问题描述
作为我学习的一部分，我认为我发现的抽象定义的最佳答案(有意义)来自 stackoverflow:
As part of my learning i think the best answer(with meaning) for definition of abstraction that i found is from stackoverflow:
除此之外，
作为当前在线课程 cs61B 伯克利 2006 年秋季的一部分，我学习了与上述定义接近的 ADT 的以下类似定义，但添加了一个额外的词不变".我可以将这个词视为上述定义的推论吗?或者这个词是定义的一部分吗?
As part of current online course cs61B Fall 2006, Berkeley, i learnt the similar below definition of ADT close to above definition but added an extra word 'invariant'. Shall i consider this word as corollary to the above definition? or Is this word part of the definition?
一个 _Abstract_Data_Type_ (ADT) 是一个具有明确定义接口的类，但它的实现细节对其他类是严格隐藏的.这样，你可以在不危及依赖于它的程序的情况下改变一个类的实现.Date 类是一个 ADT.
An _Abstract_Data_Type_ (ADT) is a class that has a well-defined interface, but its implementation details are firmly hidden from other classes. That way, youcan change the implementation of a class without jeopardizing the programs that depend on it. The Date class is an ADT.
ADT 允许您强制执行不变量.
不变量是关于数据结构的事实，它始终为真(假设代码没有错误)，无论外部类调用什么方法.例如，Date ADT 强制执行不变量，即 Date 对象始终表示有效日期.通过仅允许通过方法调用访问某些字段来强制执行不变量.
An invariant is a fact about a data structure that is always true (assuming the code is bug-free), no matter what methods are called by external classes.For example, the Date ADT enforces the invariant that a Date object always represents a valid date. An invariant is enforced by allowing access to certain fields only through method calls.
推荐答案
我最喜欢的抽象定义之一是 Robert C. Martin 的，来自 敏捷原则、模式和实践:
One of my favourite definitions of abstraction is Robert C. Martin's, from the book Agile Principles, Patterns, and Practices:
抽象是对不相关的剔除，对本质的放大.
基于该定义，抽象可以在许多不同的层次上采用多种形式:
Based on that definition, an abstraction can take many forms, on many different levels:
- 它可以是一个完整的系统
- 它可以是一个子系统
- 它可以是消息格式的定义(类似于协议)
- 它可以是一个函数(如在函数式编程中)
- 它可以是一个抽象基类
- 它可以是一个接口
- 它可以是一个具体的类
- 等
在面向对象软件构造中，Bertrand Meyer 将类描述为抽象数据类型.本书的想法是抽象数据类型的描述包括前置条件和后置条件.在 Eiffel(本书中使用的语言)中，这些被称为断言，但我们也将它们称为不变量.
In Object-Oriented Software Construction, Bertrand Meyer describes classes as Abstract Data Types. The idea in this book is that the description of an Abstract Data Type includes pre- and post-conditions. In Eiffel (the language used in the book), these are called assertions, but we also know them as invariants.
Meyer 对面向对象设计的看法包括不变量.在详细(类/对象/函数)级别上，我同意将不变量视为一种描述抽象的方式是有意义的.
Meyer's view on Object-Oriented Design includes invariants. On a detailed (class/object/function) level, I agree that it makes sense to view invariants as a way to describe an abstraction.
另一方面，如果您的抽象是对高级架构的描述，那么那些类型的不变量就没有意义.尽管如此，即使在高层次上，定义前置条件和后置条件通常也很有价值(例如系统 A 必须响应才能使系统 B 正常工作")，因此考虑不变量是有意义的与抽象的关系.
On the other hand, if your abstraction is a description of a high-level architecture, those kind of invariants doesn't make sense. Still, even at a high level, it would often be valuable to define pre- and post-condition (e.g. "System A must be responding in order for System B to work correctly"), so it does makes sense to think about invariants in relation to abstractions.
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