w2_d1_oop_ruby.md

Object Orientated Programming

##Opening

In most object-orientated languages, including Ruby, every object is an example or instance of a particular class.

Every object can do different things. A human can walk and talk, a bird can fly and a pen can write.

Why do we create objects?

We use objects in coding to represent objects in the real world. When 'modelling' objects, we try to define their characteristics and things that they can do.

We often create programmable objects in order to model the behaviour of real-world objects, like Users and Invoices. However, they can also be objects that are not physical, like Time or Friendships.

What is the difference betwen an object and a class

We use classes to define our objects. They are just a piece of code that wraps around more code that defines our object.

Data types

We have looked at some data types and some of the things they can do.

Assess: Can you name them?

• Integers
• Floats
• Strings
• Booleans
• Arrays
• Hashes
• Symbols

Let's remind ourselves and have a look at an example of a String.

$ irb
  
"".class
=> String

But what is a String? A string is an Object... this empty string is actually an instance of a String class.

Let's have a look at something called the classtree in Ruby:

Class Tree

It's not actually 100% complete but it's close.

##I Do: Inheritance

We can see the relationship between a String and an Object by doing:

"".class.superclass
=> Object

And what is an Object's superclass?

"".class.superclass.superclass
=> BasicObject

How can something be both a String and an Object and an example of a BasicObject? The answer is inheritance.

Inheritance is a kind-of downward-chaining relationship between two classes (the superclass and the subclass), whereby one class "inherits" from another.

All the instances of the subclass acquire the methods defined in the superclass.

Let's think of it in terms of a Human.

Draw on board

Human < Mammal < Animal

However inheritance, when trying to model real world objects, can get tricky because you could also write:

Human < Homosapien

It's up to you to decide how you define your classes in Ruby.

Classical Inheritance

We call this type of inheritance 'classical' inheritance.

##We Do: Creating basic Objects

From the classtree, you can see that every Object can do certain things in Ruby, the methods that are defined in the Object class.

Instantiation

Let's create a new Object using Ruby:

$ irb
  
object = Object.new
=> #<Object:0x007f97691f80a8>

This .new syntax creates a new version of that object. We call that instantiation.

We can check what methods every object can do by doing

object.methods
=> [:nil?, :===, :=~, :!~, :eql?, :hash, :<=>, :class, :singleton_class, :clone, :dup, :taint, :tainted?, :untaint, :untrust, :untrusted?, :trust, :freeze, :frozen?, :to_s, :inspect, :methods, :singleton_methods, :protected_methods, :private_methods, :public_methods, :instance_variables, :instance_variable_get, :instance_variable_set, :instance_variable_defined?, :remove_instance_variable, :instance_of?, :kind_of?, :is_a?, :tap, :send, :public_send, :respond_to?, :extend, :display, :method, :public_method, :singleton_method, :define_singleton_method, :object_id, :to_enum, :enum_for, :==, :equal?, :!, :!=, :instance_eval, :instance_exec, :__send__, :__id__]

We say that this object is an instance of the Object Class.

##I Do: Class is a template

A class is like a cookie cutter for an object. It defines how each new Object will be made and what properties it will have.

When you instantiate a new instance of an object, it is like cutting out a new cookie.

If you eat a cookie, you are eating one instance of a cookie. All the other cookies that you have cut out remain in tact.

Fist of 5

This is a really important concept, so please ask question if you don't understand that analogy.

##We Do: Instance vs class methods

Choose a person in the class

To chosen person:

"You are an instance of a Person class. Many of the basic things you can do, everyone else can do because they are also instances of a Person class."

"Can you please raise your hand?

Chosen person raises their hand.

"Other instances of the Person class haven't been asked to do anything because I've sent you an instance method."

"However, if I asked every Person to raise their hand... Class, please raise your hands..."

Everyone raises their hands.

"This is an example of a class method."

Actually, if we were being exact - all people would have to raise their hands.

##We Do: Defining an object using a class

Let's create a new file:

$ touch person.rb

To define a class, we use the keyword class followed by the class name ClassName (class names that contain more than one word are written using the CamelCase convention). We will close a class with an end, at the end of the file.

class Person 
  def talk(words)
    puts "I say, #{words}"
  end
end

bob = Person.new
puts bob.class

Using an instance method

We can call the .talk() method which we defined in the Person class. This is an instance method. We are calling it on the instance:

class Person 
  def talk(words)
    puts "I say, #{words}"
  end
end

bob = Person.new
# puts bob.class

bob.talk("hello")

##Closure

We're going to be playing a bit more with defining custom classes and making methods. This is a really core concept in Ruby and it's important that everyone understands this.