我有一个简单的服务器和客户端代码，我一直在创建。我是新来的网络，我已经被困在这个错误的绝对年龄。我知道消息就在那里，因为我可以将消息输出到控制台。我不知道为什么这不工作，我会链接服务器和客户端的代码和下面的错误。非常感谢。
服务器代码：

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package oserver;

import  java.net.*;
import  java.io.*;
import com.dosse.upnp.UPnP;
import java.util.Scanner;
import java.util.Timer;
import java.util.TimerTask;

/**
 *
 * @author sgmud
 */
public class SServer { // Server Class 
    public ServerSocket SSocket;
    public Socket CSocket;
    public PrintWriter out;
    public BufferedReader in;

    public int GetPort(){ // Gets port number for socket to be set listening to
        Scanner scan = new Scanner(System.in);
        System.out.println("please enter the port number");
        int PortNum = scan.nextInt();
        return PortNum;

    }    

    public void start(int port) { // Starts the server with the collected port
        try{
            System.out.println("Server Started");
            UPnP.openPortTCP(port);
            SSocket = new ServerSocket(port);
            CSocket = SSocket.accept();
            System.out.println("Server Connected");

            out = new PrintWriter(CSocket.getOutputStream(), true);
            in = new BufferedReader(new InputStreamReader(CSocket.getInputStream()));
            Timer timer = new Timer();
            int seconds;
            seconds = 1;

            timer.scheduleAtFixedRate(
            new TimerTask()
            {
                public void run()
                {
                    Listner();

                }
            },
            0,      // run first occurrence immediately
            1000);  // run every 1 seconds

            SServer server = new SServer();
            server.convosation();

            /**
            String input;
            while ((input = in.readLine()) != null){
                if(".".equals(input)){
                    out.println("goodbye");
                    break;
                }
                else{
                    out.println(input);
                }
            }
          **/

            }catch(IOException e){
                System.out.println("ERROR");
        }   
    }

    public String Message(){                   
            Scanner scan = new Scanner(System.in); 
            System.out.println("please enter a mesasge to be sent");
            String message = scan.nextLine();

            return message;

    }

    public void convosation(){  // method will keep letting you send a message untill you stop
        SServer client = new SServer();
        while (true){
            Scanner scan = new Scanner(System.in);
            System.out.println("Type QUIT to end the convosation or press any key to send a message");
            String qit = scan.nextLine();
            if("QUIT".equals(qit)){
                client.STOP();
            }
            else{
                client.sendMessage(client.Message()); // Runs the send message method with the output from the Message method

            }
        }       
    }

    public void stop(){ // Will close all connections
        try{
            //in.close();
            out.close();
            CSocket.close();
            SSocket.close();
        }catch(IOException e){
                System.out.println("ERROR");
        }
    }

        public void sendMessage(String msg){ 
        if(msg.equals("null")){
                SServer client = new SServer();
                client.sendMessage(client.Message());// outputs message
        }        
        else{
            out.println(msg);
        }        
                //try{
                //    Response = in.readLine();
                //}catch(IOException e){
                //    System.out.println("ERROR");
                //}
                //System.out.println(Response);
                //return Response; 
    }  // outputs message

    public static void main(String[] args){

        SServer server = new SServer(); // Creat new server class
        server.start(server.GetPort()); // Starts the server with the port number

    }

       public void STOP(){
        try{
            in.close();
            out.close();
            CSocket.close();
        }catch(IOException e){
                System.out.println("ERROR");
        }
    }

        public void Listner() {
                String message = "";
            try{
                message = in.readLine();

            }catch(IOException e){

            }
            if(message.equals("")==false){System.out.println(message);}
        }

    }

客户代码：

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package oclient;

import  java.net.*;
import  java.io.*;
import com.dosse.upnp.UPnP;
import java.util.Scanner;
import java.util.Timer;
import java.util.TimerTask;
/**
 *
 * @author sgmud
 */
public class CClient {
    public Socket CSocket;
    public PrintWriter output;
    public BufferedReader in;

    public int GetPort(){
        Scanner scan = new Scanner(System.in);
        System.out.println("please enter the port number");
        int PortNum = scan.nextInt();
        return PortNum;
    }

    public String GetAddress(){
        Scanner scan = new Scanner(System.in);
        System.out.println("Please enter the IP address of the host");
        String Address = scan.nextLine();
        return Address;
    }

    public void StartConnection(String ip, int port){
        try{
            CSocket = new Socket(ip, port);
            output = new PrintWriter(CSocket.getOutputStream(), true);
            in = new BufferedReader(new InputStreamReader(CSocket.getInputStream()));
            Timer timer = new Timer();
            int seconds;
            seconds = 1;

            timer.scheduleAtFixedRate(
            new TimerTask()
            {
                public void run()
                {
                    Listner();

                }
            },
            0,      // run first occurrence immediately
            1000);  // run every 1 seconds

            CClient server = new CClient();

        }catch(IOException e){
                System.out.println("ERROR");
        }

    }

    public void sendMessage(String msg){ 
        if(msg.equals("null")){
                CClient client = new CClient();
                client.sendMessage(client.Message());// outputs message
        }        
        else{
                System.out.println(msg);

                output.println(msg);
        }        
                //try{
                //    Response = in.readLine();
                //}catch(IOException e){
                //    System.out.println("ERROR");
                //}
                //System.out.println(Response);
                //return Response; 
    }  // outputs message

    public String receveMessage(){
        String Response = "null";
        try{
            Response = in.readLine();

        }catch(IOException e){
            System.out.println("Error");
            return Response;

        }
            System.out.println(Response);
            return Response;

    }

    public void convosation(){  // method will keep letting you send a message untill you stop
        CClient client = new CClient();
        while (true){
            Scanner scan = new Scanner(System.in);
            System.out.println("Type QUIT to end the convosation or press any key to send a message");
            String qit = scan.nextLine();
            if("QUIT".equals(qit)){
                client.STOP();
            }
            else{
                client.sendMessage(client.Message()); // Runs the send message method with the output from the Message method

            }
        }       
    }

    public String Message(){                   
            Scanner scan = new Scanner(System.in); 
            System.out.println("please enter a mesasge to be sent");
            String message = scan.nextLine();

            return message;

    }

    public void STOP(){
        try{
            in.close();
            output.close();
            CSocket.close();
        }catch(IOException e){
                System.out.println("ERROR");
        }
    }

    public void Listner() {
        String message = "";
        try{
            message = in.readLine();

        }catch(IOException e){

        }
        if(message.equals("")==false){System.out.println(message);}
    }

    /**
     *
     * @param args
     */
    public static void main(String[] args){
        CClient client = new CClient(); // Making a new client class
        client.StartConnection(client.GetAddress(), client.GetPort()); // runs the startConnection method but runs the Get address and Get port method first so the Start connection method has the IP and Port number 
        client.convosation();
        // client.STOP(); // runs the stop method which will terminate the server
    }

}

错误：

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.io.PrintWriter.println(String)" because "this.output" is null
    at oclient.CClient.sendMessage(CClient.java:79)
    at oclient.CClient.convosation(CClient.java:122)
    at oclient.CClient.main(CClient.java:169)

抱歉，如果我有点吹牛，我真的不明白为什么这是不工作。也请忽略我糟糕的错误处理，我计划在以后的工作。任何帮助都是值得的。：）