Understanding classes and instances

In the previous chapter, you learned some of the basics of the object-oriented paradigm, including classes and objects, also known as instances. Now, when you pe deep into the programming languages, the class is always going to be the type and the blueprint. The object is the working instance of the class, and one or more variables can hold a reference to an instance.

Let's move to the world of our best friends, the dogs. If we want to model an object-oriented application that has to work with dogs and about a dozen dog breeds, we will definitely have a Dog abstract class. Each dog breed required in our application will be a subclass of the Dog superclass. For example, let's assume that we have the following subclasses of Dog:

• TibetanSpaniel: This is a blueprint for the dogs that belong to the Tibetan Spaniel breed
• SmoothFoxTerrier: This is a blueprint for the dogs that belong to the Smooth Fox Terrier breed

So, each dog breed will become a subclass of Dog and a type in the programming language. Each dog breed is a blueprint that we will be able to use to create instances. Brian and Merlin are two dogs. Brian belongs to the Tibetan Spaniel breed, and Merlin belongs to the Smooth Fox Terrier breed. In our application, Brian will be an instance of the TibetanSpaniel subclass, and Merlin will be an instance of the SmoothFoxTerrier subclass.

As both Brian and Merlin are dogs, they will share many attributes. Some of these attributes will be initialized by the class, because the dog breed they belong to determines some features, for example, the area of origin, the average size, and the watchdog ability. However, other attributes will be specific to the instance, such as the name, weight, age, and hair color.