Imagine that you want to draw and calculate the areas of four different rectangles. You will end up with four rectangles drawn, with their different widths, heights, and calculated areas. It would be great to have a blueprint to simplify the process of drawing each rectangle with their different widths and heights.

In object-oriented programming, a class is a blueprint or a template definition from which the objects are created. Classes are models that define the state and behavior of an object. After defining a class that defines the state and behavior of a rectangle, we can use it to generate objects that represent the state and behavior of each real-world rectangle.

The following image shows four rectangle instances drawn, with their widths and heights specified: Rectangle #1, Rectangle #2, Rectangle #3, and Rectangle #4. We can use a rectangle class as a blueprint to generate the four different rectangle instances. It is very important to understand the difference between a class and the objects or instances generated through its usage. Object-oriented programming allows us to discover the blueprint we used to generate a specific object. Thus, we are able to infer that each object is an instance of the rectangle class.

We recognized four completely different real-world objects from the application's requirements. We need classes to create the objects, and therefore, we require the following four classes:

We already know the information required for each of the shapes. Now, it is time to design the classes to include the necessary attributes that provide the required data to each instance. In other words, we have to make sure that each class has the necessary variables that encapsulate all the data required by the objects to perform all the tasks.

Let's start with the Square class. It is necessary to know the length of side for each instance of this class, that is, for each square object. Thus, we need an encapsulated variable that allows each instance of this class to specify the value of the length of side.

The Square class defines a floating point attribute named LengthOfSide whose initial value is equal to 0 for any new instance of the class. After you create an instance of the Square class, it is possible to change the value of the LengthOfSide attribute.

For example, imagine that you create two instances of the Square class. One of the instances is named square1, and the other is square2. The instance names allow you to access the encapsulated data for each object, and therefore, you can use them to change the values of the exposed attributes.

Imagine that our object-oriented programming language uses a dot (.) to allow us to access the attributes of the instances. So, square1.LengthOfSide provides access to the length of side for the Square instance named square1, and square2.LengthOfSide does the same for the Square instance named square2.

You can assign the value 10 to square1.LengthOfSide and 20 to square2.LengthOfSide. This way, each Square instance is going to have a different value for the LengthOfSide attribute.

Now, let's move to the Rectangle class. We can define two floating-point attributes for this class: Width and Height. Their initial values are also going to be 0. Then, you can create two instances of the Rectangle class: rectangle1 and rectangle2.

You can assign the value 10 to rectangle1.Width and 20 to rectangle1.Height. This way, rectangle1 represents a 10 x 20 rectangle. You can assign the value 30 to rectangle2.Width and 50 to rectangle2.Height to make the second Rectangle instance, which represents a 30 x 50 rectangle.

The following table summarizes the floating-point attributes defined for each class:

The following image shows a UML (Unified Modeling Language) diagram with the four classes and their attributes:

So far, we have designed four classes and identified the necessary attributes for each of them. Now, it is time to add the necessary pieces of code that work with the previously defined attributes to perform all the tasks. In other words, we have to make sure that each class has the necessary encapsulated functions that process the attribute values specified in the objects to perform all the tasks.

Let's start with the Square class. The application's requirements specified that we have to calculate the areas and perimeters of squares. Thus, we need pieces of code that allow each instance of this class to use the LengthOfSide value to calculate the area and the perimeter.

The Square class defines the following two parameterless methods. Notice that we declare the code for both methods in the definition of the Square class:

Imagine that, our object-oriented programming language uses a dot (.) to allow us to execute methods of the instances. Remember that we had two instances of the Square class: square1 with LengthOfSide equal to 10 and square2 with LengthOfSide equal to 20. If we call square1.CalculateArea, it would return the result of 102, which is 100. On the other hand, if we call square2.CalculateArea, it would return the result of 202, which is 400. Each instance has a diverse value for the LengthOfSide attribute, and therefore, the results of executing the CalculateArea method are different.

If we call square1.CalculatePerimeter, it would return the result of 4 * 10, which is 40. On the other hand, if we call square2.CalculatePerimeter, it would return the result of 4 * 20, which is 80.

Now, let's move to the Rectangle class. We need exactly two methods with the same names specified for the Square class. However, they have to calculate the results in a different way.

Remember that, we had two instances of the Rectangle class: rectangle1 representing a 10 x 20 rectangle and rectangle2 representing a 30 x 50 rectangle. If we call rectangle1.CalculateArea, it would return the result of 10 * 20, which is 200. On the other hand, if we call rectangle2.CalculateArea, it would return the result of 30 * 50, which is 1500. Each instance has a diverse value for both the Width and Height attributes, and therefore, the results of executing the CalculateArea method are different.

If we call rectangle1.CalculatePerimeter, it would return the result of 2 * 10 + 2 * 20, which is 60. On the other hand, if we call rectangle2. CalculatePerimeter, it would return the result of 2 * 30 + 2 * 50, which is 160.

The Circle class also needs two methods with the same names. The two methods are explained as follows:

Finally, the Ellipse class defines two methods with the same names but with different code and a specific problem with the perimeter. The following are the two methods:

The following figure shows an updated version of the UML diagram with the four classes, their attributes, and their methods:

So far, our object-oriented solution includes four classes with their attributes and methods. However, if we take another look at these four classes, we would notice that all of them have the same two methods: CalculateArea and CalculatePerimeter. The code for the methods in each class is different, because each shape uses a different formula to calculate either the area or the perimeter. However, the declarations or the contracts for the methods are the same. Both methods have the same name, are always parameterless, and both return a floating-point value.

When we talked about the four classes, we said we were talking about four different geometrical shapes or simply, shapes. Thus, we can generalize the required behavior for the four shapes. The four shapes must declare the CalculateArea and CalculatePerimeter methods with the previously explained declarations. We can create a contract to make sure that the four classes provide the required behavior.

The contract will be a class named Shape, and it will generalize the requirements for the geometrical shapes in our application. The Shape class declares two parameterless methods that return a floating-point value: CalculateArea and CalculatePerimeter. Then, we can declare the four classes as subclasses of the Shape class that inherit these definitions, but provide the specific code for each of these methods.

Object-oriented programming allows us to discover whether an object is an instance of a specific superclass. After we changed the organization of the four classes and they became subclasses of the Shape class, any instance of Square, Rectangle, Circle, or Ellipse is also an instance of the Shape class. In fact, it isn't difficult to explain the abstraction because we are telling the truth about the object-oriented model that represents the real world. It makes sense to say that a rectangle is indeed a shape, and therefore, an instance of a Rectangle class is a Shape class. An instance of a Rectangle class is both a Shape class (the superclass of the Rectangle class) and a Rectangle class (the class that we used to create the object).

When we were implementing the Ellipse class, we discovered a specific problem for this shape; there are many formulas that provide approximations of the perimeter value. Thus, it makes sense to add additional methods that calculate the perimeter using other formulas.

We can define the following two additional parameterless methods, that is, two methods without any parameter. These methods return a floating-point value to the Ellipse class to solve the specific problem of the ellipse shape. The following are the two methods:

This way, the Ellipse class implements the methods specified in the Shape superclass. The Ellipse class also adds two specific methods that aren't included in any of the other subclasses of Shape.

The following diagram shows an updated version of the UML diagram with the abstract class, its four subclasses, their attributes, and their methods:

Python, JavaScript, and C# support object-oriented programming, also known as OOP. However, each programming language takes a different approach. Both Python and C# support classes and inheritance. Therefore, you can use the different syntax provided by each of these programming languages to declare the Shape class and its four subclasses. Then, you can create instances of each of the subclasses and call the different methods.

On the other hand, JavaScript uses an object-oriented model that doesn't use classes. This object-oriented model is known as prototype-based programming. However, don't worry. Everything you have learned so far in your simple object-oriented design journey can be coded in JavaScript. Instead of using inheritance to achieve behavior reuse, we can expand upon existing objects. Thus, we can say that objects serve as prototypes in JavaScript. Instead of focusing on classes, we work with instances and decorate them to emulate inheritance in class-based languages.

There are other important differences between Python, JavaScript, and C#. They have a great impact on the way you can code object-oriented designs. However, you will learn different ways throughout this book to make it possible to code the same object-oriented design in the three programming languages.

In this chapter, you learned how to recognize real-world elements and translate them into the different components of the object-oriented paradigm: classes, attributes, methods, and instances. You understood the differences between classes (blueprints or templates) and the objects (instances). We designed a few classes with attributes and methods that represented blueprints for real-life objects. Then, we improved the initial design by taking advantage of the power of abstraction, and we specialized in the Ellipse class.

Now that you have learned some of the basics of the object-oriented paradigm, you are ready to start creating classes and instances in Python, JavaScript, and C# in the next chapter.