Java网络“连接被拒绝：连接”

Question

我正在尝试用socket做一个基本的网络程序 服务器：

    import java.io.*;
import java.net.*;
class Socketserver{
    public static void main(String[]z)throws IOException{
        System.out.println("Server is started");
        ServerSocket ss=new ServerSocket(9999);
        System.out.println("Waiting for client request");
        Socket s=ss.accept();
        System.out.println("client connected");
        InputStreamReader a=new InputStreamReader(s.getInputStream());
        BufferedReader b=new BufferedReader(a);
        String str=b.readLine();
        System.out.println("Client data"+str);
        String nickname=str.substring(0,3);
        OutputStreamWriter os=new OutputStreamWriter(s.getOutputStream());
        PrintWriter out=new PrintWriter(os);
        out.write(str);
        os.flush();
        System.out.println("data sent from server to client");
}}

客户

import java.io.*;
import java.net.*;
class Socketclient{
    public static void main(String[]z)throws IOException{
        String ip="localhost";// for same machine
        int port=9999;
        Socket s=new Socket(ip,port);
        String str="Rujhaan";
        OutputStreamWriter os=new OutputStreamWriter(s.getOutputStream());
        PrintWriter out=new PrintWriter(os);
        out.write(str);
        os.flush();
        InputStreamReader a=new InputStreamReader(s.getInputStream());
        BufferedReader b=new BufferedReader(a);
        String nickname=b.readLine();
        System.out.println("data from server"+nickname);
    }
}

程序可以编译，服务器上没有问题，但运行客户端时总是出现连接被拒绝或连接超时异常。 我尝试了不同的端口名，也没有防火墙问题。 请建议我该怎么做