13 X 11 Java Lecture 5 CS 1311X Our Story So Far The Story of O Inheritance, Polymorphism and Death in the Afternoon

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Java Lecture 5

CS 1311X

Our Story So Far

The Story of O

Inheritance, Polymorphism and Death in the Afternoon

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Our Story So Far

• Object Oriented Programming Features– Encapsulation– Reusability– Adaptability

• Object Oriented Programming Benefits– Generic "Drop In" Components– Modeling Real World Objects– Handling Collections Easily

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Our Story So Far

• Java Language Syntax• Class Structure

– Fields– Constructors– Methods

• Accessors• Modifiers

– Special Concepts• toString• main• static

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Our Story So Far

• All objects are manipulated using references

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The Story of O

CS Version

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The Story of O

Object Oriented

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The Story of O

• Operator Overloading

int a = 3;

int b = 2;

int c;

String x = "Hello ";

String y = "World!";

String z;

c = a + b;

z = x + y;

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The Story of O

• Other Overloading– Constructor

public Student(String name, double gpa)

public Student(String name)– Method

public int max(int a, int b)

public int max(int a, int b, int c)

public int max(short a, short b)

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The Story of O

• Overriding– Also called redefinition

class A {

int someMethod()

}

class B extends A

int someMethod()

}

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The Story of O

Everything

is an

Object!

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Inheritanceclass Animal

{

String name;

public Animal(String name)

{

this.name = name;

}

}

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Inheritanceclass Dog extends Animal

{

int fleas;

public Dog(String name, int fleas)

{

super(name);

this.fleas = fleas;

}

}

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Inheritanceclass Cat extends Animal

{

int hairBalls;

public Cat(String name, int hairBalls)

{

super(name);

this.hairBalls = hairBalls;

}

}

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Inheritance (Deceptive Diagram)

Animal

CatDog

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Inheritance (True Diagram)

Animal

CatDog

Object

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Truth Be Known

• Class Object exists• It defines lots of useful methods

– e.g. toString– see the API

• Thus every class is either – a direct subclass of Object (no extends)

or– a subclass of a descendant of Object (extends)

• So what?

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Motivation

• Repetitive tasks

• Collections of things (objects)– Baseball cards– Library items– Shapes– Animals– Vehicles– Students

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Motivation

• Collections are seldom uniform

• Desire method of holding a collection of dissimilar items

• Need to change the type mismatch rules

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Recall

float f;

double d ;

int i;

String s;

CokeMachine cm;

f = d; // illegal

d = f; // legal

i = d; // illegal

d = i; // legal

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Recall

float f;

double d ;

int i;

String s;

CokeMachine cm;

f = (float)d; // legal

d = f; // legal

i = (int)d; // legal

d = i; // legal

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Recall

float f;

double d ;

int i;

String s;

CokeMachine cm;

s = cm; // illegal

cm = s; // illegal

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Recall

float f;

double d ;

int i;

String s;

CokeMachine cm;

s = (String)cm; // illegal

cm = (CokeMachine)s; // illegal

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Inheritance Changes the Rules

Animal

CatDog

Object

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Inheritance Changes the RulesObject o;

Animal a;

Dog d = new Dog();

Cat c = new Cat();

d = c; // illegal

a = c; // OK, a Cat is an Animal

o = c; // OK, a Cat is an Object

o = a; // OK, an Animal is an Object

a = o; // Illegal, not all Objects are Animals

d = a; // Illegal, not all animals are Dogs

Confusing?

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The Keyword is Extends

Object creation process

Dog d = new Dog();

1. Create reference d

2. Start creating Dog by entering Dog constructor and making call to parent.

3. Start creating Animal by entering Animal constructor and making call to parent.

4. Create Object portion

5. Create Animal portion

6. Create Dog portion

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Step by Step

d

Ref: Dog

Dog d

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Step by Step

d

Ref: Dog

Dog d = new Dog();public Dog(){

}

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Step by Step

d

Ref: Dog


}

public Animal(){

}

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Step by Step

d

Ref: Dog


}

public Animal(){

}

public Object(){

}

Object

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Step by Step

d

Ref: Dog


}

public Animal(){

}

Object

Animal

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Step by Step

d

Ref: Dog


}Object

Animal

Dog

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Step by Step

d

Ref: Dog

Dog d = new Dog();

Object

Animal

Dog

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Step by Step

d

Ref: Dog

Dog d = new Dog();

Object

Animal

Dog

"A Dog Object"

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Quizlette

a

Ref: Animal

Animal a = new Dog();

Object

Animal

Dog

"A Dog Object"

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Quizlette

o

Ref: Object

Object o = new Dog();

Object

Animal

Dog

"A Dog Object"

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Quizlette

d

Ref: Dog

Dog d = new Animal();

Object

Animal "An Animal Object"

ILLEGAL

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Quizlette

a

Ref: Animal

Animal a = new Object();

Object

"An Object Object"

ILLEGAL

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Quizlette

Dog d = new Object();

?

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Quizlette

d

Ref: Dog

Dog d = new Object();

Object

"An Object Object"

ILLEGAL

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Same Logic

d

Ref: Dogd = o;

o = d;

o

Ref: Object

o

Ref: Object

d

Ref: Dog

?

?OK!

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Don't be confused!!!

• Primitives

double d;

float f;

d = f; // legal...no loss of information

f = d; // illegal...potential loss of

// information

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Don't be confused!!!

•References

Object o;

Dog d;

o = d; // legal (a dog is an object)

d = o; // illegal (all objects are not

// dogs)

Note: Not the same as primitives...they are all just references!!!

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Warning

• When you break the aforementioned rules...

• Java sometimes tells you that a cast is required

• Even if it's a real bad idea

Pearls p;

Swine s;

p = (Pearls)s;

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What's the Point?

• Java Philosophy: Catch errors at compile time.

• Leading to tricky concept #2: Dynamic Binding

• At run time (dynamic) when a method is invoked on a reference the ACTUAL OBJECT is examined and the "lowest" or closest version of the method is actually run.

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Dynamic Binding

• The heart of polymorphism

• Assume Animal and Dog have a toString method



Dog d = new Dog();

o.toString();

a.toString();

d.toString();

((Object)o).toString();

Animal

Object

Dog

Animal

Object

Object

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Dynamic Binding

• It even works like this...

Object toString()

Animal

Dog toString()

Animal a = new Dog();a.toString();

Ref: Animal

a

A Dog Object

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Trick #3

• Java checks types at compile time when assigning references (Run time checking is also performed).

• Java always decides the method to be invoked by looking at the object at runtime.

• At compile time Java checks method invocations to make sure that the reference type will have the correct method. This may appear contradictory to dynamic binding.

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Reference/Method Checking

x.y();

• x is a reference which has a type which is its class• That class (or a superclass) must have method y

or a compile error will result.

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More Quizlette FunObject o = new Dog();


Dog d = new Dog();

o.toString();

o.move();

o.bark();

a.toString();

a.move();

a.bark();

d.toString();

d.move();

d.bark();

Object toString()

Animal move()

Dog move() toString() bark()

Object toString()

Animal move()


Object toString()

Animal move()


d

a

o

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Back to Collections

• The simple approach

Object

Dog woof()

Cat meow()

Pig oink()

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Java Collections

• LinkedList• ArrayList• Vector• Stack• HashSet• TreeSet• HashTable• Plus you'll write lots on your own...

• They all hold objects!

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Using Generic Collections -- PoorlyLinkedList zoo = new LinkedList();


Pig p = new Pig();

zoo.add(o);

zoo.add(new Cat());

zoo.add(p);

while(zoo.size() > 0) {

o = zoo.removeFirst();

if(o instanceOf Dog)

((Dog)o).bark();

if(o instanceOf Cat)

((Cat)o).meow();

if(o instanceOf Pig)

((Pig)o).oink();

}

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Can We Do Better?

• A first try

Object

Dog talk()

Cat talk()

Pig talk()

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Using Generic Collections -- Betterly???LinkedList zoo = new LinkedList();


Pig p = new Pig();

zoo.add(o);

zoo.add(new Cat());

zoo.add(p);



o.talk(); // Does this work???

}

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Using Generic Collections -- Betterly???LinkedList zoo = new LinkedList();


Pig p = new Pig();

zoo.add(o);

zoo.add(new Cat());

zoo.add(p);



((???))o.talk(); // Does this work???

}

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Mystery Partially Solved

Object

Dog talk()

Cat talk()

Pig talk()

Animal talk()

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Using Generic Collections -- CorrectlyLinkedList zoo = new LinkedList();



Pig p = new Pig();

zoo.add(a);

zoo.add(o);

zoo.add(new Cat());

zoo.add(p);



((Animal))o.talk();

}

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Software Engineering

• What should the talk method in class Animal do?

public void talk()

{

/*

* Make sure you redefine this method in

* your individual animal subclasses

*/

}

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How many will forget?

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Solution: Abstract Methods and Classesclass Animal

{

public abstract void talk();

// Note: No body!

}

Result: Error!!!

Sometimes called a deferredmethod...

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Solution: Abstract Methods and Classesabstract class Animal

{

public abstract void talk();

// Note: No body!

}

Result: Okay.

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Java Never Forgets

• Any subclass of class Animal has two choices:

– Define a talk method (i.e. { })

– Be abstract

• Note: Abstract classes may not be used to instantiate or make objects (new)

• References to abstract classes are legal (and quite desireable).

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Concreteclass Dog extends Animal

{

public void talk()

{

bark();

}

}

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Orclass Dog extends Animal

{

public void talk()

{

System.out.println("Bow wow!);

}

}

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Just for the Recordabstract class Canine

{

// define other stuff?

}

Object

Dog talk()

Cat talk()

Pig talk()

abstract Animal abstract talk()

abstract Canine

Wolf talk()

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Using Generic Collections -- CorrectlyLinkedList zoo = new LinkedList();



Pig p = new Pig();

zoo.add(a);

zoo.add(o);

zoo.add(new Cat());

zoo.add(p);



((Animal))o.talk();

}

Object

Dog talk()

Cat talk()

Pig talk()

abstract Animal abstract talk()

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Design Guidelines

• Class hierarchy design is important– Move common methods up the tree– Use abstract methods appropriately– May be iterative process

• Container classes should generally hold Objects• Learn and understand the Java rules concerning

– Type checking– Reference checking– Dynamic binding

• You'll be glad you did!

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Back to the Problem at Hand

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Coke Machines Available

•CokeMachine

•Standard Model•Requires Exact Change•Low Cost

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•CokeMachine2000

•Deluxe Model•Makes Change•Intermediate Cost

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•CokeMachineUA (Urban Assault)

•Secure Model•Makes Change•High Cost

•Tamper Proof

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The Vending Machine Hierarchy

VendingMachine vend() load() vandalize() toString()

CokeMachine vandalize() toString()

CokeMachine2000 vend() vandalize() toString()

CokeMachineUA vandalize() toString()abstract

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The Players

• Students– Normally have change & dollar bills

• Instructors– Just have change (and not much of that)

• Deans– Have lots of money– If machine is out of coke, will vandalize machine

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The Players

Person interact() toString()

Student interact()

Instructor interact()

Dean interact()

abstract

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Algorithm• Create population of Students, Instructors and Deans

adding each to population LinkedList• Create group of machines adding to machines LinkedList• Loop in time

– Dequeue a person– Dequeue a machine– Allow them to interact (capture statistics)– If the person is still living

• Enqueue them back into population– Enqueue machine– If time for service

• Randomly select and service one machine

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The Simulation

machinespopulation

p.interact(v)

p v

if stillalive

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Heart of Simulationp = (Person)population.removeFirst();

v = (VendingMachine)machines.removeFirst();

result = p.interact(v);

stats[result + 2]++;

if(result >= -1)

{

population.addLast(p);

}

else

{

System.out.println("Another one bites the dust!");

}

machines.addLast(v);

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Without Polymorphism?• Imagine what would have to happen in interaction without polymorphism

if Studentif CokeMachineelse if CokeMachine2000else if CokeMachineUA

else if Instructorif CokeMachineelse if CokeMachine2000else if CokeMachineUA

elseif Deanif CokeMachineelse if CokeMachine2000else if CokeMachineUA

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Serviceif((time % SERVICE) == 0)

{

int chg;

int random;

int siz = machines.size();

VendingMachine vs;

random = (int)(siz * Math.random());

vs = (VendingMachine)machines.get(random);

vs.load(CASE);

chg = STARTUPCHANGE - vs.getChange();

chg = vs.addChange(chg);

startup += chg;

}

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Demo

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Documents

13 X 11 Java Lecture 5 CS 1311X Our Story So Far The Story of O Inheritance, Polymorphism and Death in the Afternoon