Public method vs Private method in Ruby

In the world of Ruby, all the methods are either public method or private/protected method. Protected methods act pretty much the same with private methods other than the fact that it can be called in subclasses of the class where it is defined.

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Here is a table showing the differences between public methods and private methods:

Public method vs Private method

Let’s see an example of class!

class Dog
  attr_accessor :name, :breed, :gender
  @@name = ["Littles", "Ada", "Katie", "Rex", "Rukie"]
  @@breed = ["chihuahua", "pug", "bulldog", "beagle", "pomeranian"]
  @@gender = ["male", "female"]
  def favorite
    puts "I am everyone's favorite dog!"
    my_name
    my_breed
    my_gender
  end
  private
 
  def my_name
    @name = @@name.sample
    puts "My name is #{@name}!"
  end
  
  def my_breed
    @breed = @@breed.sample
    puts "My breed is #{@breed}."
  end
  def my_gender
    @gender = @@gender.sample
    if @gender == "male"
       puts "I am a boy!"
    else
       puts "I am a girl!"
    end
  end
end

• This Dog class has one public method: favorite
• This Dog class has three private methods: my_name, my_breed, my_gender
• These three private methods are called in the context of class, called in the method favorite which is located in the scope of class

new_dog = Dog.new
new_dog.favorite
=> I am everyone's favorite dog!
   My name is Littles!
   My breed is bulldog.
   I am a girl!

• new_dog is a new instance of Dog class and favorite method was able to be called on new_dog since favorite is a public method
• new_dog is an explicit receiver of the favorite method

new_dog = Dog.new
new_dog.my_breed
=> in `<main>': private method `my_breed' called for 
   #<Dog:0x00007fe67b853b58> (NoMethodError)

• my_breed method was not able to be called on the instance new_dog since it is a private method. It could not be called on explicit receiver, also outside the scope of class

When can private methods be useful?

• When to limit the scope of method within a class: excluding to call the method from outside class
• When the method should be only able to function with help of other methods in class
• When to increase the flexibility of codes in class

How can we make methods to private methods?

1. By placing the methods in the ‘private’ area such as an example of the Dog class above
2. By declaring the methods with ‘private’ command

class Dog
  attr_accessor :name, :breed, :gender
  @@name = ["Littles", "Ada", "Katie", "Rex", "Rukie"]
  @@breed = ["chihuahua", "pug", "bulldog", "beagle", "pomeranian"]
  @@gender = ["male", "female"]
  def favorite
    puts "I am everyone's favorite dog!"
    my_name
    my_breed
    my_gender
  end
  def my_name
    @name = @@name.sample
    puts "My name is #{@name}!"
  end
  
  def my_breed
    @breed = @@breed.sample
    puts "My breed is #{@breed}."
  end
  def my_gender
    @gender = @@gender.sample
    if @gender == "male"
       puts "I am a boy!"
    else
       puts "I am a girl!"
    end
  end
  private :my_name, :my_breed, :my_gender
end

3. Exception — initialize method

: although it is not specified, initialize method of class is always private method

new_dog = Dog.new
new_dog.initialize
=> in `<main>': private method `initialize' called for 
   #<Dog:0x00007f869284f9e8> (NoMethodError)