Inheritance(继承)

客官°小女子只卖身不卖艺 2022-08-18 02:08 188阅读 0赞

By itself, the idea of an object is a convenient tool.对象这种观念,本身就是十分方便的工具.

It allows you to package data and functionality together by concept, so you can represent an approriate problem-space idea rather than being forced to use the idioms of the underlying machine.使得你可以通过概念将数据和功能封装到一起,因此可以对问题空间的观念给出恰当的表示,而不用受制于必须使用底层机器语言.

These concepts are expressed as fundamental units in the programming language by using the class keyword.这些概念用关键字class来表示,它们形成了编程语言中的基本单位.

It seems a pity, however, to go to all the trouble to create a class and then be forced to create a brand new one that might have similar functionality.遗憾的是,这样做还是有很多麻烦:在创建了一个类之后,即使另一个新类与其具有相似的功能,你还是得重新创建一个新类.

It's nicer if we can take the existing class, clone it, and then make additions and modifications to the clone.如果我们能够以现有的类为基础,复制它,然后通过添加和修改这个副本来创建新类那就要好多了.

This is effectively what you get with inheritance, with the exception that if the original class (called the base class or superclass or parent class) is changed, the modified "clone" (called the derived class or inherited class or subclass or child class) alse reflects those changes.通过继承便可以达到这样的效果,不过也有例外,当源类(被称为基类,超类或父类)发生变动时,被修改的"副本"(被称为导出类,继承类或子类)也会反映出这些变动.

A type does more than describe the constraints on a set of objects; it also has a relationship with other types.类型不仅仅是描述了作用于一个对象集合上的约束条件,同时还有与其他类型之间的关系.

Two types can have characteristics and behaviors in common, but one type may contain more characteristics than another and may also handle more messages (or handle them differently).两个类型可以有相同的特性和行为,但是其中一个类型可能比另一个含有更多的特性,并且可以处理更多的消息(或以不同的方式来处理消息).

Inheritance expresses this similarity between types by using the concept of base types and derived types.继承使用基类型和导出类型的概念表示了这种类型之间的相似性.

A base type contains all of the charateristics and behaviors that are shared among the types derived from it. 一个基类型包含所有导出类型所共享的特性和行为.

You create a base type to represent the core of your ideas about some objects in your system. 可以创建一个基类型来表示系统中某些对象的核心概念.

From the base type, you derive other types to express the different ways that this core can be realized.从基类型中导出其他类型,来表示此核心可以被实现的各种不同方式.

For example, a trash-recycling machine sorts pieces of trash.以垃圾回收机为例,它用来归类散落的垃圾.

The base type is "trash", and each piece of trash has a weight, a value, and so on, and can be shredded, melted, or decomposed."垃圾"是基类型,每一件垃圾都有重量,价值等特特性,可以被切碎,熔化或分解.

From this, more specific types of trash are derived that may have additional characteristics(a bottle has a color) or behaviors(an aluminum can may be crushed, a steel can is magnetic).在此基础上,可以通过添加额外的特性(例如瓶子有颜色)或行为(例如铝罐可以被压碎,铁罐可以被磁化)导出更具体的垃圾类型.

In addition, some behaviors may be different(the value of paper depends on its type and condition).此外,某些行为可能不同(例如纸的价值取决于其类型和状态).

Using inheritance, you can build a type hierarchy that expresses the problem you're trying to solve in terms of its type.可以通过使用继承来构建一个类型层次结构,以此来表示待求解的某种类型的问题.

A second example is the classic "shape" example, perhaps used in computer-aided design system or game simulation.第二个例子是经典的几何形的例子,这在计算机辅助设计系统或游戏仿真系统中可能被用到.

The base type is "shape", and each shape has a size, a color, a position, and so on.基类是几何形,每一个几何形都具有尺寸,颜色,位置等.

Each shape can be drawn, erased, moved, colored, etc.同时每一个几何形都可以被绘制,擦除,移动和着色等.

From this, specific types of shapes are derived(inherited)--circle, square, triangle, and so on -- each of which may have additional characteristics and behaviors.在此基础上,可以导出(继承出)具体的几何形状--原形,正方形,三角形等--每一种都具有额外的特性和行为.

Certain shapes can be flipped, for example.例如某些形状可以被翻转.

Some behaviors may be different, such as when you want to calculate the area of a shape.某些行为可能并不相同,例如计算几何形状的面积.

The type hierarchy embodies both the similarities and differences between the shapes.类型层次结构同时体现了几何形状之间的相似性和差异性.

Casting the solution in the same terms as the problem is very useful because you don't need a lot of intermediate models to get from a description of the problem to description of the solution.以同样的术语将解决方案转换成问题是大有裨益的,因为不需要在问题描述和解决方案描述之间建立许多中间模型.

With objects, the type hierarchy is the primary model, so you go directly from the description of the system in the real world to the description of the system in code.通过使用对象,类型层次结构成为了主要模型,因此,可以直接从真实世界中对系统的描述过度到用代码对系统进行描述.

Indeed, one of the difficulties people have with object-oriented design is that it's too simple to get from the begining to the end.事实上,对使用面向对象的人来说,困难之一是从开始到结束过于简单.

A mind trained to look for complex solutions can initially be stumped by this simplicity.对于训练有素,善于寻找复杂的解决方案的头脑来说,可能会在一开始被这种简单性给难倒.

When you inherit from an existing type, you create a new type.当继承现有类型时也就创造了新类型.

This new type contains not noly all the members of the existing type(although the private ones are hidden away and inaccessible),but more importantly it duplicates the interface of the base class.这个新类型不仅包括现有类型的所有成员(尽管private成员被隐藏起来了,且不能访问),而且更重要的是它继承了基类的接口.

That is, all the messages you can send to objects of the base class you can also send to objects of the derived class.也就是说所有可以发送给基类的消息同时也可以发送给导出类对象.

Since we know the type of a class by the messages we can send to it, this means that the derived class is the same type as the base class.由于通过可以发送给类的消息我们知道类的类型,所以这也意味着导出类与基类具有相同类型.

In the previous example, "A circle is a shape." 在前面的例子中,"一个圆形也就是一个几何图形".

This type equivalence via inheritance is one of the fundamental gateways in understanding the meaning of object-oriented programming.通过继承而产生的类型等价性是理解面向对象程序设计方法内涵的重要门槛.

Since both the base class and derived class have the same fundamental interface, there must be some implementation to go along with that interface.由于基类和导出类具有相同的基础接口,所以伴随此接口的必定有某些具体实现.

That si, there must be some code to execute when an object receives a particular message.也就是说,当对象接收到特定消息时,必须有某些代码去执行.

If you simply inherit a class and don't do anything else, the methods from the base-class interface come right along into the derived class.如果只是简单的继承一个类,而不做其他任何事,那么在基类接口中的方法将会直接继承到导出类中.

That means objects of the derived class have not only the same type, they also have the same behavior, which isn't particularly interesting.这意味着导出类与基类不仅拥有相同的类型,还拥有相同的行为,这样做没有什么特别的意义.

You have two ways to differentiate your new derived class from the original base class.有两种方式可以使基类和导出来产生差异.

The first is quite straightforward: You simply add brand new methods to the derived class.第一种方法非常直接:直接在导出类中添加新方法.

These new methods are not part of the base-class interface.这些新方法并不是基类接口的一部分.

This means that the base class simply didn't do as much as you wanted it to, so you added more methods.这意味着基类并不能直接满足你所有需求,因此必须得添加新方法.

This simple and primitive use for inheritance is, at times, the perfect solution to your problem. 这种对继承简单而基础的使用方式,有时问题来说确实是一种完美的解决方式.

However, you should look closely for the possibility that your base class might alse need these additional methods.但是,应该仔细考虑是否存在类也需要这些额外方法的可能性.

This process of discovery and iteration of your design happens regularly in object-oriented programming.这种设计的发现与迭代过程在面向对象程序设计中会经常发生.

Although inheritance may sometimes imply(especially in Java, where the keyword for inheritance is extends) that you are going to add new methods to the interface, that's not necessarily true. 虽然继承有时可能意味着在接口中添加新方法(尤其是在以extends关键字表示继承的java中),但并非总需如此.

The second and more important way to differentiate your new class is to change the behavior of an existing base-class method.第二种也是更重要的一种使导出类和基类之间产生差异的方法是改变现有基类的方法的行为.

This is referred to as overriding that method.这被称之为覆盖(overriding)那个方法.

To override a method, you simply create a new definition for the method in the derived class.要想覆盖某个方法,可以直接在导出类中创建该方法的新定义即可.

You're saying,"I'm using the same interface method here, but I want it to do something different for my new type."你可以说:"此时,我正在使用相同的接口方法,但是我想在新类型中做些不同的事情."