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Inheritance and Polymorphism

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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!

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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

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More on classes vs. objects

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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

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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

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Motivation

Trains

Automobiles

Planes

Jet

Helicopter

Electric

Monorail

Diesel

Space shuttle

Car

Truck

Motorcycle

Vehicle

Provides:Provides:move()move()

getLocation()getLocation()setSpeed()setSpeed()isBroken()isBroken()

Provides:fly()

takeOff()land()

setAltitude()setPitch()

Provides:derail()

getStation()

Provides:oilChange()isInTraffic()

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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

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Inheritance code

class Vehicle { ...}

class Train extends Vehicles { ...}

class Monorail extends Train { ...}

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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

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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

Monorail Train Vehicle

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Another example Consider shapes in a graphics program

Shape class Circle class Cube class Dodecahedron class

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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

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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

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Controlling access Class access rights

Member Restriction

this Subclass Package General

public

protected

default

private

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

Obj ectShapeCi r cl e

Class representing

a circle

Superclass of all J ava objects.

Class representing

any shape

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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

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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

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Overriding Consider the following code:

class Foo { // automatically extends Objectpublic 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?

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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

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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