The four basics of OOP

 

The four basics of object-oriented programming

Object-oriented programming has four basic concepts: encapsulation, abstraction, inheritance, and polymorphism. Even if these concepts seem incredibly complex, understanding the general framework of how they work will help you understand the basics of an OOP computer program. Below, we outline these four basic principles and what they entail:

1. Encapsulation

2. Abstraction

3. Inheritance

4. Polymorphism

1. Encapsulation

The word, “encapsulate,” means to enclose something. Just like a pill "encapsulates" or contains the medication inside of its coating, the principle of encapsulation works in a similar way in OOP: by forming a protective barrier around the information contained within a class from the rest of the code.

In OOP, we encapsulate by binding the data and functions which operate on that data into a single unit, the class. By doing so, we can hide private details of a class from the outside world and only expose functionality that is important for interfacing with it. When a class does not allow calling code access to its private data directly, we say that it is well encapsulated.

Example: Elaborating on the person class example from earlier, we might have private data in the class, such as "socialSecurityNumber," that should not be exposed to other objects in the program. By encapsulating this data member as a private variable in the class, outside code would not have direct access to it, and it would remain safe within that person’s object.

If a method is written in the person class to perform, say, a bank transaction called "bankTransaction()," that function could then access the "socialSecurityNumber" variable as necessary. The person’s private data would be well encapsulated in such a class.

2. Abstraction

Often, it’s easier to reason and design a program when you can separate the interface of a class from its implementation, and focus on the interface. This is akin to treating a system as a “black box,” where it’s not important to understand the gory inner workings in order to reap the benefits of using it.

This process is called “abstraction” in OOP, because we are abstracting away the gory implementation details of a class and only presenting a clean and easy-to-use interface via the class’ member functions. Carefully used, abstraction helps isolate the impact of changes made to the code, so that if something goes wrong, the change will only affect the implementation details of a class and not the outside code.

Example: Think of a stereo system as an object with a complex logic board on the inside. It has buttons on the outside to allow for interaction with the object. When you press any of the buttons, you're not thinking about what happens on the inside because you can't see it. Even though you can't see the logic board completing these functions as a result of pressing a button, it's still performing them., albeit hidden to you.

This is the concept of abstraction, which is incredibly useful in all areas of engineering and also applied to great effect in object-oriented programming.

Example: In OOP, we might have a class defined to represent the human body. One might define some functions as part of its publicly facing interface such as “walk()” or “eatFood().” Calling code could call these functions and remain completely oblivious to the complex inner workings of the human body and its necessary functions to perform the act of walking or eating. These details are completely hidden in the implementation of the walk() and eatFood() body functions and are, therefore, us abstracted away from the end user. In these cases, it’s not important for calling code to understand how the brain coordinates walking or how the stomach manages digesting the food, but rather simply that a human walked or ate.

3. Inheritance

Object-oriented languages that support classes almost always support the notion of “inheritance.” Classes can be organized into hierarchies, where a class might have one or more parent or child classes. If a class has a parent class, we say it is derived or inherited from the parent class and it represents an “IS-A” type relationship. That is to say, the child class “IS-A” type of the parent class.

Therefore, if a class inherits from another class, it automatically obtains a lot of the same functionality and properties from that class and can be extended to contain separate code and data. A nice feature of inheritance is that it often leads to good code reuse since a parent class’ functions don’t need to be re-defined in any of its child classes.

Consider two classes: one being the superclass—or parent—and the other being the subclass—or child. The child class will inherit the properties of the parent class, possibly modifying or extending its behavior. Programmers applying the technique of inheritance arrange these classes into what is called an “IS-A” type of relationship.

Example: For instance, in the animal world, an insect could be represented by an Insect superclass. All insects share similar properties, such as having six legs and an exoskeleton. Subclasses might be defined for grasshoppers and ants. Because they inherit or are derived from the Insect class, they automatically share all insect properties.

4. Polymorphism

In OOP, polymorphism allows for the uniform treatment of classes in a hierarchy. Therefore, calling code only needs to be written to handle objects from the root of the hierarchy, and any object instantiated by any child class in the hierarchy will be handled in the same way.

Because derived objects share the same interface as their parents, the calling code can call any function in that class’ interface. At run-time, the appropriate function will be called depending on the type of object passed leading to possibly different behaviors.