le an application is executed, it can be foundout if the message sent is applicable to the object. If not the application ends up with arun-time error. The fact that the piece of code that should be executed is bound at run-timeis called late binding.The hierarchies of the classes are based on the principle of inheritance which is consideredone of the most basic of the object-oriented systems. Inheritance is an antisymmetric,transitive, binary relationship that can exist between two classes A and B from which the A iscalled a subclass of B and B is called a superclass of A. The relationship has many commoncharacteristics with the ancestor/descendant relationship since a class has direct and indirectsubclasses as an ancestor has direct and indirect descendants. In general a superclass canhave one or more direct subclasses, although the number of direct superclasses that asubclass can have is not the same for all the models. In fact, in all the models, all theclasses have at least one superclass but there are some models that do not allow classes tohave more than one. These are called single inheritance models and the rest multipleinheritance models. According to the concept of inheritance, the subclasses can inheritmethods and attributes from their superclasses. That means that inheritance is themechanism that allows the generation of new software modules from existing softwaremodules. There are four kinds of inheritances that have slightly different semantics: Substitution inheritance: if class A is a subclass of class B, then any object of class B can besubstituted by an object of the class A. That means that the set of messages that constitutethe interface of class A is a superset of the set of messages of class B. Inclusion inheritance: if class A is a subclass of class B, then objects of A and B have thesame internal structure although they may share the same methods and messages. This kindof inheritance corresponds to the notion o...
