System.in属于哪个输入类，为什么？ [英] What input class does System.in belongs to and why?

问题描述

代码

import java.io.*;

class ioTest1{
   public static void main(String args[]){
      System.out.println(System.in.getClass());
   }
}

返回 System.in 属于 BufferedInputStream 类。为什么会这样？

returns that System.in belongs to the BufferedInputStream class. Why is that?

因为类系统定义 System.in 要 InputStream ，我可以看到结果并非不可能。但是为什么以前的代码不能返回从 InputStream 继承的另一个类，例如 DataInputStream ？

Since class System defines System.in to be InputStream, I can see how the result isn't impossible. But why can't the previous code return another class inherited from InputStream,like for example DataInputStream?

推荐答案

正如评论中所解释的那样，没有什么可以阻止 System.in 来自是 DataInputStream ，但鉴于 BufferedInputStream 是一个简单的 InputStream 包括缓冲数据（总是一个好主意），给出最通用的类​​型更有意义。

As explained in the comments, there's nothing that would have prevented System.in from being DataInputStream, but given that BufferedInputStream is a no-frills InputStream that includes buffering of data (always a good idea), it makes more sense to give the most generic type possible.

然后开发人员可以用缓冲流包装例如a DataInputStream 如果程序是通过管道二进制数据提供的，或者是 InputStreamReader 如果程序正在接收文本数据用户。

Developers can then wrap the buffered stream with for example a DataInputStream if the program is fed piped binary data, or an InputStreamReader if the program is receiving text data from the user.

System.in 是一个 BufferedInputStream 因为它不需要任何其他东西。

System.in is a BufferedInputStream because it doesn't need to be anything else.

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