Question

I’m learning to code in Java. I want to write simple chat with gui. So far my application works through command line. I’m interested to build up gui to client part. I have trouble connectiong gui to it. My question is do I have to write special class for gui and than construct such an object in client class and operate on it? In particular I have a problem with establishing communication between client and server via gui. My command line application code as for client part goes as follows. I would appreciate any advice on this matter.

public class Client {
    public static void main(String[] args) {
        try {
            Socket socket = new Socket("localhost", 4444);
            System.out.println("CLIENT: Server connected on port 4444");

            PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
            BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            System.out.println("CLIENT: IN and OUT streams opened. Starting sending data...");

            ClientInputThread thread = new ClientInputThread(socket);
            thread.start();

            String serverResponse;
            while ((serverResponse = in.readLine()) != null) {
                System.out.println("Server: " + serverResponse);
                if (serverResponse.equals("koniec")) {
                    break;
                }
            } 
            System.out.println("CLIENT: Ending server connection. Closing client streams and socket.");
            out.close();
            in.close();
            socket.close();
            System.exit(0);
        } 
        catch (UnknownHostException e) {
            System.err.println("CLIENT: Trying to connect to unknown host: " + e);
            System.exit(1);
        } 
        catch (Exception e) {
            System.err.println("CLIENT: Exception:  " + e);
            System.exit(1);
        }
    }
}

and

public class ClientInputThread extends Thread {
    private PrintWriter out;

    public ClientInputThread(Socket clientSocket) {
        try {
            out = new PrintWriter(clientSocket.getOutputStream(), true);
        } 
        catch (IOException e) {
            e.printStackTrace();
            System.exit(1);
        }
    }

    public void run() {
        try {    
            BufferedReader console = new BufferedReader(new InputStreamReader(System.in));
            String userInput="";    
            while (userInput != null) {
                userInput = console.readLine();
                out.println(userInput);
                out.flush();
                if (userInput.equals("koniec")) {
                    break;
                }
            }
            System.exit(0);
        } 
        catch (IOException e) {
            e.printStackTrace();
            System.exit(1);
        }
    }
}

Answer 1

The usual practice is to separate your logic from the GUI as much as possible. I’d create a class (or a number of classes) that implements the send/receive message part (looks like you’ve done that already).

These classes should provide public methods to send/receive messages, and probably be able to register listeners and notify them of incoming messages.

Then write a GUI class(es), register it as a listener with your server class and update the text once a MessageReceived event has occured. Event handling basics can be found here and here is an example of creating and handling custom events.

Example

//an interface that will let your server work with its listeners
interface MessageListener {
    public void messageSent();
}


class Server {
    List<MessageListener> listeners = new ArrayList<MessageListener>();

    //method to register listeners to be notified of incoming messages
    public void addListener(MessageListener toAdd) {
        listeners.add(toAdd);
    }

    public void sendMessage() {
        //code your logic here
        System.out.println("Message sent");

        // Notify everybody that may be interested.
        for (MessageListener hl : listeners)
            hl.messageSent();
    }

}

class GuiImplementation implements MessageListener {
    @Override
    public void messageSent() {
         System.out.println(message);
    }
}

and the main class:

class Test {
    public static void main(String[] args) {
        Server server = new Initiater();
        GuiImplementation gui = new Responder();

        //register gui as a listener for incoming/outgoing messages
        server.addListener(gui);

        //this will trigger the gui method to process incoming message
        server.sendMessage(); 
    }
}