1. Advanced Programming in Java SadeghAliakbary Sharif University of Technology Fall 2012

2. Agenda • Object Oriented Programming • Characteristics of objects • Interface • Encapsulation • The notes are mainly extracted from TIJ book • Codes of this slide are pseudocodes • they may have syntactic or logical errors Sharif University of Technology

3. Abstraction • Machine language • Assembly: an abstraction of the machine language • Many languages are abstraction of assembly language • Fortran, Basic, C • Big improvement • But they still require you to think in terms of the structure of the computer • Rather than the structure of the problem Sharif University of Technology

4. Different Contexts • Problem Space • the place where the problem exists • such as a business • Solution Space • the place where you’re implementing that solution • such as a computer • The effort required to perform this mapping Sharif University of Technology

5. Alternatives to model the machine • An alternative is to model the problem • Some early languages chose particular views of the world: • LISP  All problems are ultimately lists • APL  All problems are algorithmic • Prolog  Casts all problems into chains of decisions • Each of these approaches may be a good solution for particular class of problems • But not for all problems… Sharif University of Technology

6. Library Problem • Suppose you want to write a library program • What are the elements of your program? • We think about functions and variables… Sharif University of Technology

7. Object Oriented Approach • OO approach goes a step further • Lets the programmer represent problem space elements • This representation is general enough • The programmer is not constrained to any particular type of problem. • The elements in the problem space and their representations in the solution space are referred to as “objects” Sharif University of Technology

8. OOP • The program is allowed to adapt itself to the lingo of the problem • by adding new types of objects • when you read the code, you’re reading words that also express the problem. • This is a more flexible and powerful language abstraction Sharif University of Technology

9. OOP (2) • OOP allows you to describe the problem in terms of the problem • Rather than in terms of the computer • Objects in your code are similar to real objects • Recall the sample programs: phonebook and library Sharif University of Technology

10. Object Oriented Languages • Smalltalk • The first successful object-oriented language • One of the languages upon which Java is based • Java • C++ • C## Sharif University of Technology

11. OOP vs. Procedural Approach • Elements of OOP • Objects • Message passing between objects • Elements of procedural programming • Functions • Variables • Function invocation • The way of thinking • Thinking about objects and relations • Thinking about functions and computer structure Sharif University of Technology

12. OOP Characteristics • Alan Kay summarized five basic characteristics of Smalltalk • Everything is an object • A program is a bunch of objects telling each other what to do • by sending messages • Each object has its own memory • made up of other objects • Every object has a type • All objects of a particular type can receive the same messages Sharif University of Technology

13. Everything is an object • You can take any conceptual component in the problem • dogs, buildings, books, people, … • And represent it as an object in your program. • Example Person p; Book book; • Think of an object as a variable • It stores data • But you can make requests to that object • asking it to perform operations on itself. Sharif University of Technology

14. Object Messages • To make a request of an object, you send a message to that object. • Message = invoking a method of an object. • Example Book b; …. if(b.isReserved())… Person p; …. p.setPhoneNumber(66166601) Sharif University of Technology

15. Each object has its own memory • You can create a new kind of object • by making a package containing existing objects • Thus, you can build complexity into a program • while hiding it behind the simplicity of objects Book{ String name; Person reservedTo; } Sharif University of Technology

16. Every object has a type • Each object is an instance of a class • class is synonymous with type • The most important distinguishing characteristic of a class is What messages can you send to it? Person p; Person q; Person[] people; Sharif University of Technology

17. Substitutability • All objects of a particular type can receive the same messages • An object of type circle is also an object of type shape • A circle is guaranteed to accept shape messages • You can write code that talks to shapes and automatically handle anything that fits the description of a shape • This substitutability is one of the powerful concepts in OOP. • Inheritance • Polymorphism Sharif University of Technology

18. Booch’s description of an Object • An object has state, behavior and identity • Booch added identity to the description • An object (may) have internal data • which gives it state • An object (may) have methods • to produce behavior • And each object can be uniquely distinguished from every other object • Each object has a unique address in memory Sharif University of Technology

19. Abstract Data Types • Each programming language has some predefined data types • int, double, char, … • Creating abstract data types is a fundamental concept in object-oriented programming • Abstract Data Type = Class • Programmer defines a class to fit a problem • You extend the programming language by adding new data types specific to your needs Sharif University of Technology

20. Messages • E.g. • Assign a book to a person • Set phone number of a person • Calculate area of a shape • complete a transaction • draw something on the screen • turn on a switch Sharif University of Technology

21. Interface • Each object can satisfy only certain requests • The requests you can make of an object are defined by its interface • The type is what determines the interface Sharif University of Technology

22. Representation of a light bulb: UML Diagram Sharif University of Technology

23. Person in an Education System Sharif University of Technology

24. New names in OOP • Function  Method, Service • Variable  Property, State Sharif University of Technology

25. Encapsulation • Commercial products are encapsulated • Remote control • TV • Cell phone • They are Black Boxes • Hidden Implementations • Public interface Sharif University of Technology

26. Why Encapsulation? • Simplified use • Even for the producer • Open implementation  bad use • Hiding the implementation reduces bugs • It is more beautiful! Sharif University of Technology

27. Object Encapsulation • Encapsulation of a problem-space concept into a class of objects • Define interface • Hide the implementation • Black box • The client may see the implementation • But can not use it directly • This is better even for the producer (programmer) Sharif University of Technology

28. Access Control • Access to some parts of the class is restricted • Public and Private area of the class • The client of a class can use only public area • Public area = class interface • Public methods • Public variables Sharif University of Technology

29. Example: Rectangle • Lets encapsulate a rectangle • What is a rectangle? • An object • Which has length and width (properties) • Lets you specify its length and width • Can calculate its area and perimeter Sharif University of Technology

30. Class Declaration Private area: hidden implementation publicclass Rectangle { privateintwidth, length; publicvoidsetWidth(int w) { width = w; } publicvoidsetLength(int l) { length = l; } publicintcalculateArea(){ returnwidth*length; } publicintcalculatePerimeter(){ return (width+length)*2; } } Public area :the interface Sharif University of Technology

31. How to Use Rectangle? Rectangle rect = new Rectangle(); rect.setWidth(2); rect.setLength(7); System.out.println(rect.calculateArea()); System.out.println(rect.calculatePerimeter()); Object creation or instantiation Sharif University of Technology

32. More Exercises • Encapsulate these concepts: • Student • Footbal-Player • Dog • name, age : properties • bite, bark : messages (actions) Sharif University of Technology

33. Conclusion • OOP brings us a new abstraction • OOP allows you to describe the problem in terms of the problem • Think in terms of the structure of the problem • Rather than the structure of the computer • Object has state, behavior and identity • Object has data and interface Sharif University of Technology

34. Further Reading • Google these queries and read some pages (Wikipedia is preferred) • Object Oriented Programming • History of Object Oriented Programming • Smalltalk • UML • Encapsulation • Inheritance • Polymorphism Sharif University of Technology

35. Sharif University of Technology