Principles of Object-Oriented Programming Why OOP is popular? Language and thoughts A new way of viewing the world Eleme

1. Principles of Object-Oriented Programming • Why OOP is popular? • Language and thoughts • A new way of viewing the world • Elements of OOP • Coping with complexity

2. Difficult Questions • What is Object-Oriented Programming? • OOP is a revolutionary idea, totally unlike anything that has come before in programming languages • OOP is an evolutionary step, following naturally on the heels of earlier programming abstractions

3. Why is OOP Popular? A few possible reasons why OOP is popular: • Hope that it will quickly and easily lead to increased productivity and increased reliability (solve the software crises) • Similarity to techniques of thinking about problems in other domains • Hope for easy transition from existing languages (e.g., C or Pascal)

4. A New Paradigm We start by considering the definition of the term ``paradigm'': Par a digmn.1. A list of all the inflectional forms of a word taken as illustrative example of the conjugation or declension to which it belongs. An example or model. [Late Latein paradigma, from Greek paradeigma, modern paradeiknunai, to compare, exhibit.] OO programming is a new paradigm • New way of thinking about what it means to compute, and about what we can structure information inside a computer Let’s first see the relationship between languages and thoughts

5. Imperative Programming Paradigm Imperative programming is the ``traditional'' model of computation. • State • Variables • Assignment • Loops A processing unit is separate from memory, and ``acts'' upon memory • Computer is the data manager • wandering through memory, pulling values in memory slots • transforming them in some manner, and pushing results back in some other slots • Although this is exactly what happens inside a computer, it is not the way people do for solving problems

6. Visualization of Imperative Programming Sometimes called the ``pigeon-hole'' model of computation.

7. Object-Oriented Programming Paradigm OO programming is based on the following principles [Kay] • Everything is an object • Objects perform computation by making requests of each other through the passing of messages (bundled with arguments) • Every object has it's own memory, which consists of other objects. • Every object is an instance of a class. A class groups similar objects. • The class is the repository for behaviors associated with an object • Classes are organized into singly-rooted tree structure, called an inheritance hierarchy. We can illustrate these principles by considering how I go about solving a problem in real life.

8. Illustration of OOP Concepts - Sending Flowers to a Friend To illustrate the concepts of OOP in an easily understood framework, consider the problem of sending flowers to a friend who lives in a different city. I can't deliver them myself. So I use my local Florist. I tell my Florist (named Flo) the address for my friend, how much I want to spend, and the type of flowers I wish to send. Flo contacts a florist in my friend’s city, who arranges the flowers, then contacts a driver, who delivers the flowers. If we start to think about it, there may even be other people involved in this transaction. There is the flower grower, perhaps somebody in charge of arrangements, and so on. And so we see, that to solve my problem requires the interaction of an entire community of individuals.

9. Elements of OOP – Objects 1. Everything is an object. Actions in OOP are performed by agents, called instances or objects. There are many agents working together in my scenario. We have myself, my friend, the florist, the florist in my friends city, the driver, the flower arranger, and the grower. Each agent has a part to play, and the result is produced when all work together in the solution of a problem.

10. Elements of OOP – Messages 2. Objects perform computation by making requests of each other through the passing of messages Actions in OOP are produced in response to requests for actions, called messges. An instance may accept a message, and in return will perform an action and return a value. To begin the process of sending the flowers, I give a message to Flo. She in turn gives a message to the florist in my friends city, who gives another message to the driver, and so on.

11. Information Hiding Notice that I, as a user of a service being provided by an object, need only know the name of the messages that the object will accept. I need not have any idea how the actions performed in response to my request will be carried out. Having accepted a message, an object is responsible for carrying it out.

12. Elements of OOP – Receivers Messages differ from traditional function calls in two very important respects: • In a message there is a designated receiver that accepts the message (in function calls, there is no receiver) • The interpretation of the message may be different, depending upon the receiver

13. Different Actions var Flo : Florist; Beth : Wife; Ken : Dentist; begin Flo.sendFlowersTo(myFriend); { will work } Beth.sendFlowersTo(myFriend); { will also work but in different way} Ken.sendFlowersTo(myFriend); { will probably not work } end;

14. Behavior and Interpretation Although different objects may accept the same message, the actions (behavior) the object will perform will likely be different. Usually, the specific receiver for any given message will not be known until run-time, so the determination of what behavior (code) to perform may be made at run-time, a form of late binding. • Different from early binding of name to code in function calls The fact that the same name can mean two entirely different operations is one form of polymorphism.

15. Responsibility and Non-interference • Behavior is described in terms of responsibilities • My message indicates only the desired outcome • I do not want to interfere with how the florist achieves the outcome • Responsibility is closely related to non-interference • Implies greater independence between agents, which is a critical factor in solving complex systems • OOP is responsibility-driven software design process ㄷ

16. Elements of OOP - Recursive Design 3. Every object has it's own memory, which consists of other objects. Each object is like a miniature computer itself - a specialized processor performing a specific task.

17. Elements of OOP – Classes 4. Every object is an instance of a class. A class groups similar objects. 5. The class is the repository for behavior associated with an object. The behavior I expect from Flo is determined from a general idea I have of the behavior of Florists. We say Flo is an instance of the class Florist. Behavior is associated with classes, not with individual instances. All objects that are instances of a class use the same method in response to similar messages.

18. Hierarchies of Categories There is more that I know about Flo than just that she is a Florist. I know she is a ShopKeeper, and a Human, and a Mammal, and a Material Objects, and so on. At each level of abstraction I have certain information recorded. That information is applicable to all lower (more specialized) levels.

19. Class Hierarchies

20. Elements of OOP – Inheritance 6. Classes are organized into a singly-rooted tree structure, called an inheritance hierarchy Information (data and/or behavior) I associate with one level of abstraction in a class hierarchy is automatically applicable to lower levels of the hierarchy.

21. Elements of OOP – Overriding Subclasses can alter or override information inherited from parent classes for exceptional cases • All mammals give birth to live young • Some mammals lays eggs

22. Object-Oriented Languages In linguistics there is a hypothesis that the languages we speak directly influence the way in which we view the world • Sapir-Whorf hypothesis • If this is true, OO programming is possible only with OO languages In computer science, we have a directly opposite assertion • Church's Conjecture: any computation for which there exists an effective procedure can be realized by a Turing machine • Any programming language can simulate Turing machine • All languages are equivalent, so anything can be done in any language Using OO language can naturally lead one to view the world in OO fashion, but does not force one to do

23. Summary: OOP View of Computing The OOP view of computation is similar to creating a universe of interacting computing objects Because the OOP view is similar to the way in which people go about solving problems in real life (finding another agent to do the real work!), intuition, ideas, and understanding from everyday experience can be brought to computing. On the other hand, common sense was seldom useful when computers were viewed in the process-state model, since few people solve their everyday problems using pigeon-holes.