麻烦大家说说这句 PrintWriter out=new PrintWriter(server.getOutputStream());在程序中的作用

import java.net.*;
import java.io.*;
public class Client {
static Socket server;
public static void main(String[] args)throws Exception {
System.out.println("我是客户端");
server=new Socket(InetAddress.getLocalHost(),5678);
BufferedReader in=new BufferedReader(new InputStreamReader(server.getInputStream()));
PrintWriter out=new PrintWriter(server.getOutputStream());
BufferedReader wt=new BufferedReader(new InputStreamReader(System.in));
while(true){
String str=wt.readLine();
out.println(str);
out.flush();
if(str.equals("end")){
break ;
}
System.out.println(in.readLine());
}
server.close();
}

}

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