A class that inherits variables and methods from an existing class.
The class from which variables and methods are inherited.
The class that is the root of the tree-structured class hier- archy.
Specifying a method in a subclass for the same message as a method in a superclass.
A pseudo-variable that refers to the receiver of a message; differs from self in where to start the search for methods.
A class that specifies protocol, but is not able to fully im- plement it; by convention, instances are not created of this kind of class.
A message to report the error that a subclass should have implemented one of the superclass’s messages.
A message to report the error that this is a message inherited from a superclass but explicitly not available to instances of the subclass.