Question

I am implementing a tcp java client/server app based on a tut on the internet
I changed some methods and get error of nullpointerexception

The error comes from the run(). Its in a Connection class that extends the Thread class. run() is supposed to listen to incoming communication. I start the server class, listen to a port. So I have a succefful connection. Then the app reports error when I try to listen to incoming communication through the run() method

establish a connection and start:

Connection c = new Connection(host,port);
                Scanner chatConsole = new Scanner(System.in);
                String text = "";

                while (!text.equalsIgnoreCase("halt")){         
                    text = chatConsole.nextLine();
                    c.start();

Exception in thread “Thread-1” java.lang.NullPointerException

The run()

public void run(){//watch for incoming communication
    String msg;

    try{//loop reading lines and display msg
        while ((msg = in.readLine()) != null) {
            System.out.println(msg);
        }
    }catch (IOException e) {
        System.err.println(e);
    }   
}

This is the constructor for the class if its helpful to debug.

public Connection(String iniName, int iniPort){
        host = iniName;
        port = iniPort;

        try {
            server = new Socket(host, port);
        }catch (UnknownHostException e){
            System.err.println(e);
            System.exit(1);
        }

        try{
            stdIn = new BufferedReader(new InputStreamReader(System.in));
            PrintWriter out = new PrintWriter(server.getOutputStream(), true); //output stream to server
            BufferedReader in = new BufferedReader(new InputStreamReader(server.getInputStream()));
        }catch(Exception e){
            System.err.println(e);
        }
    }

Thank you

Answer 1

Your in variable is null, where you try to use it:

while ((msg = in.readLine()) != null) {

possibly because you’re shadowing it where you construct it.

BufferedReader in = new BufferedReader(new InputStreamReader(server.getInputStream()));

Instead don’t re-declare the variable locally:

in = new BufferedReader(new InputStreamReader(server.getInputStream()));