How can I connect Android devices using Socket Programming?
<p>I want to create an application for connecting multiple android devices.Here able to connect from device (Using Wifi Network) to another device(3G Network).I need to connect from device using 3G network to device using Wifi Network.How can I connect devices using 3G network to WiFi Network.</p>

<pre><code>Server Side:  
</code></pre>

<p>package com.server.org;</p>

<pre><code>import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.util.Log;
import android.widget.TextView;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.InetAddress;
import java.net.NetworkInterface;
import java.net.ServerSocket;
import java.net.Socket;
import java.net.SocketException;
import java.util.Enumeration;

public class ServerActivity extends Activity {

    private TextView serverStatus;

    // default ip
    public static String SERVERIP = "10.0.2.15";

    // designate a port
    public static final int SERVERPORT = 9191;

    private Handler handler = new Handler();

    private ServerSocket serverSocket;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        serverStatus = (TextView) findViewById(R.id.server_status);

        SERVERIP = getLocalIpAddress();

        Thread fst = new Thread(new ServerThread());
        fst.start();
    }

    public class ServerThread implements Runnable {

        public void run() {
            try {
                if (SERVERIP != null) {
                    handler.post(new Runnable() {

                        public void run() {
                            serverStatus.setText("Listening on IP: " + SERVERIP);
                        }
                    });
                    serverSocket = new ServerSocket(SERVERPORT);
                    while (true) {
                        // listen for incoming clients
                        Socket client = serverSocket.accept();
                        handler.post(new Runnable() {

                            public void run() {
                                serverStatus.setText("Connected.");

                            }
                        });

                        try {
                            BufferedReader in = new BufferedReader(new InputStreamReader(client.getInputStream()));
                            String line = null;
                            while ((line = in.readLine()) != null) {
                                Log.d("ServerActivity", line);
                                handler.post(new Runnable() {

                                    public void run() {
                                        // do whatever you want to the front end
                                        // this is where you can be creative
                                    }
                                });
                            }
                            break;
                        } catch (Exception e) {
                            handler.post(new Runnable() {

                                public void run() {
                                    serverStatus.setText("Oops. Connection interrupted. Please reconnect your phones.");
                                }
                            });
                            e.printStackTrace();
                        }
                    }
                } else {
                    handler.post(new Runnable() {

                        public void run() {
                            serverStatus.setText("Couldn't detect internet connection.");
                        }
                    });
                }
            } catch (Exception e) {
                handler.post(new Runnable() {

                    public void run() {
                        serverStatus.setText("Error");
                    }
                });
                e.printStackTrace();
            }
        }
    }

    // gets the ip address of your phone's network
    private String getLocalIpAddress() {
        try {
            for (Enumeration&lt;NetworkInterface&gt; en = NetworkInterface.getNetworkInterfaces(); en.hasMoreElements();) {
                NetworkInterface intf = en.nextElement();
                for (Enumeration&lt;InetAddress&gt; enumIpAddr = intf.getInetAddresses(); enumIpAddr.hasMoreElements();) {
                    InetAddress inetAddress = enumIpAddr.nextElement();
                    if (!inetAddress.isLoopbackAddress()) { return inetAddress.getHostAddress().toString(); }
                }
            }
        } catch (SocketException ex) {
            Log.e("ServerActivity", ex.toString());
        }
        return null;
    }

    @Override
    protected void onStop() {
        super.onStop();
        try {
             // make sure you close the socket upon exiting
             serverSocket.close();
         } catch (IOException e) {
             e.printStackTrace();
         }
    }

}


Client Side:
</code></pre>

<p>package com.client.org;</p>

<pre><code>import java.io.BufferedWriter;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.InetAddress;
import java.net.Socket;
import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;

public class ClientActivity extends Activity {

    private EditText serverIp;

    private Button connectPhones;

    private String serverIpAddress = "";

    private boolean connected = false;

    private Handler handler = new Handler();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        serverIp = (EditText) findViewById(R.id.server_ip);
        connectPhones = (Button) findViewById(R.id.connect_phones);
        connectPhones.setOnClickListener(connectListener);
    }

    private OnClickListener connectListener = new OnClickListener() {

        @Override
        public void onClick(View v) {
            if (!connected) {
                serverIpAddress = serverIp.getText().toString();
                if (!serverIpAddress.equals("")) {
                    Thread cThread = new Thread(new ClientThread());
                    cThread.start();
                }
            }
        }
    };

    public class ClientThread implements Runnable {

        public void run() {
            try {
                InetAddress serverAddr = InetAddress.getByName(serverIpAddress);
                Log.d("ClientActivity", "C: Connecting...");
                int SERVERPORT = 9191;
                Socket socket = new Socket(serverAddr,SERVERPORT );
                connected = true;
                while (connected) {
                    try {
                        Log.d("ClientActivity", "C: Sending command.");
                        PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(socket
                                    .getOutputStream())), true);
                            // where you issue the commands
                            out.println("Hey Server!");
                            Log.d("ClientActivity", "C: Sent.");
                    } catch (Exception e) {
                        Log.e("ClientActivity", "S: Error", e);
                    }
                }
                socket.close();
                Log.d("ClientActivity", "C: Closed.");
            } catch (Exception e) {
                Log.e("ClientActivity", "C: Error", e);
                connected = false;
            }
        }
    }
</code></pre>
