by Alexander Petkov
Have you noticed how the same cliche questions always get asked at job interviews — over and over again?
I’m sure you know what I mean.
For example:
Where do you see yourself in five years?
or, even worse:
- What do you consider to be your greatest weakness?. I consider answering this question a great weakness! Anyway, not my point.
As trivial as questions like these may be, they are important because they give clues about you. Your current state of mind, your attitude, your perspective.
When answering, you should be careful, as you may reveal something you later regret.
Today I want to talk about a similar type of question in the programming world:
What are the main principles of Object-Oriented Programming?
- I’ve been on both sides of this question. It’s one of those topics that gets asked so often that you can’t allow yourself to not know.
Junior and entry-level developers usually have to answer it. Because it’s an easy way for the interviewer to tell three things:
Did the candidate prepare for this interview?
Bonus points if you hear an answer immediately — it shows a serious approach.
Is the candidate past the tutorial phase?
- Understanding the principles of Object-Oriented Programming (OOP) shows you’ve gone beyond copy and pasting from tutorials — you already see things from a higher perspective.
Is the candidate’s understanding deep or shallow?
The level of competence on this question often equals the level of competence on most other subjects. Trust me.
The four principles of object-oriented programming are encapsulation, abstraction, inheritance, and polymorphism.
These words may sound scary for a junior developer. And the complex, excessively long explanations in Wikipedia sometimes double the confusion.
That’s why I want to give a simple, short, and clear explanation for each of these concepts. It may sound like something you explain to a child, but I would actually love to hear these answers when I conduct an interview.
- Encapsulation
Say we have a program. It has a few logically different objects which communicate with each other — according to the rules defined in the program.
Encapsulation is achieved when each object keeps its state private, inside a class. Other objects don’t have direct access to this state. Instead, they can only call a list of public functions — called methods.
So, the object manages its own state via methods — and no other class can touch it unless explicitly allowed. If you want to communicate with the object, you should use the methods provided. But (by default), you can’t change the state.
Let’s say we’re building a tiny Sims game. There are people and there is a cat. They communicate with each other. We want to apply encapsulation, so we encapsulate all “cat” logic into a Cat class. It may look like this:
