Classes, objects, methods and properties

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1 Learning Object 1. African Virtual University Template for extracting a learning object Main Learning Objective Nature of learning object Key concept (s) Source Module information Access source module Extracted by To be able to create classes and objects using Java. A sub-topic CLASSES AND OBJECTS Module Title: Object Oriented Programming Code; CS Author: Lucy Gitau To be Provided by the AVU Lucy Gitau Date 27 th April 2016 Copy and Paste the Learning Object here Classes, objects, methods and properties Object-oriented programming is a programming style in which it is customary to group all of the variables and functions of a particular topic into a single class. Object-oriented programming is considered to be more advanced and efficient than the procedural style of programming. This efficiency stems from the fact that it supports better code organization, provides modularity, and reduces the need to repeat ourselves. However, we may still prefer the procedural style in small and simple projects. But, as our projects grow in complexity, we are better off using the object oriented style. With this tutorial, we are going to take our first steps into the world of object oriented programming by learning the most basic terms in the field: classes 1

2 objects methods properties learning objectives: How to create classes? How to add properties to a class? How to create objects from a class? How to get and set the objects' properties? How to add methods to a class? How to create classes? In order to create a class, we group the code that handles a certain topic into one place. For example, we can group all of the code that handles the users of a blog into one class, all of the code that is involved with the publication of the posts in the blog into a second class, and all the code that is devoted to comments into a third class. To name the class, it is customary to use a singular noun that starts with a capital letter. For example, we can group a code that handles users into a User class, the code that handles posts into a Post class, and the code that is devoted to comments into a Comment class. For the example given below, we are going to create a Car class into which we will group all of the code which has something to do with cars class Car { // The code We declare the class with the class keyword. We write the name of the class and capitalize the first letter. If the class name contains more than one word, we capitalize each word. This is known as upper camel case. For example, JapaneseCars, AmericanIdol, EuropeTour, etc. We circle the class body within curly braces. Inside the curly braces, we put our code. How to add properties to a class? We call properties to the variables inside a class. Properties can accept values like strings, integers, and booleans (true/false values), like any other variable. Let's add some properties to the Car class class Car { 2

3 public $comp; public $color = 'beige'; public $hassunroof = true; We put the public keyword in front of a class property. The naming convention is to start the property name with a lower case letter. If the name contains more than one word, all of the words, except for the first word, start with an upper case letter. For example, $color or $hassunroof. A property can have a default value. For example, $color = 'beige'. We can also create a property without a default value. See the property $comp in the above example. How to create objects from a class? We can create several objects from the same class, with each object having its own set of properties. In order to work with a class, we need to create an object from it. In order to create an object, we use the new keyword. For example: 1$bmw = new Car (); We created the object $bmw from the class Car with the new keyword. The process of creating an object is also known as instantiation. We can create more than one object from the same class. 1 2$bmw = new Car (); $mercedes = new Car (); In fact, we can create as many objects as we like from the same class, and then give each object its own set of properties. Objects, what are they good for? While in the procedural style of programming, all of the functions and variables sit together in the global scope in a way that allows their use just by calling their name, the use of classes makes anything inside the classes hidden from the global scope. That's because the code inside the classes is encapsulated within the class scope, outside the reach of the global scope. So, we need a way to allow the code from the global scope to use the code within the class, and we do this by creating objects from a class. I say objects, and not object, because we can create as many objects as we would like from the same class and they all will share the class's methods and properties. See the image below: 3

4 From the same Car class, we created three individual objects with the name of: Mercedes, Bmw, and Audi. Although all of the objects were created from the same class and thus have the class's methods and properties, they are still different. This is not only, because they have different names, but also because they may have different values assigned to their properties. For example, in the image above, they differ by the color property - the Mercedes is green while the Bmw is blue and the Audi is orange. Note: A class holds the methods and properties that are shared by all of the objects that are created from it. Although the objects share the same code, they can behave differently because they can have different values assigned to them. How to get an object's properties? Once we create an object, we can get its properties. For example: 1 2echo $bmw -> color; echo $mercedes -> color; In order to get a property, we write the class name, and then dash greater than (->), and then the property name. Note that the property name does not start with the $ sign; only the object name starts with a $. Result: beige beige 4

5 How to set the object's properties? In order to set an object property, we use a similar approach. For example, in order to set the color to 'blue' in the bmw object: 1$bmw -> color = 'blue'; and in order to set the value of the $comp property for both objects: 1 2$bmw -> comp = "BMW"; $mercedes -> comp = "Mercedes Benz"; Once we set the value of a property, we can get its value. In order to get the color of the $bmw object, we use the following line of code: 1echo $bmw -> color; Result: blue We can also get the company name and the color of the second car object. 1 2echo $mercedes -> color; echo $mercedes -> comp; Result: Mercedes Benz beige How to add methods to a class? The classes most often contain functions. A function inside a class is called a method. Here we add the method hello() to the class with the prefix public class Car { public $comp; public $color = 'beige'; public $hassunroof = true; public function hello() { return "beep"; We put the public keyword in front of a method. 5

6 The naming convention is to start the function name with a lower case letter. If the name contains more than one word, all of the words, except for the first word, start with an upper case letter. For example, hellouser() or flypanam(). We can approach the methods similar to the way that we approach the properties, but we first need to create at least one object from the class $bmw = new Car (); $mercedes = new Car (); echo $bmw -> hello(); echo $mercedes -> hello(); Result: beep beep Here is the full code that we have written during this tutorial: // Declare the class class Car { // properties public $comp; public $color = 'beige'; public $hassunroof = true; // method that says hello public function hello() { return "beep"; // Create an instance $bmw = new Car (); $mercedes = new Car (); // Get the values echo $bmw -> color; // beige echo "<br />"; echo $mercedes -> color; // beige echo "<hr />"; // Set the values $bmw -> color = 'blue'; $bmw -> comp = "BMW"; $mercedes -> comp = "Mercedes Benz"; // Get the values again echo $bmw -> color; // blue echo "<br />"; echo $mercedes -> color; // beige echo "<br />"; echo $bmw -> comp; // BMW echo "<br />"; echo $mercedes -> comp; // Mercedes Benz 6

7 echo "<hr />"; // Use the methods to get a beep echo $bmw -> hello(); // beep echo "<br />"; echo $mercedes -> hello(); // beep Conclusion We have learnt about classes and objects that can be created out of them. Learning object 2 Main Learning Objective Nature of learning object Key concept (s) Source Module information Access source module Extracted by To distinguish between inheritance and polymorphism. Designed summary Inheritance and polymorphism Module Title: Object Oriented Programming Code; CS Author: Lucy Gitau To be Provided by the AVU Lucy Gitau Date 27 th April 2016 Copy and Paste the Learning Object here Code of Polymorphism vs Inheritance in Java 7

8 Below is a code illustrating How Inheritance and Polymorphism works. In Java, polymorphism is type based, in order to write polymorphic code, you need to create a Type hierarchy, which is achieved using Inheritance. In this example, we have abstract class to represent a Connection and we have two sub-classes TCP and UDP. All three are related to each other via Inheritance, Connection is Parent while TCP and UDP are Child classes. Now any code, which is based upon Connection will be polymorphic and can behave differently based upon whether actual connection is of type TCP or UDP. This Polymorphism magic is constructed by method overriding in Java, but its the principle of programming for interfaces than implementation, which motivates to write polymorphic code. Why you should write Polymorphic code? Simple to be flexible, to accommodate change and to take advantage of evolution on later stage of development. A static code is fixed when written, but a Polymorphic code can evolve. public class Test { public static void main(string args[]) { Connection connection = new TCP(); connection.connect(); /** * Base class to represent a Connection. */ public abstract class Connection{ protected String data; public void connect(){ System.out.println("Connecting...") public void inputdate(string data){ this.data = data; public class TCP extends Connection { 8

9 @Override public void connect() { System.out.println("Connection reliably but slow.."); public class UDP extends Connection public void connect(){ System.out.println("Connecting fast but no guarantee of data delivery"); Output: Connection reliably but slow.. In short here are key difference between Polymorphism and Inheritance in Java : 1) Inheritance defines father-son relationship between two classes, While Polymorphism take advantage of that relationship to add dynamic behaviour in your code. 2) Inheritance is meant for code reuse, initial idea is to reuse what is written inside Parent class and only write code for new function or behaviour in Child class. On the other hand Polymorphism allows Child to redefine already defined behaviour inside parent class. Without Polymorphism it's not possible for a Child to execute its own behaviour while represented by a Parent reference variable, but with Polymorphism he can do that. 3) Polymorphism helps tremendously during Maintenance. In fact many object oriented design principles are based on Polymorphism e.g. programming for interface then implementation, which advocates to use interface everywhere in your code, to represent variable, in method parameters, in return type of method etc; so that code can take advantage of polymorphism and do more than what was expected it to do during writing. 4) Java doesn't allow multiple inheritance of classes, but allows multiple inheritance of Interface, which is actually require to implement Polymorphism. For example a Class can be Runnable, Comparator and Serializable at same time, because all three are interfaces. This makes them to pass around in code e.g. you can pass instance of this class to a method which accepts Serializable, or to Collections.sort() which accepts a Comparator. 9

10 5) Both Polymorphism and Inheritance allow Object oriented programs to evolve. For example, by using Inheritance you can define new user types in an Authentication System and by using Polymorphism you can take advantage of already written authentication code. Since, Inheritance guarantees minimum base class behaviour, a method depending upon super class or super interface can still accept object of base class and can authenticate it. 6) In UML diagram, Inheritance is represented using arrows, pointing towards Parent class. For example in this diagram, AbstractPerson is Parent class for Employee, Manager and CustomerContact class. Read more: Inheritance-java-oops.html#ixzz470oaJSpF learning object 3 Main Learning Objective Nature of learning object Key concept (s) Source Module information Access source module Extracted by To enable students to distinguish between procedural and object oriented programming Designed summary Differences between Procedural and Object Oriented Programming Module Title: Object Oriented Programming Code; CS Author: Lucy Gitau To be Provided by the AVU Lucy Gitau Date 27 th April

11 Copy and Paste the Learning Object here In this particular article we will learn about difference between Object Oriented Programming (OOP) and procedural programming. Let s learn by making some key differences: Division Of Program In procedural programming a program is divided into small functions while in oop it is divided into objects. Importance In procedural programming importance is not given to data but to procedure. But in oop importance is given to data rather than functions. Because in oop it works well with real world objects. Approach In procedural programming top down approach is followed but in oop bottom up approach is followed. Access Specifiers In procedural programming no access specifiers are used. But in oop different type of specifiers are used for different level of data protection. Data Moving 11

12 In procedural program data can move freely from function to function. But in oop data can only move from member functions. Data Access In procedural programming most of the time global data can be used. So all functions of program uses global data. In oop public or private access specifiers are used to control the data access. Data Hiding In procedural programming no particular method is used for data hiding. And therefore it is less secure. A particular method is used for data hiding in oop so it is more secure. Overloading Overloading is not possible in procedural programming. Overloading is possible in oop in the form of function overloading and overloading operator. Examples Examples of procedure oriented are c, vb, fortran. And examples of object oriented are Java, C#.Net etc. So all the above are key difference between procedural and object oriented programming. Learning object 4 Main Learning Objective Nature of learning object Key concept (s) Source Module information Access source module Extracted by To distinguish between visibility types in object oriented programming Case scenario for class student To be able to assign appropriate visibility to variables/ methods in a class/ object Module Title: Object Oriented Programming Code; CS Author: Lucy Gitau To be Provided by the AVU Lucy Gitau Date 27 th April 2016 Copy and Paste the Learning Object here 12

13 Learning Object 5 Main Learning Objective Nature of learning object Key concept (s) Source Module information Access source module Extracted by To be able to create simple desk top applications Introduction to GUI programming CLASSES AND OBJECTS Module Title: Object Oriented Programming Code; CS Author: Lucy Gitau To be Provided by the AVU Lucy Gitau Date 6 th April 2016 Introduction to object oriented programming with java, 5th edition Wu, C. Thomas,

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