Multithreaded chat-server in Java

Question

I'm trying to implement a server-client socket program in Java that can support multiple clients, but my class that performs the multithreading always crashes whenever my client connects to my server.

import java.io.*;
import java.net.*;
public class ClientWorker extends Thread{
    Socket cwsocket=null;

    public ClientWorker(Socket cwsocket){
     super("ClientWorker");
     cwsocket=cwsocket;
    }

    public void run(){
     try {
         PrintWriter out = new PrintWriter(cwsocket.getOutputStream(), true);
         BufferedReader in = new BufferedReader(new InputStreamReader(cwsocket.getInputStream()));

         String serverinput, serveroutput="";

         out.println(serveroutput);

         while ((serverinput = in.readLine()) != null) {
       out.println(serveroutput);
       if (serveroutput.equals("Terminate"))
           break;
         }
         out.close();
         in.close();
         cwsocket.close();

     } catch (IOException e) {
         e.printStackTrace();
     } 
    }
}

Whenever I create a PrintWriter object, a NullPointerException exception is thrown, and I'm not sure why it continues to happen. Below are my server and client classes. What am I doing wrong?

import java.io.*;
import java.net.*;
public class Server {

    public static void main(String[]args)throws IOException{
     ServerSocket serversocket=null;
     final int PORT_NUM=4444;
     boolean flag=true;
     try{
      System.out.println("Listening for connection");
      serversocket=new ServerSocket(PORT_NUM);
     }catch(IOException e){
      System.out.println("Could not listen to port: "+PORT_NUM);
      System.exit(-1); 
     }
     while(flag){
      new ClientWorker(serversocket.accept()).start();
     }
     System.out.println("Terminating server...");
     serversocket.close();
    }
}

import java.io.*;
import java.net.*;
public class Client {

    public static void main(String[] args){
     Socket socket=null;
     PrintWriter out=null;
     BufferedReader in=null;
     BufferedReader userInputStream=null;
     String IP="127.0.0.1";
     try{
      socket = new Socket(IP, 4444);
      out = new PrintWriter(socket.getOutputStream(), true);
      in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
     } catch (UnknownHostException e) {
      System.out.println("Unknown host:" + IP);
      System.exit(1);
     } catch  (IOException e) {
      System.out.println("Cannot connect to server...");
      System.exit(1);
     }
     String userInput, fromServer;
     try{  
      userInputStream = new BufferedReader(new InputStreamReader(System.in));
      while ((fromServer = in.readLine()) != null) {
                System.out.println("Server: " + fromServer);
                if (fromServer.equals("Terminate"))
                    break;

                userInput = userInputStream.readLine();
          if (userInput != null) {
                     System.out.println("> " + userInput);
                     out.println(userInput);
          }
            }
     }catch(IOException e){
      System.out.println("Bad I/O");
      System.exit(1);
     }
     try{
      out.close();
      in.close();
      userInputStream.close();
      socket.close();
      System.out.println("Terminating client...");
     }catch(IOException e){
      System.out.println("Bad I/O");
      System.exit(1);
     }catch(Exception e){
      System.out.println("Bad I/O");
      System.exit(1);
     }
    }
}

Answer 1

Stacktrace please

Answer 2

In

    public ClientWorker(Socket cwsocket){
        super("ClientWorker");
        cwsocket=cwsocket;
    }

You need to do

        this.cwsocket=cwsocket;

Or rename the parameter so it doesn't shadow the member of the same name.