Class vs Instance

car.rb

class Car
  class << self
    def advertise(model)
      puts "BUY IT NOW! #{model}"
    end

    def manufacture(make)
      Car.new(make)
    end
  end

  def initialize(make)
    @make = make
  end

  def drive(miles)
    puts "Driving #{miles} miles"
  end
end

civic = Car.new('Civic')
civic.drive(30)

Car.manufacture('Ford')
Car.advertise('Tesla Model S')

expense.rb

class Expense
  class << self
    @@expenses = []

    def itemize
      @@expenses.sort.each do |record|
        puts "* #{record.name}, #{record.amount}"
      end
    end

    def submit(minimum_amount = 70)
      @@expenses.each do |record|
        record.submit if record.amount >= minimum_amount
      end
    end
  end

  attr_accessor :name, :amount

  def initialize(name, amount)
    @name = name
    @amount = amount

    @@expenses << self
  end

  def <=>(other)
    other.amount <=> @amount
  end

  def submit
    puts "Submitting #{name} for #{amount}"
  end
end

gas = Expense.new('gas receipt', 70)
flight = Expense.new('southwest', 150)
hotel = Expense.new('a hilton', 100)

# Expense.itemize
Expense.submit(70)

person.rb

class Person
  @@room = []

  class << self
    def room
      @@room
    end
  end

  attr_accessor :name, :age

  def initialize(name, age)
    @name = name
    @age  = age

    @@room << self
  end

  def greet(name)
    # Hey, John, I'm Bob.
    puts "Hey, #{name}! I'm #{@name} and I'm #{@age}."
  end

  # TODO: we have to have their birthday,
  # not just their age
  def days_until_birthday
    42
  end

  def <=>(other)
    other.age <=> @age
  end
end

bob = Person.new('Bob', 82)
john = Person.new('John', 24)

puts "Bob's age: " + bob.age.to_s
puts bob.greet('John')

puts "There are #{Person.room.length} people in the room."