 11/7/16 8

9 Questions.py 11/7/16 9

10 Questions.py Creates an object of the Question class and uses methods. 11/7/16 10

11 ity to inherits transport from from one place to another. We say that the the class datapeo andple behavior the superclass. For example, consider re Vehicle. In this relationship, the Vehicle class is the super from the class ships between different kinds of vehicles depicted in Figure 1. Car classevery is thecar subclass. InCars Figure the superclass and are asjoit is a vehicle. share2,the common traits of all subclass vehicles, such from one place to another. We say that the class Car to points transport arrowity that topeo theple superclass. Vehicle. In this relationship, the Vehicle class is the superclass from the object. Beca Suppose weclass have an algorithm that manipulates a Vehicle You can always use Car class is the subclass. In Figure 2, the superclass and subclass are joined w special kind of vehicle, we can use a Car object in such an algorithm, and a subclass object arrow that points to the superclass. in place of a correctly. The substitution principle that you can always use a sua object. Because Suppose we have an algorithm thatstates manipulates a Vehicle superclass You object. can always use when special a superclass For example, sider a method object in such con an algorithm, and it wit kind of object vehicle, is weexpected. can use a Car a subclass object in place of a : argument of type correctly. TheVehicle SUBSTITUTION PRINCIPLE states that you can always use a subclas from a superclass. Programming Tip superclass object. when a superclass object is expected. For example, consider a method that t void processvehicle(vehicle v)for Varia=on in Use a Single Class for Varia=on in Values, Inheritance argument of type Vehicle: Behavior void processvehicle(vehicle v) If two vehicles only vary by fuel efficiency, use an instance variable for the varia=on, not inheritance # Car instance variable milespergallon Vehicle Vehicle If two vehicles behave differently, use inheritance Motorcycle Motorcycle Truck Truck Car Car Be careful not to over-use inheritance Sedan SUV Sedan Hierarchy of Vehicle Classes SUV Figure 1 An Inheritance /7/ Figure 1 An Inheritance Hierarchy of Vehicle Classes 11

12 The Cosmic Superclass: object In Python, every class that is declared without an explicit superclass automa=cally extends the class object 11/7/16 12

13 Implementing Subclasses Consider implemen=ng ChoiceQuestion to handle: How does ChoiceQuestion differ from Question? It stores choices (1,2,3 and 4) in addi=on to the ques=on There must be a method for adding mul=ple choices The display() method will show these choices below the ques=on, numbered appropriately In this sec4on you will see how to form a subclass and how a subclass automa4cally inherits from its superclass 11/7/16 13

14 Inheriting from the Superclass Subclasses inherit from the superclass: All methods that it does not override All instance variables The Subclass can Add new instance variables Add new methods Change the implementa=on of inherited methods Form a subclass by specifying what is different from the superclass. 11/7/16 14

15 Overriding Superclass Methods Can you re-use any methods of the Question class? Inherited methods perform exactly the same If you need to change how a method works: Write a new more specialized method in the subclass Use the same method name as the superclass method you want to replace It must take all of the same parameters This will override the superclass method The new method will be invoked with the same method name when it is called on a subclass object A subclass can override a method of the superclass by providing a new implementation. 11/7/16 15

16 Planning the Subclass Pass the name of the superclass Question as part of the defini=on of the subclass Inherits text and answer variables Add new instance variable choices class ChoiceQuestion(Question): # The subclass has its own constructor. def init (self) :... # This instance variable is added to the subclass. self._choices = [] # This method is added to the subclass def addchoice(self, choice, correct) :... # This method overrides a method from the superclass def void display(self) :... 11/7/16 16

17 Syntax 10.1: Subclass Definition The class name inside parentheses in the class header denotes inheritance. 11/7/16 17