private and protected in Ruby

private_protected.rb

class A
  def public_message
    protected_message
  end

  def self_protected_message
    self.protected_message
  end

  def self_private_message
    self.private_message
  end

  protected

  def protected_message
    puts "Protected message"
  end

  private

  def private_message
    puts "Private message"
  end
end

class B < A
  def a_protected_message
    protected_message
  end

  def a_private_message
    private_message
  end

  def b_instance_protected_message
    B.new.protected_message
  end

  def b_instance_private_message
    B.new.private_message
  end

  def a_instance_protected_message
    A.new.protected_message
  end

  def a_instance_private_message
    A.new.private_message
  end
end

class C
  def a_instance_protected_message
    A.new.protected_message
  end

  def a_instance_private_message
    A.new.private_message
  end
end

A.new.public_message
A.new.self_protected_message
A.new.self_private_message
# A.new.protected_message # protected method `protected_message' called for #<A:0x00007f9f21935a18> (NoMethodError)
# A.new.private_message # private method `private_message' called for #<A:0x00007fd449061718> (NoMethodError)

B.new.a_protected_message
B.new.a_private_message
B.new.b_instance_protected_message
# B.new.b_instance_private_message # private method `private_message' called for #<B:0x00007f84f003d6e0> (NoMethodError)
B.new.a_instance_protected_message
# B.new.a_instance_private_message # private method `private_message' called for #<A:0x00007f9cc2935240> (NoMethodError)

# C.new.a_instance_protected_message # protected method `protected_message' called for #<A:0x00007fa6d006d208> (NoMethodError)
# C.new.a_instance_private_message # private method `private_message' called for #<A:0x00007fce2d8a92f8> (NoMethodError)