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The Smalltalk-80 metaclass hierarchy as a UML diagram Diagram of the inheritance and instance relationships between classes and metaclasses in Smalltalk In Smalltalk , everything is an object . Additionally, Smalltalk is a class based system, which means that every object has a class that defines the structure of that object (i.e. the instance ...
In Python, everything is an object, even classes. Classes, as objects, have a class, which is known as their metaclass. Python also supports multiple inheritance and mixins. The language supports extensive introspection of types and classes. Types can be read and compared—types are instances of type. The attributes of an object can be ...
In knowledge representation, particularly in the Semantic Web, a metaclass is a class whose instances can themselves be classes. Similar to their role in programming languages , metaclasses in ontology languages can have properties otherwise applicable only to individuals, while retaining the same class's ability to be classified in a concept ...
Not all languages support multiple inheritance. For example, Java allows a class to implement multiple interfaces, but only inherit from one class. [22] If multiple inheritance is allowed, the hierarchy is a directed acyclic graph (or DAG for short), otherwise it is a tree. The hierarchy has classes as nodes and inheritance relationships as links.
A classifier is an abstract metaclass classification concept that serves as a mechanism to show interfaces, classes, datatypes and components.. A classifier describes a set of instances that have common behavioral and structural features (operations and attributes, respectively).
In general, the further down in the hierarchy a class appears, the more specialized its behavior. When a message is sent to an object, it is passed up the inheritance tree starting from the class of the receiving object until a definition is found for the method. This process is called upcasting.
Multiple inheritance is a feature of some object-oriented computer programming languages in which an object or class can inherit features from more than one parent object or parent class. It is distinct from single inheritance, where an object or class may only inherit from one particular object or class.
Composition over inheritance (or composite reuse principle) in object-oriented programming (OOP) is the principle that classes should favor polymorphic behavior and code reuse by their composition (by containing instances of other classes that implement the desired functionality) over inheritance from a base or parent class. [2]