البرمجة الهدفية بلغة جافا - الوراثة

OO

PInheritance

Object Oriented Programming

Prepared & Presented by: Mahmoud Rafeek Alfarra

2012

Chapter 3

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http://mfarra.cst.ps

Contents

What is Inheritance ? Advantages1

Superclasses and Subclasses2

"is-a" and the "has-a" relationship3

Mini Example: Vehicle 4

Constructors in Subclasses5

Example: Students’ types6

Single Vs Multiple inheritance 7

Important Notes8

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PPWhat is Inheritance ?

Inheritance is a form of software reuse in which a new class is created by absorbing

an existing class's members and embellishing them with new or modified

capabilities.


P_Properties P_ methods

P_Properties P_ methods Parent Class

C_Properties C_methods

C_Properties C_methods Child Class

This class has its properties, methods and that of its parent.

This class has its properties, methods and that of its parent.

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PPAdvantages of Inheritance


Using inheritance: to minimize the amount of duplicate

code. A better organization of code and smaller, simpler compilation units.

make application code more flexible to change because classes that inherit from a common superclass can be used interchangeably.

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PPSuperclasses and Subclasses


P_Properties P_ methods

P_Properties P_ methods SuperClass

C_Properties C_methods

C_Properties C_methods SubClass

When creating a class, rather than

declaring completely new members, the

programmer can designate that the new

class should inherit the members of an

existing class.

The existing class is called the

superclass, and the new class is the

subclass.

Each subclass can become the

superclass for future subclasses.

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PPSuperclasses and Subclasses


A subclass normally adds its own fields and methods.

Therefore, a subclass is more specific than its superclass.

Typically, the subclass exhibits the behaviors of its superclass and

additional behaviors that are specific to the subclass.

In Java, the class hierarchy begins with class Object (in package java.lang), which every class

in Java directly or indirectly extends.

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properties3 methods3

properties3 methods3 SubClass

Direct & Indirect Superclasses


The direct superclass is the superclass from which the subclass

explicitly inherits. An indirect superclass

is any class above the direct superclass in the

class hierarchy.


properties1 methods1 Indirect

SuperClass


properties2 methods2 Direct

SuperClass

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PP"is-a" and the "has-a" relationship


"Is-a" represents inheritance.In an "is-a" relationship, an object of a

subclass can also be treated as an object of its superclass.

For example, a car is a vehicle.

Vehicle Class

Vehicle Class SuperClass

Car ClassCar Class SubClass

Honda is a car & Honda is a vehicleHonda is a car &

Honda is a vehicle

Object of sub is also an object of superObject of sub is also an object of super

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PPMini Example: VehicleProject


class Vehicle {protected String model;protected float price;

public Vehicle(String model, float price){this.model = model;this.price = price;

}

public String print(){return "Model: "+model+"\t Price: "+price;

}

public void setModel(String model){this.model = model;}public String getModel(){

return model;} // set and get of price

}

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PPMini Example: Car class


class Car extends Vehicle{private int passengers;

public Car(String model, float price,int passengers){super( model, price);this.passengers = passengers;

}

public String print(){return "Data is:\n"+super.print()+

" # of passengers: "+passengers;}

}

A compilation error occurs if a subclass constructor calls one of its superclass constructors with arguments that do not

match the superclass constructor declarations.

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PPMini Example: using sub & super class


public class VehicleProjectInheritance { public static void main(String[] args) { Car c = new Car("Honda", 455.0f, 4); System.out.println(c.print()); }}

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PPConstructors in Subclasses

When a program creates a subclass object, the

subclass constructor immediately calls the

superclass constructor.

The superclass constructor's body executes to

initialize the superclass's instance variables that are

part of the subclass object, then the subclass

constructor's body executes to initialize the

subclass-only instance variables.


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PPConstructors in Subclasses


Public Test1(){}

Public Test1(){}

SuperClass

Public Test2(){}

Public Test2(){}

SubClass

calls

Return values

Subclass constructor invokes its direct superclass's constructor either explicitly (via the super reference) or

implicitly (calling the superclass's default constructor or no-argument constructor).

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PPExample: Students’ types


• Have part time hours • Have a field training

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PPExample: Student Class


class Student {protected String name;protected String mobile;protected float gpa;public Student(String name, String mobile, float gpa){

this.name = name;this.mobile = mobile;this.gpa = gpa;

}// set methodspublic void setName(String name){

this.name= name;}…// get methodspublic String getName(){

return name;}….// Print Data

public String showData(){return "Name: "+getName()+"\nMobile: "+getMobile()+"\nGPA: "+getGpa(); } }

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PPExample: PostGraduated Class


class PostGraduated extends Student {private int hours;

public PostGraduated(String name, String mobile, float gpa,int hours){super(name, mobile, gpa);this.hours = hours;

}// set methodpublic void setHours(int hours){

this.hours = hours;}// get methodpublic int getHours(){

return hours;} public String PrintData (){

return showData() + " Hours: "+getHours();}

}

Declare another method to calculate

the earn of post graduate if the cost of

each hour is 20$.

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PPExample: Graduate Class


class Graduate extends Student {private String trainingField;

public Graduate(String name, String mobile, float gpa, String trainingField){super(name, mobile, gpa);this.trainingField = trainingField;

}

public void setTrainingField(String trainingField){this.trainingField= trainingField;

}

public String setTrainingField(){return trainingField;

}

public String Printinfo(){return showData() + " TrainingField: "+getTrainingField();

}

}

Declare another method to calculate

the grade of the graduate student

(Excellent, V.Good, …)

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PPMethod Overridden


An instance method in a subclasssubclass with the same signature (name, plus the number and the

type of its parameters) and return type as an instance method in the superclasssuperclass overrides

the superclass's method.

public class Animal { public static void testClassMethod() { System.out.println("The class" + " method in Animal"); } public void testInstanceMethod() { System.out.println("The instance " + " method in Animal."); } }

public class Cat extends Animal { public static void testClassMethod() { System.out.println("The class method" + " in Cat."); }

public void testInstanceMethod() { System.out.println("The instance method" + " in Cat."); }

Overrid e by

Overrid e by

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PPMethod Overridden


public int calcsal(int x){sal = days*x;return sal; }

public int calcsal(int x){sal = days*x;return sal; }

public int calcsal(int x){sal = (days*x)- absent;return sal; }

public int calcsal(int x){sal = (days*x)- absent;return sal; }

Super obj1 = new Super ();int x = obj.

Super obj1 = new Super ();int x = obj. Calcsal(20);Calcsal(20);

Super class

Sub class

Sub obj2 = new Super ();int x = obj.

Sub obj2 = new Super ();int x = obj. Calcsal(20);Calcsal(20);

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PPMethod Overridden Vs Method Overloading


Using code, distinguish between overridden and overloading method.

Implement the concept of method overridden on the methods of print in example of Students’ types

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PPExample: Inherited members from Student Class


Inherited method from student class

Inherited method from student class

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PPSingle Vs Multiple inheritance

Class 1Class 1 Class 2Class 2

Class 3Class 3

Class 1Class 1

Class 2Class 2

Multiple Inheritance Single InheritanceClass can inherit members from more than

one class (more than one super class) Class can inherit members from only

one class (one super class)

Java was designed without multiple inheritance.

Java was designed without multiple inheritance.

Java was designed with single inheritance.

Java was designed with single inheritance.

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PPImportant Notes

Methods of a subclass cannot directly access private members of their superclass.

With inheritance, the common instance variables and methods of all the classes in the hierarchy

are declared in a superclass.

Use the protected access modifier when a superclass should provide a method only to its subclasses and other classes in the same package, but not to other

clients.

A subclass is more specific than its superclass and represents a smaller group of objects.

A superclass's protected members have an intermediate level of protection between public and

private access. They can be accessed by members of the superclass, by members of its subclasses and by

members of other classes in the same package.

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PPاستغفروا ربكم

: ذكره تعالى ا قال

بوا تو م ث م ك ب ر روا ف غ ت س ا ن أ ولى إ ا ن س ح عا ا ت م م عك ت م ي ه ي ل إ

ذي ل ك ت ؤ ي و مى س م ل ج أني إ ف وا ل و ت ن إ و ه ل ض ف ل ض ف

ر ي ب ك م و ي ب ذا ع م ك ي ل ع ف خا أ[ هــود [


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Thank You …Thank You …

البرمجة الهدفية بلغة جافا - الوراثة

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