Class inheritance is a mechanism for using the interface and implementation of one or more classes as the basis for another class. The inheriting class, also known as a subclass, inherits from one or more classes, known as superclasses. The subclass has all the methods defined for the inherited class definition or definitions, including any methods that the inherited definition inherited. The subclass has all the data definitions defined for the inherited class or classes, including any data definitions that the inherited class or classes inherited.
Note: This does not mean that the actual source code that describes the data is accessible or that the data items described in that source code can be directly referenced in the subclass. It means that the subclasses are treated as if their source code had a copy of the superclass definitions; in other words, the inherited data items are considered to be defined in the subclass.
The inherited data definitions define data for every instance object of the subclass and for its factory object. Each instance object has its own copy of inherited data, distinct from the copy belonging to an instance object of an inherited class. Each factory object has its own copy of inherited data, distinct from the copy belonging to a factory object of an inherited class. Names and attributes of inherited data items are not visible in the inheriting class. The inherited object data is initialized when an object is created. The inherited factory data is allocated independently from the factory data of the inherited class or classes and is initialized when the factory of the subclass is created. The inherited factory data is accessible only via methods and properties specified in the factory definition of the class that describes the data. The inherited object data is accessible only via methods and properties specified in the object definition of the class that describes the data. The subclass inherits all the file definitions in the same way as the data definitions, subject to the same provisions as data definitions. The subclass may define methods in addition to or in place of the inherited methods and may specify data definitions and file definitions in addition to, but not in place of, the inherited data definitions and file definitions.
The interface of a subclass must always conform to the interface of the inherited class, although the subclass may override some of the methods of the inherited class to provide different implementations.
User-defined words in an inherited class are not inherited in the subclass, and may be used in the subclass definition as if they were not defined in the inherited class.