In Ruby, the meaning of
private is different from other languages like Java. (They don't hide methods from inheriting classes.)
Private methods can only be called with implicit receiver. As soon as you specify a receiver, let it only be
self, your call will be rejected.
class A def implicit private_method end def explicit self.private_method end private def private_method "Private called" end end A.new.implicit => "Private called" A.new.explicit => NoMethodError: private method `private_method' called for #<A:0x101538a20>
Protected methods can be called implicitly or explicitly, as long as the receiver "
is_a? self.class". An object may call a protected method on another instance of the same class, as well as on an instance of a subclass. Invoking a protected method from a class method, even of the same class, is not possible. There are no protected class methods.
A use case is comparing two objects on private attributes, which would not be possible with a private attribute.
class B def <=>(other) foo <=> other.foo end protected attr_accessor :foo end