What are the differences between “private”, “public”, and “protected methods”?

Question

I\'m learning Ruby, and have come up to a point where I am confused.

The book I am using is talking about private, public, and protec

Answer 1

Studying the information I've taken from here, I extended explanations through errors, and for my opinion, helps to understand why and how to use protected and not private.

1) Protected:

The line num 12 crash because the parameter received is from another class, the error message is clear:

v.rb:12:in `==': undefined method `sku' for "Object of another class ==> crash":String (NoMethodError)

2) Private:

If remove self from line 8 and 12, and I change protected for private, crash because in line 12, other doesn't know what sku is:

v.rb:12:in `==': private method `sku' called for # (NoMethodError)

The program:

class Product
  attr_accessor :name, :quantity

  def initialize(name)
    @name = name
    @quantity = 1

    puts "The SKU is #{self.sku}"
  end

  def == (other)
    self.sku == other.sku
  end

  protected
    def sku
      name.crypt("yo")
    end
end

milk1 = Product.new("Milk")
milk2 = Product.new("Milk")
bread = Product.new("Bread")

puts milk1 == bread

puts milk1 == milk2

puts milk1 == "Object of another class ==> crash"