Class Compatibility - Object Oriented Programming - Lecture Slides, Slides of Object Oriented Programming

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Object-Oriented Programming

(OOP)

Lecture No. 6

Class Compatibility

• A class is behaviorally compatible with another if it supports all the operations of the other class
• Such a class is called subtype
• A class can be replaced by its subtype

…Class Compatibility

• Derived class is usually a subtype of the base class
• It can handle all the legal messages (operations) of the base class
• Therefore, base class can always be replaced by the derived class

Example – Class Compatibility

Shape color vertices move setColor draw

Circle radius draw computeArea

Line length draw getLength

Triangle angle draw computeArea

Example – Class Compatibility

File size … open print …

ASCII File … print …

PDF File … print …

PS File … print …

Polymorphism

• In general, polymorphism refers to existence of different forms of a single entity
• For example, both Diamond and Coal are different forms of Carbon

Polymorphism in OO Model

• In OO model, polymorphism means that different objects can behave in different ways for the same message (stimulus)
• Consequently, sender of a message does not need to know exact class of the receiver

Example – Polymorphism

Shape

Line Circle Triangle draw

draw

draw draw

draw View

Example – Polymorphism

File

ASCII File PDF File PS File print

print

print print

print Editor

Polymorphism – Advantages

• Messages can be interpreted in different ways depending upon the receiver class

Shape

Line Circle Triangle draw

draw

draw draw

draw View

Polymorphism – Advantages

• New classes can be added without changing the existing model

Square draw

Shape

Line Circle Triangle draw

draw

draw draw

draw View

Polymorphism – Advantages

• In general, polymorphism is a powerful tool to develop flexible and reusable systems