Socket通信

Question

下面是关于服务端和客户端通信的代码。基本都能明白，就是不明白为什么要用PrintWriter才能把消息发送给对方？
望达人解答，感谢！
import java.io.*;
import java.net.*;public class EchoClient {     public static void main(String args[]) {
         try {
             if (args.length != 1) {
                 System.out.println("USAGE: java Client servername");
                 return;
             }
             String connectto = args[0];
             Socket connection;
             if (connectto.equals("localhost")) {
                 connection = new Socket(InetAddress.getLocalHost(), 2536);
             }
             else {
                 connection = new Socket(InetAddress.getByName(connectto), 2536);
             }
             BufferedReader input = new BufferedReader(new InputStreamReader(
                     connection.getInputStream()));             PrintWriter out = new PrintWriter(connection.getOutputStream(),true );
             String info;
             info = input.readLine();
             System.out.println(info);
             boolean done = false;
             BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
             String sInput;
             while (!done) {
                 sInput = in.readLine();
                 out.println(sInput);//向对方发送消息
                 if (sInput.equalsIgnoreCase("bye"))
                     done = true;
                 info = input.readLine();
                 System.out.println(info);
             }
             connection.close();
         }
         catch (SecurityException e) {
             System.out.println("SecurityException when connecting Server!");
         }
         catch (IOException e) {
             System.out.println("IOException when connecting Server!");
         }
     }}
import java.io.*;
import java.net.*;public class EchoServer
{
     public static void main(String[] args)
     {
         try{
             ServerSocket s = new ServerSocket(2536);
             Socket incoming = s.accept();
             BufferedReader in = new BufferedReader(new InputStreamReader(incoming.getInputStream()));
             PrintWriter out = new PrintWriter(incoming.getOutputStream(), true /* autoFlush */);
             out.println("Hello! Enter BYE to exit.");
             boolean done = false;
             while (!done)
             {
                 String line = in.readLine();
                 if (line == null)
                     done = true;
                 else{
                     out.println("Echo: " + line);
                     if (line.trim().equals("BYE"))
                         done = true;
                 }
             }
             incoming.close();
         }
         catch (Exception e){
             System.out.println(e);
         }
     }}

Answer 1

没说一定要用PrintWriter这个输出字符串比较方便