使用TCP的Java套接字编程不起作用

Question

I've been trying to write a client process that will communicate with a serve.

I used the same code below but instead of scanner i used BufferedReader and the client input was a string. Furthermore, the server just changes the string to uppercase and sends it back to the client to be displayed on the screen. It worked.

However, when i changed the code so that the client can enter a double number [(ex: 1.6) the server should round it and send it back so that it'll be printed on the screen], i get no response.

And i got this error:

run:
Exception in thread "main" java.util.InputMismatchException
    at java.util.Scanner.throwFor(Scanner.java:864)
    at java.util.Scanner.next(Scanner.java:1485)
    at java.util.Scanner.nextDouble(Scanner.java:2413)
    at socketserver.SocketServer.main(SocketServer.java:38)
Java Result: 1

notes:

1. I am using NetBeans.
2. I used Scanner instead of using BufferedReader so that i'll be able to read a double number.

Client class

package socketclient;

import java.io.*; 
import java.net.*; 
import java.util.Scanner;


public class SocketClient {

    /**
     * @param args the command line arguments
     */
   public static void main(String args[]) throws Exception 
    { 
        double inputDouble; 
        double modifiedNum; 

        Scanner inFromUser = new Scanner(System.in);


        Socket clientSocket = new Socket("localhost", 6789); 

        DataOutputStream outToServer = 
          new DataOutputStream(clientSocket.getOutputStream());


          Scanner inFromServer = new Scanner(clientSocket.getInputStream());


        inputDouble = inFromUser.nextDouble(); 

        outToServer.writeDouble(inputDouble);


        modifiedNum = inFromServer.nextDouble(); 

        System.out.println("FROM SERVER: " + modifiedNum); 

        outToServer.close(); 
        clientSocket.close();

    } 

}

Server class

package socketserver;

import java.io.*; 
import java.net.*; 
import java.util.Scanner;


public class SocketServer {

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) throws Exception 
    { 
      double clientNum; 
      double roundedNum;

      ServerSocket welcomeSocket = new ServerSocket(6789); 

      while(true) { 

            Socket connectionSocket = welcomeSocket.accept(); 

            Scanner inFromClient = new Scanner(connectionSocket.getInputStream());


            DataOutputStream  outToClient = 
             new DataOutputStream(connectionSocket.getOutputStream()); 

           clientNum = inFromClient.nextDouble(); 


           roundedNum= Math.round(clientNum);


           outToClient.writeDouble(roundedNum); 
        } 
    } 


}

Answer 1

Your error is to use Scanner to read data sent by client from the server and the converse.

You use DataOutputStream to sent a double so you need a DataInputStream to read the double ; on both sides.

Change your Scanner s to DataInputStream for inputFromServer and inputFromClient . Also don't forget to flush your outputs and close the streams at the end.