Tutorial for telnet

Question

package project.robot.network;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;

import project.robot.BluetoothConnection;
import android.os.AsyncTask;

public class TCPServer extends AsyncTask<Void, Integer, Void> {
    int port;               //Port on which server is running
    String clientIP;        //IP address of remote client
    ServerSocket serverSocket;  //Server Socket
    Socket clientSocket;    //Socket connected to client
    DataOutputStream out;   //Output stream object to send data
    DataInputStream in;         //Input Stream object to receive data

    boolean flag;
    boolean videoFlag;  //Used to toggle video
    private VideoThread vthread;    //Video Thread Object

    public static BluetoothConnection conn;

    /*
     * Starts a TCP Server which listens to incoming connections
     */
    public TCPServer(int port) {
        this.port = port;
        videoFlag = false;
        vthread = null;
    }

    @Override
    protected Void doInBackground(Void... arg0) {
        int msg;    

        //Initiating server socket
        try {
            serverSocket = new ServerSocket(port);
            flag = true;
        } catch (IOException e1) {
            // TODO Auto-generated catch block
            System.out.println("Tcpserver: unable to bind socket");
            e1.printStackTrace();
            flag = false;
        }

        System.out.println("TCPServer: Server started");

        while(flag) {
            try {
                //Accepting incoming connection
                clientSocket = serverSocket.accept();
                clientIP = clientSocket.getInetAddress().getHostAddress();
                System.out.println("TCPServer: Connected to client at " + clientIP);

                //Getting input and output streams
                in = new DataInputStream(clientSocket.getInputStream());
                out = new DataOutputStream(clientSocket.getOutputStream());
                msg = 1;

                //Start VideoThread
                try {
                    vthread = new VideoThread(clientIP);
                    vthread.startVideo();
                    System.out.println("TCPServer: Video started");
                    videoFlag = true;
                } catch (Exception e) {
                    e.printStackTrace();
                    vthread = null;
                    System.out.println("TCPServer: Error while starting video");
                }

                //Reading input commands and signaling processing function
                while(flag && msg != 0) {
                    msg = in.readInt();
                    publishProgress(msg);
                }

                //Stopping video streaming if running
                if(vthread != null) {
                    vthread.stopVideo();
                    vthread = null;
                }
                System.out.println("TCPServer: Video stopped");

                //Closing Connection to client
                clientSocket.close();
                clientSocket = null;
                System.out.println("TCPServer: Closed connection to host at " + clientIP);
                clientIP = "null";

            } catch (IOException e) {
                if(clientSocket != null) {
                    clientSocket = null;
                }
                flag = false;
                System.out.println("TCPServer: Error while accepting or closing client connection");
            }
        }

        //Closing serverSocket
        if(serverSocket != null) {
            try {
                serverSocket.close();
                System.out.println("TCPServer: Server Socket Closed");
            } catch (IOException e) {
                e.printStackTrace();
                System.out.println("TCPServer: Error while closing socket");
            }
            serverSocket = null;
        }

        System.out.println("TCPServer: Server stopped");
        return null;
    }

    private void sendSignal(int signal) {
        if(conn != null)
            conn.send(signal);
        else
            System.out.println("TCPServer: null conn, can't send value");
    }


    @Override
    protected void onProgressUpdate(Integer... integers) {
        //Method is called every time a publishProgress is called from doInBackground
        for(Integer integer : integers) {
            System.out.println("TCPServer: Message received - " + integer);
            switch(integer) {
            case 1:
                //Enable SMS service
                if(vthread != null)
                    vthread.msgFlag = true;
                break;
            case 2:
                //Move backward
                sendSignal(2);
                break;
            case 4:
                //Move left
                sendSignal(4);
                break;
            case 5:
                //Stop
                sendSignal(5);
                break;
            case 6:
                //Move right
                sendSignal(6);
                break;
            case 7:
                //Buzzer toggle
                sendSignal(7);
            case 8:
                //Move forward
                sendSignal(8);
                break;
            case 9:
                //Toggle Video
                toggleVideo();
                break;
            default:
                System.out.println("TCPServer: Unrecognized instruction : " + integers[0]);
                break;
            }
        }
    }

    /*
     * Toggles video streaming state
     * if it was on then it will stop it
     * else it will start video streaming
     */
    private void toggleVideo() {
        //If videoFlag is true then stop Video else start video
        if(vthread==null)
            return;

        if(vthread.videoStream) {
            vthread.videoStream = false;
            System.out.println("TCPServer: Video streamming stopped");
        } else {
            System.out.println("TCPServer: Video streamming started");
            vthread.videoStream = true;
        }
    }


    /*
     * `s the server process
     */
    public void stop() {
        System.out.println("TCPServer: Stopping server");
        flag = false;

        //Stopping video if it is running
        if(vthread != null) {
            vthread.stopVideo();
            vthread = null;
        }
        System.out.println("TCPServer: Video stopped");

        //Closing server socket
        if(serverSocket != null) {
            try {
                serverSocket.close();
                System.out.println("TCPServer: Server Socket Closed");
            } catch (IOException e) {
                e.printStackTrace();
                System.out.println("TCPServer: Error while closing socket");
            }
            serverSocket = null;
        }
    }
}

Please someone tell me how telnet works(or tell me where me i can find a tutorial on that). I had got the above code. I have to change the above code(which works for blutooth) into telnet code(which works for wifi).

Answer 1

Your question is a bit weird, in the sense that it would be like asking "Tutorial for Browser", showing piece of code that downloads a webpage, and asking how a browser works.

Telnet is just an application (like a browser) that can connect to a server and send an receive text. This piece of code also is an application that connects to a server and sends an receives stuff (after a quick glance, mainly integers).

A lot of the code mainly deals with making the connection and passing parameters. The stuff that will interest you is the data connection:

            //Getting input and output streams
            in = new DataInputStream(clientSocket.getInputStream());
            out = new DataOutputStream(clientSocket.getOutputStream());

Later on

            //Reading input commands and signaling processing function
            while(flag && msg != 0) {
                msg = in.readInt();
                publishProgress(msg);
            }

And later

private void sendSignal(int signal) {
    if(conn != null)
        conn.send(signal);
    else
        System.out.println("TCPServer: null conn, can't send value");
}

You should learn about Socket programming in java (tutorial here) first. After you understand that, you'll see why your question is a bit weird. You can also start to figure out how to write the code to connect using another protocol. And yes, you can use telnet to try it out manually first, before writing code.

If you really need a "telnet tutorial", you can download any RFC that uses a plain-text connection, and just simulate the protocol on telnet. HTTP, FTP, SMTP and IRC are easy ones to start with. Try sending an e-mail with telnet.