理解面向对象程序设计 1

• 面向对象中的重要概念：类(class)与对象(object)

In object-oriented programming, a class is a construct that is used to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable its instances to have state and behavior.^[1] Data field members (member variables or instance variables) enable a class instance to maintain state. Other kinds of members, especially methods, enable the behavior of class instances. Classes define the type of their instances.^[2]

A class usually represents a noun, such as a person, place or thing, or something nominalized. For example, a "Banana" class would represent the properties and functionality of bananas in general. A single, particular banana would be an instance of the "Banana" class, an object of the type "Banana".

类是一个抽象的概念，包含有数据和对数据的控制

The concept of classes

A class is the structure of an object, meaning the definition of all items that an object is made up of. An object is therefore the "result" of a class. In reality, an object is an instance of a class, which is why can use interchange the terms object or instance (or even occurrence).

A class is made up of two parts:

• Attributes (often called member data): this is the data that refers to the object's state
• Methods (often referred to as member functions): these are functions that can be applied to objects

If we have a class called car, the objects Peugeot 406 and Renault 18 will be instances of this class. There may also be other objects Peugeot 406, differentiated by their model number. Furthermore: two instances of a class may have the same attributes but may be considered separate distinct objects. In a real world scenario: two T-shirts may be identical, but they are however distinct from each other. However, if we mix them together it is impossible to distinguish one from the other.