1 6inheritance and Polymorphism v0!9!101027034323 Phpapp02

8/11/2019 1 6inheritance and Polymorphism v0!9!101027034323 Phpapp02

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Contents

1. Inheritance in Java Inheriting Classes The super Reference Overriding Methods Polymorphism The instanceof Operator final Methods and Classes

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Contents (2)

1. Abstract Classes Abstract Methods

2. Interfaces Defining Interfaces Implementing Interfaces

3. Using Interfaces (Demo)

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Access Modifierspublic, private, protected, default

(package)


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Access Modifiers

The most common modifiers are the accessmodifiers:

public

private protected (default)

Access modifiers control which classesmay use a member

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Access Modifiers Rules

The variables that you declare and usewithin a classs methods may not haveaccess modifiers

The only access modifier permitted to non-inner classes is public A member may have at most one access

modifier

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public

The most generous access modifier is public A public class, variable, or method may be

used in any Java program withoutrestriction Any public method may be overridden by

any subclass

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private

The least generous access modifier is private Top-level (that is, not inner) classes may

not be declared private A private variable or method may be used

only by an instance of the class thatdeclares the variable or method

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(default)

Default is the name of the access ofclasses, variables, and methods if youdont specify an access modifier

A classs data and methods may be default,as well as the class itself

A classs default members are accessible toany class in the same package as the class

in question A default method may be overridden by any

subclass that is in the same package as thesuperclass 10


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Applying Encapsulation inJava

Instance variables should bedeclared as private

Only instance methods can

access private instancevariables

Movie mov1 = new Movie();String rating = mov1.getRating();String r = mov1.rating; // Error: private

...if (rating.equals("G"))

var

aMethod

aMethod()


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Packages


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What Are Java Packages?

A package is a container of classes that arelogically related A package consists of all the Java classes

within a directory on the file system Package names and used within a JRE to

manage the uniqueness of identifiers They segment related parts of complex

applications into manageable parts


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Grouping Classes in aPackage

Include the package keyword followed byone or more names separated by dots at thetop of the Java source file

If omitted the compiler places the class inthe default unnamed package 14

package utils;

public class MathUtils {...

}


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Grouping Classes in aPackage

To run a main() method in a packaged classrequires: The CLASSPATH to contain the directory

having the root name of the package tree The class name must be qualified by its

package name

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java cp . myPackage.MainClass


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The CLASSPATH withPackages

Includes the directory containing the top levelof the package tree

16C:\>setCLASSPATH=E:\Curriculum\courses\java\practices\les06

Package name .class location

CLASSPATH


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Inheritance andPolymorphismCode Reuse Techniques


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Key Object-OrientedComponents

InheritanceConstructors referenced by subclass

Polymorphism

Inheritance is an OO fundamental

InventoryItem

Movie Game Book

Superclass

Subclasses


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Example of Inheritance

The InventoryItem class defines methodsand variables

Movie extends InventoryItem and can:

Add new variables

Add new methods

Override methods in InventoryItem class

InventoryItem

Movie

f h


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Specifying Inheritance inJava

Inheritance is achieved by specifying whichsuperclass the subclass extends

Movie inherits all the variables and methodsof InventoryItem

public class InventoryItem {

}

public class Movie extends InventoryItem {

}

Wh D S b l


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What Does a SubclassObject Look Like?

A subclass inherits all the instancevariables of its superclass

Movie

titlelength

pricecondition


private String title; private int length;

}


private float price; private String condition;

}


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Default Initialization

What happens when asubclass object is created?

If no constructors are defined:

First, the defaultparameterless constructor is

called in the superclass

Then, the defaultparameterless constructor is

called in the subclass

Movie movie1 = new Movie();

Movie

titlelength

pricecondition


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The super Reference

Refers to the base classIs useful for calling base classconstructors

Must be the first line in the derived classconstructor

Can be used to call any base class

methods public class Movie extends InventoryItem {

public Movie() {super("Matrix 8");

}}

h f


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The super ReferenceExample

public class InventoryItem {InventoryItem(String cond) {

System.out.println("InventoryItem");

}}class Movie extends InventoryItem {

Movie(String title) {super(title); System.out.println("Movie");

}}

Base classconstructor

Calls baseclass

constructor


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Using SuperclassConstructors

Use super() to call a superclassconstructor: public class InventoryItem {

InventoryItem(float p, String cond) {

price = p;condition = cond;}

public class Movie extends InventoryItem{

Movie(String t, float p, String cond) {super(p, cond);title = t;

}

S if i Additi l


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Specifying AdditionalMethods

The superclass defines methods for alltypes of InventoryItemThe subclass can specify additional

methods that are specific to Movie public class InventoryItem {

public float calcDeposit() public String calcDateDue()

public class Movie extends InventoryItem { public void getTitle() public String getLength()

O idi S l


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Overriding SuperclassMethods

A subclass inherits all the methods of itssuperclass

The subclass can override a method with itsown specialized version

Must have the same signature and semanticsas the superclass method

In Java all methods are virtual (unlessdeclared as final )

This forces the l ate b ind ing mechanism oneach method call

O idi g S l


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Overriding SuperclassMethods Example


public float calcDeposit(int custId) {return itemDeposit;

}}


public float calcDeposit(int custId) {if (specialCustomer(custId) {

return itemDeposit / 2;} else {return itemDeposit;

}}

}

I ki g S l


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Invoking SuperclassMethods

If a subclass overrides a method, it can stillcall the original superclass method

Use super.method() to call a superclassmethod from the subclass

public class InventoryItem { public float calcDeposit(int custId) {

if return 33.00;

}

public class Movie extends InventoryItem { public float calcDeposit(int custId) {

itemDeposit = super.calcDeposit(custId);return (itemDeposit + vcrDeposit);

}


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Acme Video and


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Acme Video andPolymorphism

Acme Video started renting only videosAcme Video added games and VCRs

Whats next?

Polymorphism solves the problem


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ShoppingBasketvoid addItem(InventoryItem item) {

// this method is called each time// the clerk scans in a new itemfloat deposit = item.calcDeposit();

}

InventoryItem

VCR Movie

calcDeposit(){}

calcDeposit(){} calcDeposit(){}

How It Works

Using the instanceof


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Using the instanceof Operator

The true type of an object can bedetermined by using an instanceof operator

An object reference can be downcast to thecorrect type, if needed

public void aMethod(InventoryItem i) {if (i instanceof Movie)

((Movie)i).playTestTape();}

Limiting Methods and


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Limiting Methods andClasses with final

A method can be marked as final to preventit from being overridden

A whole class can be marked as final to

prevent it from being extended public final class Color {

}

public final boolean checkPassword(String p) {

}


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Abstract Classes andInterfaces


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Defining Abstract Classes

Abstract classes model abstractconcepts from the real worldCannot be instantiated directly

Should be subclasses to be instantiatedAbstract methods must be implementedby subclasses

Abstractsuperclass

Concretesubclasses

InventoryItem

Movie VCR

Creating Abstract Classes


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Creating Abstract Classesin Java

Use the abstract keyword to declare aclass as abstract

public abstract class InventoryItem { private float price; public boolean isRentable()

}

public class Movieextends InventoryItem { private String title; public int getLength()

public class Vcrextends InventoryItem { private int serialNbr; public void setTimer()

What Are Abstract


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What Are AbstractMethods?

An abstract method:Is an implementation placeholder

Is part of an abstract class

Must be overridden by a concrete subclass

Each concrete subclass can implement themethod differently

Defining Abstract Methods


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Defining Abstract Methodsin Java

Use the abstract keyword to declare a methodas abstract :

Provide the method signature only

The class must also be abstract

Why is this useful?

Declare the structure of a class withoutproviding complete implementation of everymethod

public abstract class InventoryItem { public abstract boolean isRentable();

Defining and Using


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Defining and UsingInterfaces

An interface is like a fully abstract class:All of its methods are abstract

All variables are public static final

An interface lists a set of method signatures,without any code details

A class that implements the interface must

provide code details for all the methods ofthe interface

A class can implement many interfaces butcan extend only one class


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Creating an Interface

Use interface keyword

All methods public abstractAll variables public static final

public interface Steerable {int MAXTURN_DEGREES = 45;void turnLeft(int deg);void turnRight(int deg);

}


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Implementing an Interface

Use implements keyword

public class Yacht extends Boatimplements Steerable

public void turnLeft(int deg) {} public void turnRight(int deg) {} }


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Sort: A Real-World Example

Is used by a number of unrelated classesContains a known set of methods

Is needed to sort any type of objects

Uses comparison rules known only to thesortable object

Supports good code reuse


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Overview of the Classes

Created by the sort expert:

Created by the movie expert:

public class

MovieSortApplication

public class Movie

implements Sortable

public interfaceSortable

public classSort


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MyApplication passesan array of movies to

Sort.sortObjects()1

23sortObjects() asks amovie to compare itself

with another movie

The movie returnsthe result of the

comparison

4sortObjects()

returns thesorted list

Sort

Movie

MyApplication

How the Sort Works?

e Sortable


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e Sortable Interface

Specifies the compare() method public interface Sortable {

// compare(): Compare this object// to another object// Returns:// 0 if this object is equal to obj2// a value < 0 if this object < obj2// a value > 0 if this object > obj2int compare(Object obj2);

}

Th S Cl


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The Sort Class

Implements sorting functionality: themethod sortObjects() public class Sort {

public static void sortObjects(Sortable[] items) {// Perform "Bubble sort" algorithmfor (int i = 1; i < items.length; i++) {

for (int j = 0; j < items.length-1; j++) {if (items[j].compare(items[j+1]) > 0) {

Sortable tempItem = items[j+1];items[j+1] = items[j];

items[j] = tempItem;}}

}}

}

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public class Movie extends InventoryItemimplements Sortable {

private String title;

public int compare(Object movie2) {String title1 = this.title;String title2 = ((Movie)movie2).getTitle();

return(title1.compareTo(title2));}}

Implements Sortable

The Movie Class

U i h S


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Using the Sort

Call Sort.sortObjects(Sortable []) with an array of Movie as the argument

public class MovieSortApplication {

Movie[] movieList;

public static void main(String[] args) {// Build the array of Movie objectsSort.sortObjects(movieList);

}}


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Sorting with Interfaces

Live Demo


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Inner Classes


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Defining Static Inner


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Defining Static InnerClasses

public class Outer {

private static float varFloat = 3.50f; private String varString; static class InnerStatic {

}

Defined at class level Can access only static members of the outer

class

Defining Member Inner


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Defining Member InnerClasses

public class Outer {

private static float varFloat = 3.50f; private String varString;

...class InnerMember {

...Outer.this

...

}

Defined at class level Instance of the outer is needed Keyword this used to access the outer instance

Defining Local Inner


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gClasses

Are defined at the method level Are declared within a code block Have access only to final variables

Cannot have a constructor public class Outer {

... public void outerMethod(final int var1){

final int var2=5; ...

class InnerLocal { private int localVar = var1 + var2; ...}}

}

Defining Anonymous Inner


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g yClasses

Defined at method level Declared within a code block Missing the class , extends , and

implements keywords Cannot have a constructor

public class Outer { public void outerMethod(){

myObject.myAnonymous(new SomeClass(){...} )

}}

Using instanceof with


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g instanceofInterfaces

Use the instanceof operator to check ifan object implements an interface

Use downcasting to call methods definedin the interface

public void aMethod(Object obj) {if (obj instanceof Sortable) {

result = ((Sortable)obj).compare(obj2); }

}


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Cl Di g E l


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Class Diagrams Example

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Shape

#mPosition:Point

structPoint

+mX:int+mY:int

+Point

interfaceISurfaceCalculatable

+CalculateSurface:int

Rectangle

-mWidth:int-mHeight:int

+Rectangle+CalculateSurface:int

Square

-mSize:int

+Square+CalculateSurface:int

FilledSquare

-mColor:Color

+FilledSquare

structColor

+mRedValue:byte+mGreenValue:byte+mBlueValue:byte

+Color

FilledRectangle

-mColor:Color

+FilledRectangle

Inheritance and Polymorphism


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Questions?

Inheritance and Polymorphism


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Problems


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Problems

1. Inherit from the base abstract class Cat andcreate subclasses Kitten and Tomcat . Theseclasses should fully implement the IAnimal interface and define an implementation for the

abstract methods from the class Cat .2. Write a class TestAnimals that creates an

array of animals ( IAnimal[] ): Dog, Frog ,Kitten , Tomcat and calls their methodsthrough IAnimal interface to ensure theclasses are implemented correctly.

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Homework


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Homework

1. Create a class diagram for the hierarchyIAnimal , Dog, Frog , Cat , Kitten , Tomcat .

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Problems


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Problems

1. Create a project, modeling a restaurantsystem. You need to model the Restaurant,People (Personnel and Clients), Orders from aclient and Products for orders. Products could

be Meals or Beverages.2. Client has one order and could buy many

products. Orders are limited to a maximumbill. No products could be added above themaximum.

3. Clients could pay with tip included (bill = price+ tip) 67

Problems


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Problems

1. Tasks: Model the scheme and conventions Find the top 5 clients with largest bills

Sort clients by name in a collection Design a function to find the day revenue of

the restaurant

Find the waiter with most tips

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