Dev Concepts: Object-Oriented Programming

Editor's Notes

• #2 Hello, I am Svetlin Nakov from SoftUni and I am here for the next episode from my "Dev Concepts" series. In this lesson I will briefly explain and demonstrate the concept of “Oh-Oh-Pee” (the object-oriented programming), which is basically the concept of using classes and objects to model the real world in your programming language.I will explain the concepts of classes (which define the structure for the objects), data fields (which hold data) and properties (which provide access the data fields), and the concept of object state, as well as the actions, defined in the classes (which in “Oh-Oh-Pee” are called methods). I will show some live coding examples in C# and JavaScript, where I will do the following:I will define a class, I will instantiate objects from this class, then I will access the object's data and call the methods of the object. Later, I will explain the concepts of inheritance, interfaces and abstract classes in “Oh-Oh-Pee” and give you a real-world example. I will define an abstract class with an abstract method in it and I will inherit this base class into two child classes, where I will override the inherited abstract methods to provide a specific implementation. Finally, I will demonstrate how to use these abstract class and the child classes.Watch this video lesson to learn the main concepts of object-oriented programming, explained briefly, concisely, and in simple words.
• #3 Object-oriented programming (OOP) is the concept of using classes and objects to model the real world. Classes are sets of data fields, together with methods (which are functionality to interact with the data fields and other objects). Classes define the structure of information objects: the data they holds and the operation they can perform. Objects are instances of the classes, holding certain values in their data fields. At the example we have a definition of the class "Rectangle". It holds two data fields: width and height. – integer values. It defines a method, holding the code to calculate the area of the rectangle. This is the class definition … and the programming language here doesn’t matter. These are the definitions of the data fields, which the class holds in each object. These are the methods of the class: the operations or actions that objects of this class can do. And now we have several objects of this class "Rectangle". The first object is a rectangle of width 5 and height 6. Another object has width 6 and height 4. Some other object has width 7 and height 3. We have one class "Rectangle" and 3 objects (or instances) of this class. The class holds the definition (the specification, the model, the template) for the objects. It defines the data fields and methods and more details (in some cases). Classes don't hold data. They hold data definitions and operation definitions. Objects hold values for the data fields in the class. Objects of class "Rectangle" hold data about certain rectangle. Objects are information structures, holding data. Typically, one class has multiple objects (or instances). Classes and objects are the building blocks of the object-oriented programming (OOP) and they come together with some other OOP concepts like abstraction, interfaces, data encapsulation, inheritance, polymorphism and exception handling.
• #4 I have prepared examples of classes and objects in C# and JavaScript. We can open the JavaScript example. Wait a bit for the code to load. It takes time… Now we click [Run] and we see several rectangles (objects of class "Rectangle") together with their data field values and the calculated area. The rectangle area is calculated by a method "calcArea" in the class. This is the class definition. And this is how we create objects. I will not be able to get into more details at this moment. Now the concept is important, not the implementation and the code. The concept is that in programming we can define classes, which model real-world entities. They hold data (properties) and operations, just like in the real world. Objects are instances of the class definition with certain data characteristics, just like objects in the real world. We shall learn more about defining and using classes in the "Object-Oriented Programming" course at SoftUni. We have the same code in C#, which works in a very similar way. If you are curious, you can look at the sample code in detail and play with it.
• #5 In this section I will explain the concepts of inheritance, interfaces and abstract classes in the “Oh-Oh-Pee”, together with a real-world example. I will define an abstract class Figure with an abstract method "calcArea" in it, and I will inherit this base class into two child classes (Circle and Rectangle), where I will override the inherited abstract method to provide a specific implementation for the area calculation. Finally, in a live coding demo, I will demonstrate how to use this abstract class and the derived child classes.
• #6 Inheritance and interfaces are two other major concepts in the object-oriented programming. Inheritance allows classes to inherit data and functionality from a parent class (also called "base class"). When a class inherits another class, the parent class fields are merged with the child class fields and they together form the set of data fields for the child class. Interfaces defines abstract actions. These are actions to be implemented in the descendent classes. Interfaces define a set of empty (or abstract) methods (or actions), which shall be obligatory implemented in the child classes. Interfaces are also called "contracts", because they define certain set of functionalities, a contract to implement certain methods. Abstract classes are used to model abstractions. For example, the class Figure is not a concrete figure like square or rectangle, but the concept or the abstraction of "figure". Abstract classes defines data + actions (or normal methods) + abstract actions (or empty methods). Abstract classes are designed to be inherited (or extended). Concrete classes like Circle and Rectangle represent real entities, not abstractions. Concrete classes define data fields + concrete functionality (methods). They can implement interfaces and inherit abstract and other classes.
• #7 In this example we demonstrate abstract classes and concrete classes. This is an example of abstract class, which models an abstraction "Figure". It defines two data fields: x and y. It defines also an abstract action (or method) for calculating the area of the figure. This method is empty (or abstract), because it is specific to the concrete figure, like "circle" or "rectangle". In the child (or descendent) classes this abstract action will become concrete, it will hold the code to calculate a circle area or rectangle area or other, depending on the concrete figure. This abstract class models the generic abstraction "Figure" and child classes will determine the type of the figure. This is the definition of the base abstract class. Don't focus on the programming language, we talk about concepts now. This is the definition of an abstract method, which returns an integer value. Again, don't care about the programming language. Different programming languages have different syntax, but the concepts remain the same. This is an example of child class "Circle" – a class which inherits from the abstract class "Figure". It inherits the fields "x" and "y" from "Figure" and appends an additional field "radius". This way the child class has 3 fields: two inherited from the parent class and one defined additionally. The class Circle defines a concrete implementation of the abstract method "calcArea", which calculates the circle area using the well-known formula from the school-level math. This is an example definition of another child class "Rectangle", which inherits from the same base class Figure. The "Rectangle" class defines two additional fields: width and height. It provides different concrete implementation of the "calcArea" abstract method, which calculates the rectangle area. This is a very good example of abstract and concrete classes. Abstract classes model common (or generic) data and functionality, and concrete classes model concrete entities and concrete implementations of the abstract actions from the parent class. The programming languages doesn’t matter. This configuration of parent class + child classes is an important concept from the object-oriented programming, which we shall learn in detail later in SoftUni. This example illustrates two main object-oriented programming concepts: inheritance and polymorphism. These are complex concepts and here I just want to mention them. We shall learn them later with a lot of practice.
• #8 We can play with the example that I have prepared to illustrate abstract classes and inheritance: Abstract class Figure + two child classes: Circle and Rectangle. Let's see this in repl.it. I will open the first code link. And it takes some time to load… Now I run it and the result is shown on the right. We have two objects: a rectangle and a circle, which have certain location and size, and their area is also calculated and printed. This is the base class "Figure". This is the child class "Rectangle", and the child class "Circle". And this is how we use these classes. I will not spend more time to explain this sample code in detail, because I just want to briefly introduce the idea of inheritance in the object-oriented programming, not to teach you writing object-oriented code in JavaScript, C# or other language. For those of you, who are more curious, you can look at the sample code in detail and play with it. This is the same code in C#. And it produces similar results.
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