deliverMessage():
public interface Postman {
    void deliverMessage(String msg);
}

public class PostmanImpl implements Postman {

    PRivate PrintStream output;
    
    public PostmanImpl() {
        output = System.out;
    }
    
    public void deliverMessage(String msg) {
        output.println(" Postman  " + msg);
        output.flush();
    }
}

public class PostmanApp {

    public static void main(String[] args) throws Exception {
        BufferedReader sysin = new BufferedReader(new InputStreamReader(System.in));

        // OBTain a Postman instance
        Postman postman = getPostman();

        while (true) {
            System.out.print("Enter a message: ");
            String msg = sysin.readLine();
            postman.deliverMessage(msg);
        }
    }

    private static Postman getPostman() {
        // Omit for now, will come back later
    }
}

// MODIFIED VERSION
public class PostmanImpl implements Postman {

    private PrintStream output;
    
    // Start of modification
    public PostmanImpl() throws IOException {
        output = new PrintStream(new FileOutputStream("msg.txt"));
    }
    // End of modification

    public void deliverMessage(String msg) {
        output.println(" Postman  " + msg);

        output.flush();
    }
}