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Hierarchy is everything

I strongly believe that the smartest people share only one trait: they understand that everything is hierarchy.

Our world is full of hierarchies. They're either one-to-one, one-to-many or many-to-many. The most important skill to have is to traverse this tree-structure in your mind.

Let's take a school for example. You're one of many students in a class. You have one teacher.

Teacher
Student Student Student

But from the teacher's perspective, he's just one of many teachers. He has one director to report to, though.

Director
Teacher Teacher
Student Student Student Student Student Student

Let's say you were to design an application. By looking at this diagram it's clear that you need three classes: Director, Teacher and Student.

But what do these people have in common?

Director Dr. A
Teacher Mr. B Mr. C, PhD
Student Anakin Luke Leia Han Padme Student

We can deduct that everyone has a name. And some have a gender. So there are some shared properties. In our schema it looks like only teachers have a gender. And if you look more closely Teachers and Directors can have a degree.

Our class schema therefore looks like this:

  • Person
    • .name (String)
    • .gender (Male/Female/null)
    • Director
      • .degree (Dr, PhD, null)
    • Teacher
      • .degree (Dr, PhD, null)
    • Student

Can you see the duplicated data? This means that both Teacher and Director can share a degree. They have something in common, and therefore share a common parent class. Because they share this property we can move them one down in the hierarchy:

  • Person
    • .name (String)
    • .gender (Male/Female/null)
    • Employee
      • .degree (Dr, PhD, null)
      • Director
      • Teacher
    • Student

What if a student should also be able to have a degree? Well, you could throw everything aboard:

  • Person
    • .name (String)
    • .gender (Male/Female/null)
    • .degree (Dr, PhD, null)
    • .role (Director, Teacher, Student)

But experience might tell you that an Employee has a few other properties than Students. For example salary:

  • Person
    • .name (String)
    • .gender (Male/Female/null)
    • Employee
      • .degree (Dr, PhD, null)
      • .salary (Number)
      • Director
      • Teacher
    • Student

Adding salary to a student and setting it to 0 or null is just ugly.

Learn to view everything in the world as a hierarchy and you will become a great programmer.