Object-Oriented Principals

1. www.espirity.com Object-Oriented Principals Dwight Deugo (dwight@espirity.com) Nesa Matic (nesa@espirity.com)

2. Additional Contributors • None as of Septembe, 2004

3. Module Overview Object-Oriented Principals

4. Module Road Map • Object-Oriented Principals • Objects • Classes, instances, fields and methods • Encapsulation • Polymorphism • Inheritance • Dynamic binding

5. What is Object-Oriented Technology? • It is a technology that allows modeling of software as the real world • This is achieved through objects and interactions between them • It is a way of thinking of software as reusable components • Every object is a component on its own and could be reused in different context

6. Why Use Object-Oriented Systems? • Benefits of Object-Oriented systems: • Greater flexibility • Reusability of objects • Higher quality control • Lower maintenance

7. Developing Object-Oriented Software • Software is modeled by objects • Software objects represent real-world objects • Each object contains: • State, i.e., data • Behavior, i.e., methods/functions

8. Object-Oriented Software • Developing object-oriented software is identifying: • Objects • Characteristics of individual objects • Relationships between objects • Objects interact by sending messages to each other • Interacting objects make an object-oriented system

9. Objects and Loose Coupling • Changing an object’s data does not lead to changes in an object’s external behavior • An object’s external interface stays the same • Promotes loose coupling between objects object2 object1 state state behavior behavior object3 state behavior

10. Objects • Every object has: • State • Behavior • State represents data - what an object knows, or what an object contains • Behavior represents what an object can do

11. Object State and Behavior • Person object • State: age, name, children • Behavior: addChild, getAge, setAge aPerson state age name children behavior getAge setAge addChild …

12. Interactions between Objects • Object interact by sending messages to each other • Objects and interactions between them make up an object-oriented system aBroker: aPolicyFactory: aPerson: createPolicy aPolicy aPolicy: new(“Joe Smith”, “35 years”) aPerson setOwner(aPerson) …

13. Messages • There are two major terms in messaging: • Message sender • Message receiver • Messages may have arguments aBroker: aPerson: receiver sender getAge() message

14. Methods • Method is concrete implementation of a message • When message is sent to a receiver: • Method is found by type of the receiver object and method signature • Method code is executed • Method represents an object’s response to a message

15. Method Signature • Method signature is unique identifier of the method • It is used to distinguish methods with same name and same number of parameters • It consists of: • Method name • Parameter name • Parameter type

16. Object’s Public Protocol • Public protocol is set of messages that can be sent to an object • It does not include messages that an object can send to itself • These are private aPerson age name children getAge setAge addChild … public protocol

17. Fields • Fields represent characteristics of an object • Fields are also known as attributes, or instance variables aPerson age name children fields getAge setAge addChild …

18. Object-Oriented Principal: Encapsulation • Objects hide implementation of the messages behind their public protocols • Object’s internal implementation is accessed by that object only • Encapsulation is also known as implementation hiding aBroker: aPerson: getName() Joe Smith getAge() 35

19. Classes • Classes are: • Factories for creating objects • Template for the same kind of objects that describes their state and behavior • Code repository for objects • Classes define objects (by defining their state and behavior) and their type Person age name children address getAge setAge addChild …

20. Instances • Every object is an instance of some class • All instances of same class have the same protocol • They have same fields and same methods that are defined by the class class Person John Smith age name children address getAge setAge addChild … Dad Smith class 35 John Smith Jimmy, Chloe 60 Dad Smith John, Tom

21. Object-Oriented Principal: Inheritance • Some classes may share commonalities • For example HomePolicy, AutoPolicy, LifePolicy classes may all have same state and behavior • Instead of repeating commonalities in each class, we can abstract them in a common place • These commonalities can be stored in a super class • Each subclass inherits state and behavior from its superclass

22. Specialization and Generalization Policy client premium policyNumber getClient setClient … specialization generalization HomePolicy AutoPolicy LifePolicy house auto getHouse setHouse getAuto setAuto

23. Why Inheritance? • Inheritance represents real-world modeling • Some objects are special cases of other objects • Inheritance promotes reuse and extensibility • Same data and behavior is shared among objects of different types (different class) • New data and new behavior that is common for those objects is easier to add

24. Object-Oriented Principal: Polymorphism • Polymorphism • different objects respond to the same message in different ways • For example when asked to talk a dog barks, and a cat meows • It is often supported by method overriding • Overriding means that subclass may implement the same method as superclass, but with different code • toString() method in the Object class is an example of often overridden method

25. Overriding Example • Consider Animal class: • Dog and Cat as subclasses • All Animal objects should know how to talk Animal talk() aCat.talk(); aDog.talk(); meows barks Cat Dog talk() talk()

26. Dynamic Binding • Dynamic binding represents runtime method binding • It is runtime binding of method invoked in the message to the method that implements that message • For example: anAnimal.talk(); aDog aCat … … talk talk

27. Labs! Lab: Object-Oriented Concept