Inheritance and Polymorphism

Inheritance and Polymorphism Chapter 9 Spring 2006 CS 101 Aaron Bloomfield

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!

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

More on classes vs. objects

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

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

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

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

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

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

Monorail Train Vehicle 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

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

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

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

Controlling access • Class access rights

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

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

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 user 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

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?

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

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

Inheritance and Polymorphism