Class Methods

Warmup

> sam = User.new("Sam")
# => #<User:0x007f94e3ab6218 @name="Sam">
> sam.say_hello("Jay")
# => "Hello, Jay!"

• What do we call the method #say_hello on? What do we call the method ::new on?
• In the method list in the Ruby docs page for Array, what is the difference between methods with a # prefix and those with a :: prefix?

Classes

• Factories for instances of a thing.
• Hold blueprints.
  • Instance Variables (State)
  • Methods (Behavior)

Classes (continued)

• Also objects themselves.
  • Class Variables (@@var_name)
  • Class Methods (def self.method_name)

Syntax

class User
  def initialize(name)
    @name = name
  end

  def self.describe_yourself
    "I'm the user class!"
  end
end

Practical Use

• Track information about instances
• Create multiple instances at once

Example

# user.rb
class User
  def initialize(name)
    @name = name
  end

  def self.create_multiple(users)
    users.map do |user|
      User.new(user[:name])
    end
  end
end

# runner.rb
users = [
    {name: "Sal"}
    {name: "Brian"}
    {name: "Megan"}
  ]

User.create_multiple(users)

Practice

• Create a House class that has a method that will return a collection of Houses when passed an array of hashes.

Exploration

• What happens if you create a class method and an instance method with the same name?
• What happens if you call an instance method from within a class method?
• What about a class method from within another class method?
• How would you explain the difference between class and instance methods to someone else?
• Can you think of a metaphor for classes that includes a description of class and instance methods?

Wrap Up

• What is the syntactic difference between class methods & instance methods?
• Why might you use a class method instead of an instance method?