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Designed and Presented by

Dr. Ayman Elshenawy Elsefy

Dept. of Systems & Computer Eng..

Al-Azhar University

Website: eaymanelshenawy.wordpress.com

Email : eaymanelshenawy@azhar.edu.eg

eaymanelshenawy@yahoo.com

eaymanelshenawy@gmail.com

Object Oriented Programming

Lecture 5

Inheritance and Polymorphism

Part 2

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IN THIS Lecture, YOU WILL:

Learn about inheritance

Learn about subclasses and superclasses

Explore how to override the methods of a

superclass

Examine how constructors of superclasses and

subclasses work

Learn about polymorphism

Examine abstract classes

Become familiar with interfaces

Learn about composition

Objectives

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Protected Members of class

Private members: are private to the class and cannot be

directly accessed outside the class. Only class methods can

access the private members directly.

The subclass cannot access the private members of the

superclass directly. ( sometimes it may be necessary for a

subclass to access a private member of a superclass).

Solution: make a private member public,

Errors: then anyone can access that member.

Another solution: Use Protected Modifier

any member of a superclass can be accessed directly

(only) by a subclass, that member is declared using the

modifier protected.

The accessibility of a protected member of a class falls

between public and private. A subclass can directly access the

protected member of a superclass.

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the protected instance variable can be

accessed directly by the methods. However, DClass objects cannot

directly access bCh. 9

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Dynamic Binding and Inheritance

• A committee that consists of four

people that are either students or

employees.

• Make the array of type Person so it can

accommodate any class derived fromit.

• we are assigning an object of a derived

class (either Student or

Undergraduate) to a variable defined

as an ancestor of the derived class

(Person).11

Dynamic Binding and Inheritance

• As a result, even though people[0]

is declared to be of type Person, the

method associated with the class

used to create the object is invoked.

This is called dynamic binding or

late binding.

• What Happen if we write:

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Dynamic Binding and Inheritance

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Dynamic Binding and Inheritance

Conclusion

when an overridden method is invoked, its action is the one

defined in the class used to create the object using the new

operator (not by the type of the variable naming the object).

A variable of any ancestor class can reference an object of a

descendant class, but the object always remembers which

method actions to use for every method name. The type of the

variable does not matter. What matters is the class name when

the object was created.

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Dynamic Binding and Inheritance

Output will be

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Dynamic Binding with tostring()

Java uses dynamic binding with the toString

method.

The various println methods that belong to the

object System.out were written long before we

defined the class Student.

Interface and Abstract Classes Class interface consists of the headings for the public methods

and public named constants of the class, along with any

explanatory comments.

Knowing only a class’s interface, a programmer can write code

that uses the class (no need to know about the class’s

implementation).

Until now, Class interface is included in its definition.

Java Enables you to write class interface in a separate file

away from the implementation.

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Interface and Abstract Classes A person calling his pets for dinner

Pets are able to be :

Named

Eat

Respond to command

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Dog, Bird and Fish are 3 classes that implement these

behaviours

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Interface and Abstract Classes Java interface is a program component that contain the

headers and comments of public methods (All or subset) or

named constants.

You name and store an interface as you would a class

An interface has no instance variables, constructors, or method

definitions

To define the interface:

Use Implements ( one or more interfaces).

Define each method declared in the interface(s)

Several Classes can implement the same interface

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Interface and Abstract Classes

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Interface and Abstract Classes

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Interface and Abstract Classes

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Interface as a type

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Extending an Interface

A class that implements callable must be implement the method

come,setName, and getName

You can combine several interfaces into new one and add to

them new methods.23

Extending an Interface

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Trainable will have setName, getName, come, respond, sit,

speak, lieDown

Class Object If a user-defined class include the definition of the function

toString() this function is executed when ever it is called.

If a user-defined class does not provide its own definition of the

method toString, then the default definition of the method

toString is invoked.

the default definition of the method toString returns the class

name followed by the hash code of the object.

You might ask, where is the method toString defined?

toString() defined in the class Object, and it is a public member.

If your defined class is not inherited from any class, the class

you define is automatically considered to be derived from the

class Object.

The class Object becomes the superclass of every class in

Java. 25

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class Object

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Equivalent to

Constructors and Methods of the class Object

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Java Stream Classes Hierarchy

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Scanner Class Example

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Scanner( InputStream )

This constructor create an object of Scanner class by talking an

object of InputStream class. An object of InputStream class is

called in which is created as a static data member in the System

class

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Scanner Class Example

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Scanner Class Example

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Input Stream Reader

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turning the byte based input stream into a character based

Reader.

Java InputStreamReader interprets the bytes of a Java

InputStream as text instead of numerical data.

is often used to read characters from files (or network

connections) where the bytes represents text .

PrintWriter

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Java PrintWriter class is the implementation of Writer class. It is

used to print the formatted representation of objects to the text-

output stream.

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Polymorphism

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Polymorphism

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In a class hierarchy, several methods may have the same name

and the same formal parameter list.

A reference variable of a class can refer to either an object of its

own class or an object of its subclass.

A reference variable can invoke, that is, execute, a method of

its own class or of its subclass(es).

Binding means associating a method definition with its

invocation, (determine which method definition gets executed).

Early binding, a method’s definition is associated with its

invocation when the code is compiled.

Late binding, a method’s definition is associated with the

method’s invocation at execution time, that is, when the

method is executed (java).

Term polymorphism means associating multiple (potential)

meanings with the same method name.

Final

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If a method of a class is declared final, it cannot be overridden

with a new definition in a sub-class.

If a class is declared final, then no other class can be derived

from this class; that is, it cannot be the superclass of any other

classes.

Final

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Instance of Operator

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To determine whether a reference variable that points to an

object is of a particular class type, Java provides the operator

instanceof.

p instanceof BoxShape

Abstract Methods and Classes

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An abstract class is a class that is declared with the reserved

word abstract in its heading.

An abstract class is a class that is declared with abstract

keyword.

An abstract method is a method that is declared without an

implementation.

An abstract class may or may not have all abstract methods.

Some of them can be concrete methods

A method defined abstract must always be redefined in the

subclass,thus making overriding compulsory OR either make

subclass itself abstract.

Any class that contains one or more abstract methods must

also be declared with abstract keyword.

There can be no object of an abstract class.That is, an abstract

class can not be directly instantiated with the new operator.

An abstract class can have parametrized constructors and

default constructor is always present in an abstract class.

Example of Abstract Classes

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You must define the abstract method in all inherited class

Example of Abstract Classes

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Banking Example

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Banks offer various types of accounts Savings account :

InterestMinimum Balance

number of monthly checking writing

Monthly service charge

NoNoLimitedYesType 1

LowerLowerUn-LimitedNoType 2

HigherHigherUn-LimitedNoType 3

Certificate of deposit (CD) is used to save money for the long term

InterestMinimum Balance

LowerNoType 1

HigherYesType 2

Checking accounts:

money is left for some time and for higher interest ratesSuppose that you purchase a CD for six months. Then we say that the CD will

mature in six months. Furthermore, the penalty for early withdrawal is stiff.

inheritance hierarchy of these bank accounts.

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abstract.

abstract.No abstract.

No abstract.

No abstract.No abstract. No abstract.

No abstract.

name,

accountNumber balance

inheritance hierarchy of these bank accounts. inheritance hierarchy of these bank accounts.

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inheritance hierarchy of these bank accounts. inheritance hierarchy of these bank accounts.

inheritance hierarchy of these bank accounts. inheritance hierarchy of these bank accounts.

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inheritance hierarchy of these bank accounts. inheritance hierarchy of these bank accounts.