Code snippets showing how to define public and private instance and class methods in Ruby.

1_public_and_private_instance_methods.rb

class Car
  attr_reader :mileage

  def initialize
    @mileage = 0
  end

  def drive
    update_mileage
  end

  private

  def update_mileage
    @mileage += 1
  end
end

# => car = Car.new
# #<Car:0x005..>
#
# => car.drive
# 1
#
# => car.update_mileage
# private method `update_mileage' called for #<Car:0x005..>
#
# => car.mileage
# 1

2_public_class_method_v1.rb

class Toyota < Car
  def self.name
    "Toyota #{superclass}"
  end
end

# => Toyota.name
# "Toyota Car"

3_public_class_method_v2.rb

class Toyota < Car
  class << self
    def name 
      "Toyota #{superclass}"
    end
  end
end

# => Toyota.name
# "Toyota Car"

4_private_class_method_v1.rb

class Toyota < Car
  def self.name
    self.i_am
  end

  private

  def self.i_am
    "Toyota #{superclass}"
  end
end

# => Toyota.name
# "Toyota Car"

5_private_class_method_v1_output.rb

# => Toyota.i_am # expecting an error
# "Toyota Car"

6_private_class_method_v2.rb

class Toyota < Car
  class << self
    def name 
      i_am
    end
 
    private
 
    def i_am
      "Toyota #{superclass}"
    end
  end
end

# => Toyota.name
# "Toyota Car"
#
# => Toyota.i_am # expecting an error
# private method `i_am' called for Toyota:Class

7_private_class_method_v3a.rb

class Toyota < Car
  def self.name
    self.i_am
  end
 
  private_class_method def self.i_am
    "Toyota #{superclass}"
  end
end

# => Toyota.name
# "Toyota Car"
#
# => Toyota.i_am # expecting an error
# private method `i_am' called for Toyota:Class

8_private_class_method_v3b.rb

class Toyota < Car
  def self.name
    self.i_am
  end

  def self.i_am
    "Toyota #{superclass}"
  end

  def self.do_something
    # I want to be private too
    puts "I'm private too! I do something."
  end
  
  # I added do_something to show that
  # private_class_method takes an array
  # of symbols corresponding to method
  # names.
  private_class_method :i_am, :do_something
end

# => Toyota.name
# "Toyota Car"
#
# => Toyota.i_am # expecting an error
# private method `i_am' called for Toyota:Class
#
# => Toyota.do_something # expecting an error
# private method `do_something' called for Toyota:Class