State and behaviour

Data and methods

Lets have another look our class definition for Person:

class Person
  def initialize(name)
    @name = name

  def name

  def password=(password)
    @password = password

Do you notice something?

Our class demonstrates an important thing about objects:

There’s a way to ask a person for their name, but no way to set a new name. On the other hand there’s a way to set a new password to the person, but no way to ask for it.

If you think about it, that makes sense, doesn’t it?

If you join our Ruby beginners study group for the first time, and we ask you for your name, you’ll happily tell it. But if we ask for your Gmail password, you will probably just laugh at us, or stare at us, or show some other error message. ;) In any case, you won’t tell us your email password, because that’s private information.

The same is also true for objects.

Every object has its own object scope that might hold a bunch of instance variables. These are private to the object. Our person object knows their password, once it has been given to them. But from then on, they won’t tell anyone the password, because there’s no method for that. On the other hand, there’s a method name, which is an attribute reader, so we can ask our person object for their name. But there’s no way for others to give a new name to the person, because there’s no method for that, no attribute writer name=.

This concept is called encapsulation, and it is one of the main motivations behind the whole paradigm of object-oriented programing:

We can say that an object encapsulates state (data, knowledge), which is private to the object, and exposes behaviour by the way of having publicly accessible methods.

Objects have state (instance variables) and behaviour (methods).

So, we have now created our first little class, and it’s one that you could actually see in real applications.