public class TcpClient {

    public static final String SERVER_IP = "192.168.1.8"; //server IP address
    public static final int SERVER_PORT = 1234;
    // message to send to the server
    private String mServerMessage;
    // sends message received notifications
    private OnMessageReceived mMessageListener = null;
    // while this is true, the server will continue running
    private boolean mRun = false;
    // used to send messages
    private PrintWriter mBufferOut;
    // used to read messages from the server
    private BufferedReader mBufferIn;

    /**
     * Constructor of the class. OnMessagedReceived listens for the messages received from server
     */
    public TcpClient(OnMessageReceived listener) {
        mMessageListener = listener;
    }

    /**
     * Sends the message entered by client to the server
     *
     * @param message text entered by client
     */
    public void sendMessage(final String message) {
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                if (mBufferOut != null) {
                    Log.d(TAG, "Sending: " + message);
                    mBufferOut.println(message + "rn");
                    mBufferOut.flush();
                }
            }
        };
    Thread thread = new Thread(runnable);
    thread.start();
}

    /**
     * Close the connection and release the members
     */
    public void stopClient() {

        mRun = false;

        if (mBufferOut != null) {
            mBufferOut.flush();
            mBufferOut.close();
        }

        mMessageListener = null;
        mBufferIn = null;
        mBufferOut = null;
        mServerMessage = null;
    }

    public void run() {

        mRun = true;

        try {
            //here you must put your computer's IP address.
            InetAddress serverAddr = InetAddress.getByName(SERVER_IP);

            Log.e("TCP Client", "C: Connecting...");

            //create a socket to make the connection with the server
            Socket socket = new Socket(serverAddr, SERVER_PORT);

            try {

                //sends the message to the server
                mBufferOut = new PrintWriter(new BufferedWriter(new OutputStreamWriter(socket.getOutputStream())), true);

                //receives the message which the server sends back
                mBufferIn = new BufferedReader(new InputStreamReader(socket.getInputStream()));


                //in this while the client listens for the messages sent by the server
                while (mRun) {

                    mServerMessage = mBufferIn.readLine();

                    if (mServerMessage != null && mMessageListener != null) {
                        //call the method messageReceived from MyActivity class
                        mMessageListener.messageReceived(mServerMessage);
                    }

                }

                Log.e("RESPONSE FROM SERVER", "S: Received Message: '" + mServerMessage + "'");

            } catch (Exception e) {

                Log.e("TCP", "S: Error", e);

            } finally {
                //the socket must be closed. It is not possible to reconnect to this socket
                // after it is closed, which means a new socket instance has to be created.
                socket.close();
            }

        } catch (Exception e) {

            Log.e("TCP", "C: Error", e);

        }

    }

    //Declare the interface. The method messageReceived(String message) will must be implemented in the MyActivity
    //class at on asynckTask doInBackground
    public interface OnMessageReceived {
        public void messageReceived(String message);
    }

}

public class MainActivity extends Activity {

    TcpClient mTcpClient;

    //............

public class ConnectTask extends AsyncTask<String, String, TcpClient> {

    @Override
    protected TcpClient doInBackground(String... message) {

        //we create a TCPClient object
        mTcpClient = new TcpClient(new TcpClient.OnMessageReceived() {
            @Override
            //here the messageReceived method is implemented
            public void messageReceived(String message) {
                //this method calls the onProgressUpdate
                publishProgress(message);
            }
        });
        mTcpClient.run();

        return null;
    }

    @Override
    protected void onProgressUpdate(String... values) {
        super.onProgressUpdate(values);
        //response received from server
        Log.d("test", "response " + values[0]);
        //process server response here....

}

new ConnectTask().execute("");

//sends the message to the server
if (mTcpClient != null) {
    mTcpClient.sendMessage("testing");
}

if (mTcpClient != null) {
    mTcpClient.stopClient();
}

mRun = true;

try {
    InetAddress serverAddr = InetAddress.getByName(SERVER_IP);
    Log.e("TCP Client", "C: Connecting...");
    Socket socket = new Socket(serverAddr, SERVER_PORT);
    try {
        mBufferOut = new PrintWriter(socket.getOutputStream());
        Log.e("TCP Client", "C: Sent.");
        mBufferIn = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        int charsRead = 0; char[] buffer = new char[1024]; //choose your buffer size if you need other than 1024

        while (mRun) {
            charsRead = mBufferIn.read(buffer);
            mServerMessage = new String(buffer).substring(0, charsRead);
            if (mServerMessage != null && mMessageListener != null) {
                mMessageListener.messageReceived(mServerMessage);}
            mServerMessage = null;
        }
        Log.e("RESPONSE FROM SERVER", "S: Received Message: '" + mServerMessage + "'");

@Override
protected void onProgressUpdate(String... values) {
        super.onProgressUpdate(values);
        Log.d("test", "response " + values[0]);
        response.setText(response.getText() + "/n" +values[0]);
}

public class MainActivity extends AppCompatActivity {
TextView response; //...so on