1 Inheritance and Polymorphism Chapter 11 Spring 2007 CS 101 Aaron Bloomfield

2 This section is not required material!!!!  A note about inheritance… It’s not normally covered in 101 It will be gone over in more detail in CS 201  Ask questions if you are confused about inheritance You aren’t the only one!

3 Motivation  Consider a transportation computer game Different types of vehicles:  Planes Jets, helicopters, space shuttle  Automobiles Cars, trucks, motorcycles  Trains Diesel, electric, monorail  Ships …  Let’s assume a class is written for each type of vehicle

4 More on classes vs. objects

5 Motivation  Sample code for the types of planes: fly() takeOff() land() setAltitude() setPitch()  Note that a lot of this code is common to all types of planes They have a lot in common! It would be a waste to have to write separate fly() methods for each plane type  What if you then have to change one – you would then have to change dozens of methods

6 Motivation  Indeed, all vehicles will have similar methods: move() getLocation() setSpeed() isBroken()  Again, a lot of this code is common to all types of vehicles It would be a waste to have to write separate move() methods for each vehicle type  What if you then have to change one – you would then have to change dozens of methods  What we want is a means to specify one move() method, and have each vehicle type inherit that code Then, if we have to change it, we only have to change one copy

7 Motivation Provides:move()getLocation()setSpeed()isBroken() Provides: fly() takeOff() land() setAltitude() setPitch() Provides: derail() getStation() Provides: oilChange() isInTraffic()

8 Motivation  What we will do is create a “parent” class and a “child” class  The “child” class (or subclass) will inherit the methods (etc.) from the “parent” class (or superclass)  Note that some classes (such as Train) are both subclasses and superclasses

9 Inheritance code class Vehicle {... } class Train extends Vehicles {... } class Monorail extends Train {... }

10 About extends  If class A extends class B Then class A is the subclass of B Class B is the superclass of class A A “is a” B A has (almost) all the methods and variables that B has  If class Train extends class Vehicle Then class Train is the subclass of Vehicle Class Vehicle is the superclass of class Train Train “is a” Vehicle Train has (almost) all the methods and variables that Vehicle has

11 Object-oriented terminology  In object-oriented programming languages, a class created by extending another class is called a subclass  The class used for the basis is called the superclass  Alternative terminology The superclass is also referred to as the base class The subclass is also referred to as the derived class MonorailTrainVehicle

12 10 dimensions Disclaimer: it doesn’t have much scientific validity Disclaimer: it doesn’t have much scientific validity

13 End of lecture on 27 April 2007

14 Another example  Consider shapes in a graphics program Shape class  Circle class  Cube class  Dodecahedron class

15 Inheritance  Organizes objects in a top-down fashion from most general to least general  Inheritance defines a “is-a” relationship A mountain bike “is a” kind of bicycle A SUV “is a” kind of automobile A border collie “is a” kind of dog A laptop “is a” kind of computer

16 Packages  Allow definitions to be collected together into a single entity— a package  The classes in our game could be added to a package  Classes and names in the same package are stored in the same folder  Classes in a package go into their own “namespace” and therefore the names in a particular package do not conflict with other names in other packages For example, a package called OtherGame might have a different definition of Map

17 Controlling access  Class access rights Member Restriction thisSubclassPackageGeneral public protected  default  private 

18 Java’s Mother-of-all-objects—Class Object

19 Thus, everything extends Object  Either directly or indirectly  So what does that give us?  Object contains the following methods: clone() equals() toString() and others…  Thus, every class has those methods

20 More about clone()  It’s protected in class Object Which means that a Circle class also has a protected version Recall that protected means that code outside the Circle class can NOT call it  If we wanted a clone() method, we would declare it as such: public Object clone() { … }  Otherwise, we would get the following error message: Method clone() has protected visibility in class Cirlce

21 A note about equals()  Why does the equals() method always have to have the following prototype: boolean equals(Object obj)  Many other class in the Java SDK require the use of equals() Such as the Vector class  Those classes need to know how the equals() method will work in order for them to work properly Thus, it must have the same prototype  Same for toString() This method allows Java to “print” out an object

22 Overriding  Consider the following code: class Foo { // automatically extends Object public String toString () { return “Foo”; } }... Foo f = new Foo(); System.out.println (f);  Now there are two toString() method defined One inherited from class Object One defined in class Foo  And they both have the same prototype!  Which one does Java call?

23 Overriding  Java will call the most specific overriden method it can toString() in Foo is more specific than toString() in Object  Consider our transportation hierarchy: Assume each class has its own toString() method Car extends Automobile extends Vehicle (extends Object) Assume each defines a toString() methods  The toString() method in Vehicle is more specific (to vehicles) than the one in Object  The toString() method in Automobiles is more specific than the ones in Vehicle or Object  The toString() method in Car is more specific than the ones in Automobile, Vehicle, or Object  Thus, for a Car object, the Car toString() will be called There are ways to call the other toString() methods  This has to be specifically requested

24 Overriding  This is called overriding, because the toString() in Foo “overrides” the toString() in Object  Note that the prototype must be EXACTLY the same With overloading, the parameter list must be DIFFERENT  Overriding only works with inheritance In particular, you can only override a method already defined in a parent (or grandparent, etc.) class