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In object-oriented programming (OOP), an inner class or nested class is a class declared entirely within the body of another class or interface. It is distinguished from a subclass. ==Overview== An instance of a normal or top-level class can exist on its own. By contrast, an instance of an inner class cannot be instantiated without being bound to a top-level class. Let us take the abstract notion of a Car with four Wheels. Our Wheels have a specific feature that relies on being part of our Car. This notion does not represent the Wheels as Wheels in a more general form that could be part of a vehicle. Instead, it represents them as specific to this one. We can model this notion using inner classes as follows:We have the top-level class Car. Instances of class Car are composed of four instances of the class Wheel. This particular implementation of Wheel is specific to the car, so the code does not model the general notion of a wheel that would be better represented as a top-level class. Therefore, it is semantically connected to the class Car and the code of Wheel is in some way coupled to its outer class. The wheel rather is a composition unit of a car. The wheel for a particular car is unique to the car, but for generalization, the wheel is an aggregation unit to the car. Inner classes provide us with a mechanism to accurately model this connection. We say that our Wheel class is Car.Wheel, Car being the top-level class and Wheel being the inner class.Inner classes therefore allow for the object orientation of certain parts of the program that would otherwise not be encapsulated into a class. Larger segments of code within a class might be better modeled or refactored as a separate top-level class, rather than an inner class. This would make the code more general in its application and therefore more re-usable but potentially might be premature generalization. This may prove more effective, if code has many inner classes with the shared functionality. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「inner class」の詳細全文を読む スポンサード リンク
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