Java Lecture 5

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Java Lecture 5

• CS 1311X
• Our Story So Far
• The Story of O
• Inheritance, Polymorphism and Death in the
  Afternoon

13X11
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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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Inheritance

• class Animal
• String name
• public Animal(String name)
• this.name name

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Inheritance

• class Dog extends Animal
• int fleas
• public Dog(String name, int fleas)
• super(name)
• this.fleas fleas

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Inheritance

• class Cat extends Animal
• int hairBalls
• public Cat(String name, int hairBalls)
• super(name)
• this.hairBalls hairBalls

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Inheritance (Deceptive Diagram)
Animal
Dog
Cat
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Inheritance (True Diagram)
Object
Animal
Dog
Cat
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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
Object
Animal
Dog
Cat
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Inheritance Changes the Rules

• Object 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
Dog d
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Step by Step
Dog d new Dog()
public Dog()
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Step by Step
Dog d new Dog()
public Dog()
public Animal()
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Step by Step
Dog d new Dog()
public Dog()
Object
public Animal()
public Object()
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Step by Step
Dog d new Dog()
public Dog()
Object
Animal
public Animal()
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Step by Step
Dog d new Dog()
public Dog()
Object
Animal
Dog
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Step by Step
Dog d new Dog()
Object
Animal
Dog
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Step by Step
Dog d new Dog()
Object
Animal
"A Dog Object"
Dog
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Quizlette
Animal a new Dog()
Object
Animal
"A Dog Object"
Dog
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Quizlette
Object o new Dog()
Object
Animal
"A Dog Object"
Dog
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Quizlette
Dog d new Animal()
Object
Animal
"An Animal Object"
ILLEGAL
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Quizlette
Animal a new Object()
Object
"An Object Object"
ILLEGAL
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Quizlette
Dog d new Object()
?
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Quizlette
Dog d new Object()
Object
"An Object Object"
ILLEGAL
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Same Logic
d o
?
o d
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
• Object o new Dog()
• Animal a new Dog()
• Dog d new Dog()
• o.toString()
• a.toString()
• d.toString()
• ((Object)o).toString()

Object
Object
Animal
Object
Animal
Dog
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Dynamic Binding

• It even works like this...

A Dog Object
Ref Animal
a
Animal a new Dog() a.toString()
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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 Fun

• Object o new Dog()
• Animal a new Dog()
• Dog d new Dog()
• o.toString()
• o.move()
• o.bark()
• a.toString()
• a.move()
• a.bark()
• d.toString()
• d.move()
• d.bark()

o
a
d
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Back to Collections

• The simple approach

Object
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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 -- Poorly

• LinkedList zoo new LinkedList()
• Object o new Dog()
• Pig p new Pig()
• zoo.add(o)
• zoo.add(new Cat())
• zoo.add(p)
• while(zoo.size() gt 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
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Using Generic Collections -- Betterly???

• LinkedList zoo new LinkedList()
• Object o new Dog()
• Pig p new Pig()
• zoo.add(o)
• zoo.add(new Cat())
• zoo.add(p)
• while(zoo.size() gt 0)
• o zoo.removeFirst()
• o.talk() // Does this work???

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Using Generic Collections -- Betterly???

• LinkedList zoo new LinkedList()
• Object o new Dog()
• Pig p new Pig()
• zoo.add(o)
• zoo.add(new Cat())
• zoo.add(p)
• while(zoo.size() gt 0)
• o zoo.removeFirst()
• ((???))o.talk() // Does this work???

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Mystery Partially Solved
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Using Generic Collections -- Correctly

• LinkedList zoo new LinkedList()
• Animal a new Dog()
• Object o new Dog()
• Pig p new Pig()
• zoo.add(a)
• zoo.add(o)
• zoo.add(new Cat())
• zoo.add(p)
• while(zoo.size() gt 0)
• o zoo.removeFirst()
• ((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 Classes

• class Animal
• public abstract void talk()
• // Note No body!
• Result Error!!!

Sometimes called a deferred method...
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Solution Abstract Methods and Classes

• abstract 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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Concrete

• class Dog extends Animal
• public void talk()
• bark()

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Or

• class Dog extends Animal
• public void talk()
• System.out.println("Bow wow!)

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Just for the Record

• abstract class Canine
• // define other stuff?

Object
abstract Animal abstract talk()
abstract Canine
Cat talk()
Pig talk()
Dog talk()
Wolf talk()
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Using Generic Collections -- Correctly

• LinkedList zoo new LinkedList()
• Animal a new Dog()
• Object o new Dog()
• Pig p new Pig()
• zoo.add(a)
• zoo.add(o)
• zoo.add(new Cat())
• zoo.add(p)
• while(zoo.size() gt 0)
• o zoo.removeFirst()
• ((Animal))o.talk()

Object
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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Coke Machines Available

• CokeMachine2000
• Deluxe Model
• Makes Change
• Intermediate Cost

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Coke Machines Available

• CokeMachineUA
• (Urban Assault)
• Secure Model
• Makes Change
• High Cost
• Tamper Proof

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The Vending Machine Hierarchy
VendingMachine vend() load() vandalize()
toString()
CokeMachine2000 vend() vandalize()
toString()
CokeMachine 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
p.interact(v)
p
v
population
machines
if still alive
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Heart of Simulation

• p (Person)population.removeFirst()
• v (VendingMachine)machines.removeFirst()
• result p.interact(v)
• statsresult 2
• if(result gt -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 Student if CokeMachine else if
CokeMachine2000 else if CokeMachineUA else if
Instructor if CokeMachine else if
CokeMachine2000 else if CokeMachineUA elseif
Dean if CokeMachine else if
CokeMachine2000 else if CokeMachineUA
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Service

• if((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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