什么是抽象类？ [英] What is an abstract class?

问题描述

当我得知抽象类是说WT（H *）！

问题：

1. 什么是不能被实例化创建一个类的点？


2. 为什么会有人想要这样的类？


3. 什么是在抽象类成为必需的情况呢？

*的如果你知道我的意思的

解决方案

1. 最常见的，作为一个基类或接口（一些语言有一个单独的接口结构，有的没有） - 它没有知道的实施（即由子类来提供/实现类）


2. 提取和再利用


3. 当基类可以提供任何有意义的默认实现的方法（但允许子类重复使用实现的非抽象的部分;任何领域，非抽象方法等）

例如：

 公共抽象类流{/ *批次code，一些抽象方法* /}
 

到底是什么流的本身？什么样的流的？一个流文件？网络？内存缓冲区？各自可以具有读/写的不同的和不相关的方式，而是提供一个通用的API。它使无感以创建的只是的一个流，而是通过摘要类，你可以的 code 的如果不知道的细节流 API

 流S = CreateStream（...）; //我不小心* *什么样的流
s.Write（新的byte [] {1,2,3,4,5}）;
S.CLOSE（）;
 

When I learned about abstract classes is said WT(H*)!!!

QUESTIONS:

1. What is the point of creating a class that can't be instantiated?
2. Why would anybody want such a class?
3. What is the situation in which abstract classes become NECESSARY?

*if you know what i mean

解决方案

1. most commonly to serve as a base-class or interface (some languages have a separate interface construct, some don't) - it doesn't know the implementation (that is to be provided by the subclasses / implementing classes)
2. abstraction and re-use
3. when the base-class can provide no meaningful default-implementation for a method (but allowing subclasses to re-use the non-abstract parts of the implementation; any fields, non-abstract methods, etc)

For example:

public abstract class Stream { /* lots of code, some abstract methods */ }

What the heck is a stream by itself? What kind of stream? a stream to a file? a network? a memory buffer? Each may have different and unrelated ways of reading / writing, but provide a common API. It makes no sense to create just a Stream, but via the abstract class, you can code to the Stream API without knowing the details:

Stream s = CreateStream(...); // I don't *care* what kind of stream
s.Write(new byte[] {1,2,3,4,5});
s.Close();

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