1. ICS 201 Inheritance Introduction to Computer Science
University Of Ha’il

2. Inheritance Hierarchy

3. Encapsulation
Data Hiding
Abstraction
Security ؟

4. Encapsulation (data hiding)
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
withDraw
Deposit
Transfer
Print
AccountNo
AccountValue
Name
Address
withDraw any value
Change Address

5. Encapsulation
Sell (1 SR, Pepsi)
Person
Vending Machine
Sell
Buy Pepsi

6. - public and private Modifiers …
Illegal because we try to access a private member (age) from outside the class Employee

7. Problem ..
It is considered good programming practice to make all instance variables private
Question: how to access and modify the instance variables of Employee objects e1, e2, e3 and e4? .. answer .. Use accessor and mutaor methods …. next slide ..

8. - 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;

9. - 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

10. 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

11. Encapsulation and Inheritance Pitfall: Use of Private 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

12. Access Modifiers public private Protected
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 )

13. What is Package ? Include all the classes in package
Way to group a related class into one unit
To resolve the name conflicts between class names
import packageName.className ;
Include all the classes in package
import packageName.*;

14. 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

15. Visibility and Inheritance
Public
default
protected
private
Clients in same package C
None
Clients in different
packages
Subclass in same
package
Subclass in different package
Note: C; Can access
default: If the access modifier is omitted

16. Access Modifiers
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17. The Object Class is the Superclass of Every Java Class
The class Object
All classes defined without an explicit extends clause automatically extend Object
The Object Class is the Superclass of Every Java Class

18. The Class Object
The class Object is in the package java.lang which is always imported automatically
public class Circle
Equivalent
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