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University Of Ha’il 1

ICS 201

Introduction to Computer Science

Inheritance

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Inheritance Hierarchy

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Encapsulation

Data Hiding Abstraction Security

؟3

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Encapsulation allows the programmer to group data and the methods that operate on them together in one place, and to hide details from the user.

In Java, hiding details is done by marking them private

Encapsulation (data hiding)

withDraw

Deposit

Transfer

Print

AccountNo

AccountValue

Name Address

Change Address

withDraw any value

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Encapsulation

Person Vending Machine

Buy Pepsi

Sell (1 SR, Pepsi)

Sell

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- public and private Modifiers …

Illegal because we try to access a private member (age) from outside the class Employee

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Problem ..

It is considered good programming practice to make all in-stance variables private

Question: how to access and modify the in-stance variables of Employee objects e1, e2, e3 and e4? .. answer .. Use accessor and mutaor methods …. next slide ..

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- Accessor and Mutator Methods … The methods that retrieve the data of fields are called accessors.

The data can be accessed but not changed The name of an accessor method starts with the word get Example: public String getName()

{ return name;

}

The methods that modify the data of fields are called mutators. The name of a mutator method starts with the word set Example: public void setName(String n)

{ name = n;

}

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- Accessor and Mutator Methods (Example)

Accessor method for instance variable name

Mutator method for instance variable name

Modifying the name of e1 using a mutator method

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Encapsulation and Inheritance Pitfall: Use of Private Instance Variables from the Base Class

An instance variable that is private in a base class is not accessible by name in the definition of a method in any other class, not even in a method definition of a derived class

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Encapsulation and Inheritance Pitfall: Use of Pri-vate Instance Variables from the Base Class

Instead, a private instance variable of the base class can only be accessed by the public accessor and mutator methods defined in that class

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Access Modifiers public

Can be used in any Java program without restriction

private may only be used by the instance of the class that

declares the variable or method

Protected available to all classes in the same package available to all subclasses( even those in different

packages )

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What is Package ?

Way to group a related class into one unit

To resolve the name conflicts between class

names

Include all the classes in package import packageName.*;

import packageName.className ; import packageName.className ;

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Some Predefined Java Packages

Package Name Contents

java.applet Classes for implementing applets

java.awt Classes for graphics, windows, and GUI’s

java.awt.event Classes supporting AWT event handling

java.awt.image Classes for image handling

java.io Classes for input and output

java.lang Basic language classes like Math (always available in any Java program)

java.net Classes for networking

java.util Useful auxiliary classes like Date

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Visibility and Inheritance

Visibility Public default protected private

Clients in same package

C C C None

Clients in different packages

C None None None

Subclass in samepackage

C C C None

Subclass in different package

C None C None

Note: C; Can access default: If the access modifier is omitted

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© 2006 Pearson Addison-Wesley. All rights reserved 7-16

Access Modifiers

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The class Object

All classes defined without an explicit extends clause automatically extend Object

The Object Class is the Superclass of Every Java Class

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The Class Object The class Object is in the package java.lang

which is always imported automatically

public class CircleEquivalent

public class Circle extends Object

Most useful methods: String toString() boolean equals(Object otherObject) Object clone()

Good idea to override these methods in your own classes