• Class acts as a template used to create objects.
• Class describes the behavior and state that an object can hold.
• Every object is an instance of a class.
• Instance methods live in the class.
• Instance variables live in the object.
• Instance variables are unique to each object.
• By default, instance variables are hidden from the outside.
• We can expose instance variable via an accessor.
• You can over-ride the to_s method to customize the inspect message.
• Everything is an object is true for Smalltalk but not for Ruby.
• Every sender and receiver in a message passing interaction is an object.
• Sender can be explicit or implicit.
• Sender is the owner of the scope where the message originated.
• The dot notation makes sending messages explicit.
• If the receiver and the sender is the same, you can omit the receiver and the dot.
• There is always a receiver.
• There is always a sender.
• There is always a message that passes between the sender and the receiver.
• You cannot provide an explicit receiver to call a private method.
• You have to call the private method in functional form.
• Everything in the inheritance hierarchy is an Object.
• Receiver and Sender in a message sending interaction are objects.
• Every class is an object. In other words, every class is an instance of a Ruby built-in class called Class.
• Every object is an instance of a class.
• Every class has a superclass.
• Everything happens by sending messages.
• Method lookup follows the inheritance chain.
• Class methods and singleton methods are the same.
• User defined classes and Ruby's built-in classes are objects.
• User defined classes and Ruby's built-in classes are instances of class called Class.