Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In object-oriented programming, a protocol or interface is what or how unrelated objects use to communicate with each other. These are definitions of methods and values which the objects agree upon in order to cooperate. For example, in Java (where protocols are termed interfaces), the Comparable interface specifies a method compareTo() which implementing classes should implement. This means that a separate sorting method, for example, can sort any object which implements the Comparable interface, without having to know anything about the inner nature of the class (except that two of these objects can be compared by means of compareTo()). Some programming languages directly support protocols or interfaces (Ada, C#, D, Java, Logtalk, Objective-C). Older languages may also have features that can support the interface concept, such as abstract base classes with pure virtual functions in C++, or object-oriented features in Perl.

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