why it returns null pointer exception (server side )

Question

Hi, this is my server class which let the clients to chat with each other but it will return nullpointer exception for this line: while (!(line = in.readLine()).equalsIgnoreCase("/quit")) would you please help me?thanks.

my ChatHandler class:

 final static Vector handlers = new Vector(10);
private Socket socket;
private BufferedReader in;
private PrintWriter out;

public ChatHandler(Socket socket) throws IOException {
    this.socket = socket;
    in = new BufferedReader(
            new InputStreamReader(socket.getInputStream()));
    out = new PrintWriter(
            new OutputStreamWriter(socket.getOutputStream()));
}

@Override
public void run() {
    String line;

    synchronized (handlers) {
        handlers.addElement(this);
    // add() not found in Vector class
    }
    try {
        while (!(line = in.readLine()).equalsIgnoreCase("/quit")) {
            for (int i = 0; i < handlers.size(); i++) {
                synchronized (handlers) {
                    ChatHandler handler =
                            (ChatHandler) handlers.elementAt(i);
                    handler.out.println(line + "\r");
                    handler.out.flush();
                }
            }
        }
    } catch (IOException ioe) {
        ioe.printStackTrace();
    } finally {
        try {
            in.close();
            out.close();
            socket.close();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            synchronized (handlers) {
                handlers.removeElement(this);
            }
        }
    }
}

a part of client class:

  String teXt = MainClient.getText();

    os.println(teXt);
    os.flush();
    try {
        String line = is.readLine();



            setFromServertext("Text recieved:"+line+"\n");

        is.close();
        is.close();
        c.close();
    } catch (IOException ex) {
        Logger.getLogger(MainClient.class.getName()).log(Level.SEVERE, null, ex);
    }

Answer 1

in.readLine() will return null when there is nothing more to read. You need to change that to

String line;
while ((line = in.readLine()) != null) {
    if (!line.equalsIgnoreCase("/quit")) {

    }
}

Answer 2

This isn't the correct idiom.The BufferedReader#readLine() will return null when end of stream is reached.

Thus, the following

while (!(line = in.readLine()).equalsIgnoreCase("/quit")) {
    // Do stuff.
}

has to be replaced by:

while ((line = in.readLine()) != null && !line.equalsIgnoreCase("/quit")) {
    // Do stuff.
}

Also see Sun's own basic Java IO tutorials how to use a BufferedReader: http://java.sun.com/docs/books/tutorial/essential/io/