Multiple inheritance
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Some programming languages allow multiple inheritance, in which a class can inherit behaviors and features from more than one superclass. This contrasts with single inheritance, where a class inherits from only one superclass.
Multiple inheritance can cause some confusing situations, so there is some debate over whether or not its benefits outweigh its risks. Java compromises: it allows a class to inherit interfaces from more than one parent (that is, one can specify that a class must have all of the same externally exposed methods of its interface-parents, and allow the compiler to enforce that), but can inherit actual methods and data from only one parent.
In object oriented programming (OOP), inheritance describes a relationship between two types, or classes, of objects in which one is said to be a "subtype" or "child" of the other. The child inherits features of the parent, allowing for shared functionality. For example, one might create a variable class "Mammal" with features such as eating, reproducing, etc.; then define a subtype "Rat" that inherits those features without having to explicitly program them, while adding new features like collecting garbage.
If, however, more than one totally orthogonal hierarchy are desired to be used simultaneously, such as allowing "Rat" to inherit from "Cartoon character" and "Disease vector" as well as "Mammal", lack of multiple inheritance often results in a very awkwardly mixed hierarchy, or forces functionality to be rewritten in more than one place (with the attendant maintenance problems).
Multiple inheritance has been a touchy issue for many, with opponents pointing to its increased complexity and ambiguity. For example, if two parents have independent implementations of a feature named X, which one is inherited? (This situation is commonly referred to as the diamond problem)
Languages have different ways of dealing with these problems. Eiffel, for example, allows subtypes to adapt their inherited features by renaming them or setting selection rules for them ahead of time. Java allows objects to inherit multiple interfaces but only a single implementation. REALbasic is similar, but also allows additional methods to "extend" a class without inheritance. Perl uses the list of classes to inherit from as an ordered list, and uses the first method it finds by depth-first searching first the current class, then the inherited classes in the order listed and their superclasses. CLOS allows for complete programmer control of method combination, and if that's not enough the meta-object protocol gives the programmer a means to modify the inheritance, method dispatch, class instantiation, and other internal mechanisms without affecting the stability of the system.