creating connection within android device using sockets

Question

I am trying to create a connection between two applications using android. I have tried to use sockets to connect. I have created two application One which accepts the connection when client wants to connect and another application that requests to connect.

I have successfully run this code in same manner in pc in java in pc network. Is the way of connecting to android also same?

My server class implementation

public class MainActivity extends AppCompatActivity {

    private Button startButton;
    private ServerSocket server;
    private Socket connection;
    private TextView statusText;
    private ObjectOutputStream output;
    private ObjectInputStream input;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
statusText=(TextView)findViewById(R.id.statusText);
        startButton=(Button)findViewById(R.id.startButton);
        startButton.setOnClickListener(
                new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {

                        startRunning();


                    }
                }
        );



    }

    private void startRunning() {

        try{

            server=new ServerSocket(8080);
            while(true)
            {

                try{

                    waitForConnection();
                    setUpStreams();
                    whileChatting();



                }catch(Exception e){}

            }



        } catch(Exception e){}
    }


    public void waitForConnection()
    {

       setMyStatus("Waiting for client to connect...");
        try {
            connection= server.accept();
           setMyStatus("Now connected to "+ connection.getInetAddress().getHostName());


        } catch (IOException e) {
            e.printStackTrace();
        }


    }


    public void setUpStreams()
    {
        try {
             output= new ObjectOutputStream(connection.getOutputStream());
             output.flush();
             input= new ObjectInputStream(connection.getInputStream());
            setMyStatus("Streams are now setup. Ready to go...");



        } catch (IOException e) {
            e.printStackTrace();
        }


    }
    public void whileChatting()
    {
setMyStatus("You can now start chatting...");


    }
    public void setMyStatus(String msg) {
        statusText.setText(msg);
    }


}

I will use the tasks like async task later. But i am just trying to set up connection in these application. My client implementation goes like this

public class MainActivity extends AppCompatActivity {
private Button startButton;
private Socket connection;
private TextView statusText;
public ObjectOutputStream output;
public ObjectInputStream input;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    statusText=(TextView)findViewById(R.id.statusText);
    startButton=(Button)findViewById(R.id.startButton);
    startButton.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            startRunning();
        }
    });

}





private void startRunning() {

   connectToServer();
    setUpStreams();
    whileChatting();

}


public void  connectToServer()
{

  setMyStatus("Connecting to server. Please wait...");
    try {
        connection= new Socket(InetAddress.getByName("127.0.0.1"),8080);
        setMyStatus("Connected to Server."+ connection.getInetAddress().getHostName());


    } catch (IOException e) {
        e.printStackTrace();
    }

}


public void setUpStreams()
{
    try {
        output= new ObjectOutputStream(connection.getOutputStream());
        output.flush();
        input= new ObjectInputStream(connection.getInputStream());
        setMyStatus("Streams are now setup. Ready to go...");



    } catch (IOException e) {
        e.printStackTrace();
    }


}
public void whileChatting()
{
    setMyStatus("You can now start chatting...");


}
public void setMyStatus(String msg) {
    statusText.setText(msg);
}



}

The error I got is in the case of client that tries to connect to server

 java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.OutputStream java.net.Socket.getOutputStream()' on a null object reference
                                                                             at com.myapp.client.clientdemo.MainActivity.setUpStreams(MainActivity.java:71)
                                                                             at com.myapp.client.clientdemo.MainActivity.startRunning(MainActivity.java:46)
                                                                             at com.myapp.client.clientdemo.MainActivity.access$000(MainActivity.java:16)
                                                                             at com.myapp.client.clientdemo.MainActivity$1.onClick(MainActivity.java:33)

Answer 1

It's because "connection" was never created. Probably because it failed to connect or it timed out and it moved on to the next code. If it timed out or didn't connect, then you need to catch that exception and retry to connect before moving on with your later code.

Socket can be tricky if you do not catch all the erroneous states they can get into.

simply wrap your try and catch with a while loop until the condition is met like this:

while(!connected & !closed){
    try {
        connection= new Socket(InetAddress.getByName("127.0.0.1"),8080);
        connected = true;
        //now it is connected and will exit the loop and continue with other code
    } catch (SocketTimeoutException socketTimeoutException) {
        //add this for safety
    } catch (IOException e) {
        e.printStackTrace();
    }
}