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In programming languages, an abstract class is a generic class (or type of object) used as a basis for creating specific objects that conform to its protocol, or the set of operations it supports. Abstract classes are not instantiated directly.
Abstract classes are useful when creating hierarchies of classes that model reality because they make it possible to specify an invariant level of functionality in some methods, but leave the implementation of other methods until a specific implementation of that class (a derived class) is needed.
In object-oriented programming (OOP) languages, classes represent objects in the domain of the problem the software is intended to solve. Classes include collections of attributes (properties) and behaviors (methods), which can be based on previously-defined classes. Programmers use inheritance to derive the specific implementation of abstract classes. Classes that are derived from abstract classes are called derived classes. When this principle is applied many times in succession, it results in a hierarchy of classes. In this context, abstract classes are at the root of this hierarchy, and is used to enforce methods that need to be overridden in the derived classes, thus avoiding potential runtime errors.
An abstract class has at least one abstract method. An abstract method will not have any code in the base class; the code will be added in its derived classes. The abstract method in the derived class should be implemented with the same access modifier, number and type of argument, and with the same return type as that of the base class. Objects of abstract class type cannot be created, because the code to instantiate an object of the abstract class type will result in a compilation error.