revisiting_classes.md

Revisiting Classes

We are going to recap the following three ideas within classes:

• Constructor (initialize)
• Instance Methods
• Attributes (stored in instance variables)

Then we're going to add one more concept to our classes: Class Methods & Self

Constructor (initialize)

The initialize method within a class is special, when the new method is called on a class a new _instance _is created and the initialize method is promptly called on it. This method is used to setup the default state of an object.

Typically within a initialize method we set any given attributes and make any data manipulations that we need to...

class Product
  def initialize(name, price)
    @name  = name.capitalize
    @price = price
    activate!
  end

  def activate!
    @status = :for_sale
  end
end

Instance Methods

Instance methods are defined with def a_snake_case_name. Instance methods are only available on instances of a class.

class Order

  def initialize(subtotal, quantity)
    @subtotal = subtotal
    @quantity = quantity
  end

  def total
    @subtotal * @quantity
  end

  def to_money
    "$#{sprintf("%0.02f", total / 100)}"
  end

end

order = Order.new(1000, 2)
order.to_money

Attributes

Attributes are really just instance variables. But Ruby provides a couple of methods to help us get and set these instance variables. attr_accessor defines methods to get and set an instance variable as the same name as the symbol we give it.

class Coordinate
  attr_accessor :x, :y

  # attr_accessor does the exact same as the four methods below
  # ----------------------------------------------------

  def x
    @x
  end

  def x=(new_x)
    @x = new_x
  end

  def y
    @y
  end

  def y=(new_y)
    @y
  end

end

Class Methods & self

Class methods are defined like instance methods, but they start with def self.make_row and are called internally with the self identifier.

Class methods are called directly by the class and not by an instance of the class.

class Pawn
  def initialize(position)
    @position = position
  end

  # This is the class method, it starts with self.
  # It is only called on the class directly Pawn.make_row
  def self.make_row(side)
    if side == "white"
      num = 2
    else
      num = 7
    end

    ("a".."h").collect do |letter|
      new("#{letter}#{num}")
    end
  end
end

pawns = Pawn.make_row("black")