Java基本控制台编程 - 可以使用hasNextLine从控制台读取输入？ [英] Java basic console programming - can use hasNextLine to read inputs from console?

问题描述

我已经看到这个代码对我看起来不自然。我通常使用 hasNextLine（），而不是一个布尔变量 done ，如 while loop，但现在我很困惑。我的问题是，我可以替换的逻辑，其中变量完成显示与 hasNextLine（）控制台，或者当输入来自文件时，我可以只使用 hasNextLine（）吗？这是一个更好的实践方法，通过使用 done 变量或 hasNextLine（）？感谢。

  // TCPClient.java 
 
 import java.net。 
 import java.io. *; 
 import java.lang。*; 
 
 public class TCPClient {
 
 public static void main（String args []）{
 Socket clientSock = null; 
 try {
 int port_num = Integer.valueOf（args [1]）。intValue（）; //获取服务器的端口号。 
 clientSock = new Socket（args [0]，（int）port_num）; // args [0]是服务器主机名
 
 / * String sock = clientSock.toString（）; 
 System.out.println（sock）; * / 
 
 PrintWriter oStream = 
 new PrintWriter（clientSock.getOutputStream（），true）; 
 
 BufferedReader iStream = 
 new BufferedReader（new InputStreamReader 
（clientSock.getInputStream（）））; 
 
 BufferedReader keyInput = 
 new BufferedReader（new InputStreamReader（System.in））; 
 
 boolean done = false; 
 String answer = iStream.readLine（）; 
 if（answer！= null）
 System.out.println（answer）; 
 
 while（！done）{
 String line = keyInput.readLine（）; 
 if（line.trim（）。equals（BYE））
 done = true; 
 oStream.println（line）; 
 answer = iStream.readLine（）; 
 if（answer！= null）
 System.out.println（answer）; 
} 
 
 clientSock.close（）; 
} catch（Exception e）{
 System.out.println（e）; 
} 
} 
} 
  

解决方案>

@ Voo的解决方案中有一个错误，因为 nextLine（）在 keyInput c $ c> null 。这是一个更正的版本：

 字符串行; 
 while（（line = keyInput.readLine（））！= null& 
！line.trim（）。equals（BYE））{
 oStream.println ）; 
 answer = iStream.readLine（）; 
 if（answer！= null）{
 System.out.println（answer）; 
} 
} 
  

---

当我们希望从控制台输入时，我可以用 hasNextLine（）来显示变量done的逻辑，或者我只能使用 hasNextLine（）当输入来自一个文件？

您可以封装任何 InputStream 或任何 Scanner 中的（其中 Reader 是子类型），允许您使用 hasNextLine（）。唯一的警告是，如果底层流来自控制台，管道，套接字等， hasNextLine（）可能会阻止无限等待输入。

这是一个更好的实践方法来实现这个代码，其中输入来自控制台，通过使用 done 变量或 hasNextLine（）？




，如上所述的第三选项。这真的是一个味道的问题...和你认为看起来最简单。 （个人而言，我不会使用 Scanner ，以便我可以调用 hasNextLine（） $ c> BufferedReader 是 Scanner 隐藏任何可能在呼叫中出现的 IOException 到 hasNext ...（）并返回 false 。这对于典型的使用情况来说是一件好事 - Scanner 作为一个轻量级的用户输入解析器（因为你在 keyInput ），但可能不在其他用例中。

I have been presented with this code which looks unnatural to me. I would normally use hasNextLine() rather than a Boolean variable done as shown in this code in the while loop, but now I'm confused. My question is, can I replace the logic where the variable done is shown with hasNextLine() when input is expected from the console, or can I only use hasNextLine() when input comes from a file? Which is a better practice way of implementing this code where input comes from console, by using the done variable or hasNextLine()? Thanks.

// TCPClient.java

import java.net.*;
import java.io.*;
import java.lang.*;

public class TCPClient{

    public static void main(String args[]){
    Socket clientSock=null;
    try{
        int port_num = Integer.valueOf(args[1]).intValue();  //get server's port no.
        clientSock = new Socket(args[0],(int)port_num);  // args[0] is the server host name

        /* String sock=clientSock.toString();
        System.out.println(sock); */

        PrintWriter oStream =
        new PrintWriter(clientSock.getOutputStream(),true);

        BufferedReader iStream =
        new BufferedReader(new InputStreamReader
                   (clientSock.getInputStream()));

        BufferedReader keyInput =
        new BufferedReader(new InputStreamReader(System.in));

        boolean done = false;
        String answer = iStream.readLine();
        if(answer != null)
        System.out.println(answer);

        while(!done){
        String line = keyInput.readLine();
        if(line.trim().equals("BYE"))
            done = true;
        oStream.println(line);
        answer = iStream.readLine();
        if(answer != null)
            System.out.println(answer);
        }

        clientSock.close();
    }catch(Exception e){
        System.out.println(e);
    }
    }
}

解决方案

There is a bug in @Voo's solution because the nextLine() on keyInput could also return null. Here's a corrected version:

String line;
while ((line = keyInput.readLine()) != null && 
       !line.trim().equals("BYE")) {
    oStream.println(line);
    answer = iStream.readLine();
    if (answer != null) {
        System.out.println(answer);
    }
}

---

Can I replace the logic where the variable done is shown with hasNextLine() when input is expected from the console, or can I only use hasNextLine() when input comes from a file?

You can wrap any InputStream or any Readable (of which Reader is a subtype) in a Scanner, allowing you to use hasNextLine() on all of them. The only caveat is that hasNextLine() can potentially block indefinitely waiting for input if the underlying stream comes from a console, pipe, socket or similar.

Which is a better practice way of implementing this code where input comes from console, by using the done variable or hasNextLine()?

Either will do, as will the third option as illustrated above. It is really a matter of taste ... and what you think looks simplest. (Personally, I'd not use Scanner just so that I can call hasNextLine() ... but that's just my opinion.)

The other significant difference between using Scanner and BufferedReader is that Scanner hides any IOExceptions that might occur in a call to hasNext...() and returns simply false. This is a good thing for typical use-cases of Scanner as a light-weight user input parser (as you are using it on keyInput), but maybe not in other use-cases.

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