M275 - Web Development using PHP and MySQL

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1 Arab Open University Faculty of computer Studies M275 - Web Development using PHP and MySQL Chapter 9 Working with Objects This is a supporting material to chapter 9. This summary will never substitute original text book. Haifaa Elayyan 2/6/2013

2 Why do programmer use objects? 2 P a g e Working with Objects Programmers use objects to store and organize data. Object-oriented programming is a type of programming in which the structure of the program (or application) is designed around these objects and their relationships and interactions. Object-oriented programming structures are found in many programming languages, and are also evident in PHP. Explaining the concept of an object is a little difficult if you ve never encountered the concept before, because it is inherently abstract: It s a sort of theoretical box of things variables, functions, and so forth that exists in a templated structure called a class. Although it s easy to visualize a scalar variable, such as $color, with a value of red, or an array called $character with three or four different elements inside it, some people have a difficult time visualizing objects. For now, try to think of an object 1. as a little box with inputs and outputs on either side of it. 2. The input mechanisms are methods, 3. and methods have properties. 4. classes, methods, and properties work together to produce various outputs. As mentioned previously, an object exists in a structure called a class. In each class, you define a set of characteristics. For example: Suppose you have created an automobile class. In the automobile class, you might have color, make, and model characteristics. Each automobile object uses all the characteristics, but each object initializes the characteristics to different values, such as silver, Mazda, and Protege5, or red, Porsche, and Boxter. The whole purpose of using objects is to create reusable code. Because classes are so tightly structured but self-contained and independent of one another, you can reuse them from one application to another.. Creating an object is simple; you just declare it to be in existence: Definition : class myclass { //code will go here } Now that you have a class, you can create a new instance of an object: $object1 = new myclass(); Example : Proof That Your Object Exists

3 3: //code will go here 4: } 5: $object1 = new myclass(); 6: echo \$object1 is an.gettype($object1)..<br/> ; 7: 8: if (is_object($object1)) { 9: echo Really! I swear \$object1 is an object! ; 10: } 11:?> OUTPUT : $object1 is an object. Really! I swear $object1 is an object! This is not a particularly useful class because it does absolutely nothing, but it is valid and shows you how the class template works in lines 2 5. Lines 8 10 use the is_object() function to test whether something is an object; in this case, the something is $object1. Because the test of is_object() evaluates to true, the string within the if statement is printed to the screen. Next, you learn about using object properties and methods within the class template. Properties of Objects The variables declared inside an object are called properties. It is standard practice to declare your variables at the top of the class. These properties can be values, arrays, or even other objects. The following snippet uses simple scalar variables inside the class, prefaced with the public keyword: class mycar { public$color = silver ; public$make = Mazda ; public$model = Protege5 ; } If you use the keyword public, protected, or private before the variable name, you can indicate if the class member (the variable) can be accessed everywhere (public), within the class itself or a parent class or an inherited class (protected), or only by the class itself (private). Now when you create a mycar object, it will always have those three properties. Listing 9.2 shows you how to access properties after they have been declared and values have been assigned to them. LISTING 9.2 Showing Object Properties 2: class mycar { 3: public$color = silver ; 4: public$make = Mazda ; 5: public$model = Protege5 ; 7: $car = new mycar(); 8: echo I drive a:.$car -> color..$car -> make..$car -> model; 9:?> 3 P a g e

4 In this instance, even if the $color, $make, and $model properties had no initial values when declared, lines 8 10 would assign a value to them. As long as the properties are declared, you can use them later (initial values or not). Methods add functionality to your objects. No longer will your objects just sit there, holding on to their properties for dear life they ll actually do something! Listing 9.4 shows just that. LISTING 9.4 A Class with a Method 3: function sayhello() { 4: echo HELLO! ; 5: } 7: $object1 = new myclass(); 8: $object1 -> sayhello(); 9:?> Although it is not the most thrilling example of action, if you save this code as helloclass.php, place it in your document root, and access it with your web browser, you will see the following on your screen: HELLO! A method looks and acts like a normal function but is defined within the framework of a class. The - > operator is used to call the object method in the context of your script. Had there been any variables stored in the object, the method would have been capable of accessing them for its own purposes, as illustrated in Listing 9.5. LISTING 9.5 Accessing Class Properties Within a Method 3: public$name = Jimbo ; 4: function sayhello() { 5: echo HELLO! My name is.$this->name; 7: } 8: $object1 = new myclass(); 9: $object1 -> sayhello(); 10:?> If you save this code as helloclass2.php, place it in your document root, and access it with your web browser, you will see the following on your screen: HELLO! My name is Jimbo The special variable $this is used to refer to the currently instantiated object as you see on line 5. Anytime an object refers to itself, you must use the $this variable. Constructors A constructor is a function that lives within a class and, given the same name as the class, is automatically called when a new instance of the class is created using new classname. Using constructors enables you to provide arguments to your class, Object Inheritance P a g e

5 which will then be processed immediately when the class is called. You see constructors in action in the next section. Object Inheritance Having learned the absolute basics of objects, properties, and methods, you can start to look at object inheritance. Inheritance with regard to classes is just what it sounds like: One class inherits functionality from its parent class. Listing 9.7 shows an example. LISTING 9.7 A Class Inheriting from Its Parent 3: public$name = Matt ; 4: function myclass($n) { 5: $this->name = $n; 7: function sayhello() { 8: echo HELLO! My name is.$this->name; 9: } 10: } 11: class childclass extends myclass { 12: //code goes here 13: } 14: $object1 = new childclass( Baby Matt ); 15: $object1 -> sayhello(); 16:?> If you save this code as inheritance.php, place it in your document root, and access it with your web browser, you will see the following on your screen: HELLO! My name is Baby Matt Example of overriding :. LISTING 9.8 The Method of a Child Class Overriding That of Its Parent 156 CHAPTER 9: Working with Objects LISTING 9.8 Continued 3: public$name = Matt ; 4: function myclass($n) { 5: $this->name = $n; 7: function sayhello() { 8: echo HELLO! My name is.$this->name; 9: } 10: } 11: class childclass extends myclass { 12: function sayhello() { 13: echo I will not tell you my name. ; 14: } 15: } 16: $object1 = new childclass( Baby Matt ); 17: $object1 -> sayhello(); 5 P a g e

6 18:?> The only changes in this code from Listing 9.7 are the new lines In these lines, a function is created called sayhello() that, instead of printing HELLO! My name is..., prints the message I will not tell you my name. Because the sayhello() function now exists in childclass, and childclass is the class called in line 16, its version of sayhello() is the one used. Although you might want to pull your hair out in frustration, you re really not in a bad situation. If you separate the compilation and storage classes from the formatting classes one for each of the various delivery methods (HTML, text, , XML) you essentially have a parent-child relationship. Consider the parent class as the one that holds the compilation and storage methods. The formatting classes are the children: They inherit the information from the parent and output the result based on their own functionality. Summary This chapter was about teaching you learned how to create and access the properties and methods of a class, how to build new classes, and how to inherit features from parent classes. 6 P a g e

Java Object Oriented Design. CSC207 Fall 2014

Java Object Oriented Design. CSC207 Fall 2014 Java Object Oriented Design CSC207 Fall 2014 Design Problem Design an application where the user can draw different shapes Lines Circles Rectangles Just high level design, don t write any detailed code