SEEM4570 System Design and Implementation Lecture 09 Object Oriented Programming

Transcription

1 SEEM4570 System Design and Implementation Lecture 09 Object Oriented Programming Programming In general, there are two kinds of programming: Procedural Programming Also known as Imperative Programming E.g. C Object Oriented Programming (OOP) E.g. Java, C++, Objective-C

2 Procedural Programming (1) Programs are made up of procedures A procedure is a sequence of statements. E.g. In C, statements can be assignments, loops, invocations of sub procedures, etc. These procedures are functions, which map arguments to return statements. The design method used in procedural programming is called Top Down Design. You start with a problem (procedure) and then systematically break the problem down into sub problems (sub procedures). a.k.a. functional decomposition, which continues until a sub problem is straightforward enough to be solved by the corresponding sub procedure. Procedural Programming (2) The difficulties with this type of programming is that software maintenance can be difficult and time consuming. When changes are made to the main procedure (top), those changes can cascade to the sub procedures of main, and the subsub procedures and so on, where the change may impact all procedures in the pyramid. Example: It is common for some of the program s data to be global. As programs grow in size, allowing any function to modify any piece of data means that bugs can have wide-reaching effects. So, writing a function that access global variable may sometimes need to understand all other functions as well Too tough!

3 From Procedure to Object Oriented (1) One alternative to procedural programming is object oriented programming. Object oriented programming is meant to address the difficulties with procedural programming. In object oriented programming, the main modules in a program are classes, rather than procedures. The objectoriented approach lets you create classes and objects that model real world objects. Classes and Objects (1) Classes A class is a data structure consisting of state (data) and behaviors (methods) Examples: We may design a class Car. Cars have states (data), such as brand and color. Cars also have behaviors (methods), such as enginestart, running. All of the state and behavior is encapsulated (contained) within the class Car. We may design a class Human. Humans have states (data), such as height and weight. Humans also have behaviors (methods), such as eating and driving. All of the state and behavior is encapsulated (contained) within the class Human.

4 Classes and Objects (2) Objects An object is an instance (example) of a particular class Example: We try to make a car, say BMW-Silver, based on the states and behaviors encapsulated in the class Car. Objects communication Objects communicate using messages. An object passes a message to another object, which results in the invocation of a method. Objects then perform the actions that are required to get a response from the system. Example: John (belongs to the class Human) passes the message drive to BMW (belongs to the class Car). Then BMW invocates the method enginestart and BMW can now be able to drive. Classes and Objects (3) Relationship between classes and objects: A class is a prototype of a collection of objects. Example: We design a class Car (a prototype) which is a collection of all cars in the world.

5 Object Oriented Programming (1) We usually use OO to denote Object Oriented OOP > Object Oriented Programming OO Concept > Object Oriented Concept With OOP: Instead of having functions, we have objects. Each object can have a list of properties and methods: Modifying one object usually will not affect other objects Each programmer only need to concentrate on his/her own object. No need to understand/take care of the other programmers works. Object Oriented Programming (2) An OO programming language usually include one or more of the following features: Encapsulation Inheritance Polymorphism Data Abstraction

6 Encapsulation (1) The ability of an object to hide its data and methods from the rest of the world One of the fundamental principles of OOP. The user of an object only need to know the services provided by the object, but do not need to known the implementation detail. If we want to drive a car, we do not need to know how a car is built. We only need to know how to drive. Encapsulation (2) We can view an object as a black box that provides services As long as the services provided by the object remain the same, then code that uses the object can continue to use without problem. Examples: If our class Car added a new method automaticparking, it won't affect how we drive the car. If the car engine is upgraded, it shouldn't affect how we drive the car as well.

7 Encapsulation (3) class Car{ private $brand; public function construct($brand) { $this->brand = $brand; public function getbrand(){ return $this->brand; $bmw = new Car("BMW"); // instance of Car echo $bmw->brand; // THIS IS WRONG! echo $bmw->getbrand(); // THIS IS CORRECT! Consider the followings: Encapsulation (4) class Car{ private $brand; : $bmw= new Car("BMW"); echo $bmw->brand! Error!!! class Car{ public $brand; : $bmw= new Car("BMW"); echo $bmw->brand;! OK!!!

8 Encapsulation (5) In the previous slides, Private and Public are called access modifier. Control how people (i.e., programs) use the resources. Private: only accessible within the same class only Protected: only accessible within the same class OR the class inherited it. Public: Can be accessible by any programs Inheritance (1) You can use one class as the basic for another class. Multiple classes may share some of the same characteristics, and have things in common with one another, but also may have certain properties that make them different. Example: I want to define two kinds of cars, Japanese Car and European Car, so that both inherits the properties (i.e., data and methods) of Car

9 Inheritance (2) class JapaneseCar extends Car{ private $madein = "Japan"; public function getmadein(){ echo "{$this->getbrand() is made in {$this->madein "; class EuropeanCar extends Car{ private $madein = "Europe"; public function getmadein(){ echo "{$this->getbrand() is made in {$this->madein "; $bmw = new EuropeanCar("BMW"); $toyota = new JapaneseCar("Toyota"); $bmw->getmadein(); $toyota->getmadein(); Polymorphism Polymorphism is the ability to create a variable, a function, or an object that has more than one form. Polymorphism can has many forms. The most common forms: Method Overriding Re-define the method in a subclass (inherited class) Method Overloading (Not in PHP) Define some different methods with the same name but different number of arguments

10 Polymorphism (Method Overriding) (1) Suppose we do not want to construct the European Car using one parameter (brand) but also want to specify the color. Hence, we want something like: $bmw = new EuropeanCar("BMW", "Silver") As a result, we need to "Override" the default service of a method Polymorphism (Method Overriding) (2) class EuropeanCar extends Car{ private $madein = "Europe"; private $model; public function construct($brand, $color){ Car::($brand); $this->model = $model; public function getmadein(){ echo" {$this->getbrand() is made in {$this->madein<br>";

11 Polymorphism (Method Overriding) (3) Suppose we want to construct another subclass UK Car which extends European Car and we want to overrides the "getmadein" method class UKCar extends EuropeanCar { public function getmadein(){ echo "{$this->getbrand() is made in UK"; Data Abstraction (1) Data abstraction refers to, providing only essential information to the outside word and hiding their background details To represent the needed information in program without presenting the details. Data abstraction is a programming technique that relies on the separation of interface (method definitions) and implementation. We separate them completely! One file is just for definition, another file is for implementation.

12 Data Abstraction (2) There are two kinds of data abstraction: Interface and Abstract Class Strictly speaking, they are not class (i.e. not object). They define how a class (i.e. an object) should look like Data Abstraction (3) Writing interface: interface XXX{ Using interface: class YYY implements XXX{

13 Data Abstraction (4) Example: interface CarLayout{ public function getbrand(); class Car implements CarLayout{ // same as before (e.g. P.14) Data Abstraction (5) Writing abstract class: abstract class XXX{ Using abstract class: class YYY extends XXX{

14 Data Abstraction Coding (5) Example: abstract class CarLayout{ abstract public function getbrand(); class Car extends CarLayout{ // same as before (e.g. P.14) Data Abstraction Coding (6) What are the differences between Interface and Abstract Class? Major differences (Very Important): Method Implementation: Interface: Does not allow specifying methods implementation detail Abstract class: Allow specifying methods implementation detail Object inheritance: Interface: An object can implement any number of interfaces Abstract class: An object can extend one and only one class

15 Data Abstraction Coding (7) This is correct: abstract class XXXX{ public function AAA(){ // some code This is incorrect: interface XXXX{ public function AAA(){ // some code Note The keyword Abstract does not appear! You can implement a method in an interface! Note You cannot implement a method in an interface! Data Abstraction Coding (8) This is correct: class Car implements CarLayout, Engine, Tire { // some codes This is incorrect: class XXXX extends CarLayout, Engine, Tire { // some codes Note You cannot extends multiple abstract class!