public, private, protected

explained.txt

bob = Person.new('Bob', 'Smith', 'bob@example.com', '444-333-4444')
john = Person.new('John', 'Smith', 'john@example.com', '555-444-3333')

# full_nane is a public method 
bob.full_name
# => "Smith, Bob"
john.full_name
# => "Smith, John"

# email is a protected method.
bob.email
# => protected method `email' called for #<Person:0x0000559fb5b50e00>

# ssn is a private method.
bob.ssn
# => private method `ssn' called for #<Person:0x000055a1f6f21550>

bob.talk_to(john, 'Hey friend')
# => mailto:john@example.com
#    Hey friend

# in the #talk_to method we have access to friend.email because email is a protected method and we are calling it
# from within the class it was created.  We would also have access to this from any class that subclasses the 
# Person class.  Since both Bob and John are of the same class then they can share email with eachother.  
# We would not have access to #ssn though.  #ssn is private.  John and Bob may be of the same class, but they 
# cannot share their ssn with eachother.  Each of their #ssn methods are only accessible to their own instances.

person.rb

class DummyEmail
  def self.send_mail(email, message)
    puts "mailto:#{email}\n#{message}"
  end
end

class Person 
  def initialize(first_name, last_name, email, ssn, mailer = DummyEmail)
    @first_name = first_name
    @last_name = last_name
    @email = email
    @ssn = ssn
    @mailer = mailer
  end

  def full_name
    "#{@last_name}, #{@first_name}"
  end

  def talk_to(friend, message)
    @mailer.send_mail(friend.email, message)
  end

  protected 
  
  attr_reader :email

  private 

  attr_reader :ssn
end