public_and_protected.rb

class Base

  def public_method
    puts 'public method called'
  end

  # This works as we are calling the private method
  # from within the object
  def call_private_method
    private_method
  end

  private

  def private_method
    puts 'private method called'
  end

  protected

  def protected_method
    puts 'protected method called'
  end
end

class Child < Base

  # This will fail as we cannot call a private
  # method in a child class
  def call_private_method
    private_method
  end

  # This will work however as a protected method
  # we can call in the child class
  def call_protected_method
    protected_method
  end
end

base, child = Base.new, Child.new

# These will succeed
base.public_method
child.public_method
base.call_private_method

# This will fail as we are calling it from
# outside the class
base.private_method

# This will fail as we are calling it from
# outside the class
base.protected_method

# Will fail, see notes in class method
child.call_private_method

# Will succeed
child.call_protected_method