The Basics. Concepts of Class and Object

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1 Web Programming Week 11 COSC2453-SP2, 2011 OO- Part I 1 2 What are we covering now? The Basics Concepts of class and objects, How to write a class in PHP? Constructors, Visibility Modifiers, Abstract classes, Inheritance, Object Interfaces, Overloading, Polymorphism and other concepts The basics of OO Concepts of Class and Object 3 4

2 Objects and Classes An object represents an abstraction or thing with a crisp boundary for the problem at hand. eg, a car, a car fleet, a client. An object contains: data: capturing attributes and relationships with other objects (also called data members or variables), and methods for creating, accessing, transforming the data A class is a (design) template for an object (data + methods). For instance Car is a class. My car, the car QPD764 are objects (instances of the class Car) Objects are instantiated at runtime from their classes and communicate via messages Object and Real world The real-world objects share two characteristics: They all have state and behaviour. For example, dogs have state (name, color, breed, hungry) and behaviour (barking, fetching, and wagging tail). Bicycles have state (current gear, current pedal cadence, two wheels, number of gears) and behaviour (braking, accelerating, slowing down, changing gears). 5 6 What do objects consist of? The following illustration is a common visual representation of a software object (a bicycle modelled as a software object): Methods (behaviour) Variables (state) Representation of an object Methods surround and hide the object's nucleus from other objects in the program. Packaging an object's variables within the protective custody of its methods is called encapsulation. This conceptual picture of an object - a nucleus of variables packaged within a protective membrane of methods - is an ideal representation of an object and is the ideal that designers of object-oriented systems strive for. 7 8

3 Encapsulation Encapsulating related variables and methods into a neat software bundle is a simple yet powerful idea that provides two primary benefits to software developers: Modularity: The source code for an object can be written and maintained independently of the source code for other objects. Also, an object can be easily passed around in the system. You can give your bicycle to someone else, and it will still work. Information hiding: An object has a public interface that other objects can use to communicate with it. The object can maintain private information and methods that can be changed at any time without affecting the other objects that depend on it. You don't need to understand the gear mechanism on your bike to use it. Objects do talk A single object alone is generally not very useful. Instead, an object usually appears as a component of a larger program or application that contains many other objects. Through the interaction of these objects, programmers achieve higher-order functionality and more complex behaviour. Your bicycle hanging from a hook in the garage is just a bunch of titanium alloy and rubber; by itself, the bicycle is incapable of any activity. The bicycle is useful only when another object (you) interacts with it (pedal). Software objects interact and communicate with each other by sending messages to each other. When object A wants object B to perform one of B's methods, object A sends a message to object B 9 10 Messages Sometimes, the receiving object needs more information so that it knows exactly what to do; for example, when you want to change gears on your bicycle, you have to indicate which gear you want. This information is passed along with the message as parameters. The next figure shows the three components that comprise a message: 1.The object to which the message is addressed (YourBicycle) 2.The name of the method to perform (changegears) 3.Any parameters needed by the method (lowergear) Looking inside a class In object-oriented software, it's also possible to have many objects of the same kind that share characteristics: rectangles, employee records, video clips, and so on. Like the bicycle manufacturers, you can take advantage of the fact that objects of the same kind are similar and you can create a blueprint for those objects. A class is a blueprint, or prototype, that defines the variables and the methods common to all objects of a certain kind

4 More about a class The class for our bicycle example would declare the instance variables necessary to contain the current gear, the current cadence, and so on, for each bicycle object. The class would also declare and provide implementations for the instance methods that allow the rider to change gears, brake, and change the pedaling cadence, as shown in the next figure. Class and an Instance After you've created the bicycle class, you can create any number of bicycle objects from the class. When you create an instance of a class, the system allocates enough memory for the object and all its instance variables. Each instance gets its own copy of all the instance variables defined in the class Class versus Object You probably noticed that the illustrations of objects and classes look very similar. The difference between classes and objects is often the source of some confusion. In the real world, it's obvious that classes are not themselves the objects they describe: A blueprint of a bicycle is not a bicycle. However, it's a little more difficult to differentiate classes and objects in software. This is partially because software objects are merely electronic models of real-world objects or abstract concepts in the first place. But it's also because the term "object" is sometimes used to refer to both classes and instances. An interlude Before we go on to write some code in PHP, let us have a look at a real life example - A banking system. to understand how classes are born. If you understand the concept, the code writing is very simple 15 16

5 A Real World Example: A Bank system The Bank stores data about its customers. A customer has a name, a customer number, a password and up to 5 accounts. An account is identified by an account number and has a balance. A customer can perform withdrawals, deposits and queries on his/her account balance. How to discover classes (and their relationships)? How to discover attributes (for a class)? How to discover methods (for a class)? Discovering classes A concrete entity, eg. Student, Course An abstract concept, eg. Shape, Comparable Rule of thumb: look for nouns in the problem specs Relationship: Association? Composition? Inheritance? What are these? Discuss it with your lecturer/tutor. We will elaborate upon these later on. How many instances of one class are there in the other? stores owns Bank Customer Account 1 1..M Discovering Relationships 1 1..M Bank Customer Account customers : Customer [ ] accounts : Account [ ] Representing association or composition as an attribute: Discovering Attributes Ask questions such as I am an Account, what should I know? Should I know: my account number? my account balance? my owner? my bank? For each attribute you need to decide on: name, type, accessibility (more later); Also on constant or variable, having instance or class scope (later- it will be covered under the topic Visibility Modifiers ) Usually the Many (1..M) end is held in the One (1) end 19 20

6 Attributes Account accountnumber : integer balance : float Attributes (data) Discovering methods Ask questions such as: I am an Account, what services should I provide? Should I provide services for others to: query my account number? query my account balance? change my account number? change my account balance? withdraw money? deposit money? Rule of thumb: look for verbs in the problem specs Methods data + methods = class Methods Account accountnumber : integer balance : float withdraw (amount : float) : boolean deposit (amount : float) getaccountnumber () : integer getbalance () : float Account accountnumber : integer balance : float getaccountnumber () : integer getbalance () : float withdraw (amount : float) : boolean deposit (amount : float) etc. Each class encapsulates its own data and methods owns Customer customernumber : integer name : String password : String accounts : Account [ ] getcustomernumber () : integer getname () : String getpassword () : String getaccount (index:integer) : Account getaccounts () : Account [ ] setname (aname:string) setpassword (apasswd:string) addaccount (ac:account) : boolean etc

7 Object-orientation in PHP OO in PHP PHP is NOT a purely object-oriented language: (almost) unlike Java You have a choice of writing non-oo or OO code. In the professional world PHP websites/systems, the code is most of the time Object-Oriented. In order to build larger websites/systems in PHP, it is always advisable to write code in an Object-oriented manner A class is.. Do you remember what a class is? A class is technically defined as a representation of an abstract data type. In laymen's terms a class is a blueprint from which new will construct our box. It's made up of variables and functions that allow our box to be self-aware. With the blueprint, we can build our box exactly to our specifications. Let us now write a class in PHP. OOExample1.php <?php class SimpleClass { // member declaration public $var = 'a default value'; // method declaration public function displayvar() { echo $this->var; NOTE: $this refers to an internal pointer which is used to access class members -> is known as Qualifier operator

8 Is that code enough? A class on its own is of no use. You need to create an instance of the class - object in order to use it. That can be done as: $instance = new SimpleClass(); $instance -> displayvar(); As you can see the keyword new is used to create an object of the class. -> is used to access a method from the class. OOExample2.php <?php class Dog { public $hungry = Oh yeah'; function eat($food) { $this->hungry = 'not so much'; OOExample2.php $dog = new Dog(); echo $dog->hungry; echo <br>\n ; $dog->eat('cookie'); echo $dog->hungry;?> What will the program print? Did you notice that even though hungry is a variable, we do not put $ in front of it? Output of OOExample2.php The output will be: Oh yeah.<br> not so much. $this is a reserved variable name, referring to the current instantiation of the object. By instantiating, we create a new Dog. Don t mind the public keyword for now. The initial state of this dog is that it is pretty hungry. So we feed our newly instantiated dog a treat, to modify its state. After eating cookie, doggie isn t as hungry as before

9 Constructor A Constructor is a special method that is invoked to create a new object. Its purpose is to initialise the instance variables for the new object. Constructors A Constructor usually has the same name as the class (in older versions of PHP and other languages it does). In PHP 5, it is named << construct>> It is ALWAYS invoked with the new operator. For eg, Account a = new Account(). It NEVER has a return type. By default every class is provided with a default constructor that takes no parameters (called the default constructor). For eg, public Account(). If a class provides its own constructor (that takes parameters) then the default constructor is NOT automatically provided. In this case the class MUST explicitly define the default constructor if required. Many classes have more than one constructor (with different parameter lists), ie. constructors are overloaded Constructor PHP 5 allows developers to declare constructor methods for classes. Classes which have a constructor method call this method on each newly-created object, so it is suitable for any initialization that the object may need before it is used. <?php class BaseClass { function construct() { print "Inside constructor\n"; Example 1- class with a constructor <?php class person { //class variables var $name; //class constructor function construct($persons_name) { $this->name = $persons_name; //functions function set_name($new_name) { $this->name = $new_name; 35 36

10 Example 1- continued function get_name() { return $this->name; //end of the class $brook = new person("brook Lynn"); echo "Brook's full name: ". $brook->get_name();?> Explanation Always remember- Functions = methods Variables = properties Constructors allow you to initialise your object's properties (translation: give your properties values,) when you instantiate (create) an object. An object is created with a constructor Now that we've created a constructor method, we can provide a value for the $name property when we create our person objects. You 'feed' the constructor method by providing a list of arguments (like you do with a function) after the class name Explanation This saves us from having to call the set_name() method reducing the amount of code. This is just an example of how the mechanisms built into OO PHP can save you time and reduce the amount of code you need to write. Less code means less bugs. Lecture 11.2 : OO- Part II 39 40

11 Access Modifier Visibility/Access Modifiers One of the fundamental principles in OOP is 'encapsulation'. The idea is that you create cleaner better code, if you restrict access to the data structures (properties) in your objects. More so, you may want to hide some of the data in the class, you may not want to provide access to your properties- instead methods may provide an indirect access to class properties. You can restrict access to class properties using something called 'access modifiers Access Modifiers In PHP, there are 3 access modifiers: public private protected Public is default access modifier. When you declare a property with the 'var' keyword, it is considered 'public'. Access modifiers When you declare a property as 'private', only the same class can access the property. When a property is declared 'protected', only the same class and classes derived from that class can access the property - this has to do with inheritance more on that later. Properties declared as 'public' have no access restrictions, meaning anyone can access them

12 Example2.php (private) <?php class person { //class variables var $name; private $pinn_number; //class constructor function construct($persons_name) { $this->name = $persons_name; //functions function set_name($new_name) { $this->name = $new_name; Example3.php <?php class phpclass{ private $var="hello"; private function format( ){ $this->var = $this->var." Shekhar"; public function getvar( ){ $this->format(); return $this->var; $obj = new phpclass( ); echo $obj->getvar( );?> 45 Example2.php $brook = new person("brook Lynn"); echo "Brook's full name: ". $brook->get_name(); echo "<br>"; echo "Tell me private stuff: ". $brook->pinn_number;?> Output: Brook's full name: Brook Lynn Fatal error: Cannot access private property person::$pinn_number in Example2.php Static keyword Declaring class members or methods as static makes them accessible without needing an instantiation of the class (i.e. you do not need to declare object to access them) A member declared as static can not be accessed with an instantiated class object (though a static method can). Because static methods are callable without an instance of the object created, the pseudo variable $this is not available inside the method declared as static. Static properties cannot be accessed through the object using the arrow operator ->

13 Static property(example4.php) <?php class Foo { public static $my_static = 'foo'; public function staticvalue() { return self::$my_static; print Foo::$my_static. "\n";?> In order to access something static, the scope-resolution (::) operator is used. Static method <?php class Foo { public static function astaticmethod() { //...?> Foo::aStaticMethod(); Inheritance Inheritance Subclasses are more specialized versions of a class, which inherit attributes and behaviours from their parent classes, and can introduce their own. For example, the class Dog might have subclasses called Collie, Chihuahua, and GoldenRetriever. Suppose the Dog class defines a method called bark() and a property called furcolor. Each of its sub-classes (Collie, Chihuahua, and GoldenRetriever) will inherit these members, meaning that the programmer only needs to write the code for them once

14 Inheritance Inheritance promotes reusability (code reuse) and extensibility. Remember the Bank example? What if the bank introduces a new product- Credit Account, which is a type of Account. Inheritance: The Credit Account example Account is-a CreditAccount accountnumber : integer balance : float creditlimit : float INTEREST : float duedate : Date outstanding : float withdraw (amount:float): boolean deposit (amount:float) calculateoutstanding ()... The Bank introduces a new product, Credit Account. A Credit Account has a credit limit, a fixed interest rate, a due date, and an outstanding amount. A Customer may withdraw up to the credit limit plus account balance. Every month, any outstanding amount after the due date will be charged to the account Inheritance in PHP (Example5.php) Often you need classes with similar variables and functions to another existing class. In fact, it is good practice to define a generic class which can be used in all your projects and adapt this class for the needs of each of your specific projects. To facilitate this, classes can be extensions of other classes. The extended or derived class has all variables and functions of the base class (this is called 'inheritance') and what you add in the extended definition. Inheritance in PHP (Example5.php) Keyword <<extends>> is used to implement Inheritance in PHP You can inherit parent/base classes constructor in a child class. Have a look at Example5.php 55 56

15 Protected keyword (Example6.php) When a property is declared 'protected', only the same class and classes derived from that class can access the property - this has to do with inheritance more on that later. Let us now look at Example6.php Class Constants Constants It is possible to define constant values on a perclass basis remaining the same and unchangeable. Constants differ from normal variables in that you don't use the $ symbol to declare or use them. The value must be a constant expression, not (for example) a variable, a class member, result of a mathematical operation or a function call. Constants in a class <?php class MyClass { const constant = 'constant value'; function showconstant() { echo self::constant. "\n"; As of PHP 5.3.0, it's possible to reference the class using a variable. The variable's value can not be a keyword (e.g. self, parent and static)

16 Constants in class (Example7.php) Let us now look at Example7.php References php information: php tutorial: Abstract Class An Abstract Class An abstract class represents a concept; classes derived from it represent implementations of the concept. In other words you can call it a template class that contains such things as variable declarations and methods, but cannot contain code for creating new instances. You cannot create an object of an abstract class As a result, you must create a sub-class to implement an abstract class

17 Abstract Class An abstract class isn't intended to be instantiated, but to serve as a parent to other classes, partly dictating how they should behave. Abstract classes can have abstract methods, these are required in the child classes. Abstract Class Example <?php abstract class Animal{ public $hungry = 'hell yeah.'; abstract public function eat($food); class Dog extends Animal{ //overriding the abstract method in the child class function eat($food){ if($food == 'cookie'){ $this->hungry = 'not so much.'; else { echo 'barf, I only like cookies!'; Abstract Class Example - cont $dog = new Dog(); echo $dog->hungry; //echoes "hell yeah." $dog->eat('peanut'); //echoes "barf, I only like cookies!?> Interface 67 68

18 Interface Interfaces are different from abstract classes. For one, they're not actually classes. They don't define properties, and they don't define any behaviour. The methods declared in an interface must be declared in classes that implement it. Interface Because an interface in the more general sense is a definition of how an object interacts with other code, all methods must be declared public. Using abstract classes, an abstract method can have any visibility, but the extending classes must have their implementations use the same (or weaker) visibility. Interfaces can be looked upon as a contract, a certificate of compliance if you will. Other code is guaranteed that a class implementing it will use certain methods to interact Interface Example <?php interface Animal { public function eat($food); interface Mammal { public function givebirth(); Interface Example class Dog implements Animal, Mammal { public $gender = 'male'; function eat($food) { if($food == 'cookie') { $this->hungry = 'not so much.'; else { echo 'barf, I only like cookies!'; 71 72

19 Interface Example function givebirth() { if($this->gender == 'male'){ echo 'I can\'t, I am a boy'; else { echo 'I\'m not even pregnant yet.';?> Overloading Overloading Object overloading in PHP refers to the mechanism where a call to a method or property will 'overload' the call to a different property or method. The call ultimately made can depend on the type of the arguments or the context. Note: If you're coming from a different OO language, the term 'overloading' likely has a very different meaning to you: defining different method with the same name having different signatures. This has nothing to do with that. Overloading These magic methods allow you catch calls to methods and properties that haven't been defined, because you didn't know (or didn't want to specify) the exact name. The magic methods are executed only if the object doesn't have the method or property declared

20 Overloading Overloading uses the following magic methods: call get set isset unset Overloading Example It is too long to be replicated here Have a look at example- Overloading.php. Note: Constructor construct() is also considered as a magic method Exception handling Remember the good-old try-catch and throw? Creating a custom exception class Try - A function using an exception should be in a "try" block. If the exception does not trigger, the code will continue as normal. However if the exception triggers, an exception is "thrown". Throw - This is how you trigger an exception. Each "throw" must have at least one "catch". Catch - A "catch" block retrieves an exception and creates an object containing the exception information 79 80

21 Creating a custom exception class Creating a custom exception handler is quite simple. We simply create a special class with functions that can be called when an exception occurs in PHP. The class must be an extension of the exception class. Example Have a look at the example- CreatingCustomException.php. The custom exception class inherits the properties from PHP's exception class and you can add custom functions to it Explanation of the example 1. The customexception() class is created as an extension of the old exception class. This way it inherits all methods and properties from the old exception class. 2. The errormessage() function is created. This function returns an error message if an address is invalid. 3. The $ variable is set to a string that is not a valid address. 4. The "try" block is executed and an exception is thrown since the address is invalid. What else is there? We have not covered the following concepts in this subject: Destructors Autoload property Final keyword Object Serialization Object Iteration and Polymorphism These are left for a more advanced subject. 5. The "catch" block catches the exception and displays the error message 83 84

22 References php information: php tutorial: 85