[Java for Android App Development] Yusif Nurizade:
Hello,

I am a first time poster new to Android app development so please bear with me. I am currently working off of two great TCP/IP Client examples. My goal is to create a simple TCP/IP Client that connects to a server and, once a connection is established, continuously updates a TextView with strings passed from the server. If the user presses a button, the client sends a stop command to the server and I would ultimately like to expand the TextView into a graph with the converted string values. I am able to establish the connection to the server and send the stop command from the client but my attempts at adding the read capability have, so far, been unsuccessful with the app crashing as soon as it starts up.

Since it is my first time posting, I am not allowed to link the examples I am using. If anyone is interested in seeing them, however, they are posts from the android-er blog titled: "Android Server/Client example - client side using Socket" and "Bi-directional communication between Client and Server, using ServerSocket, Socket, DataInputStream and DataOutputStream".

Here is the code in my MainActivity:

package com.example.androidclient;

import java.io.ByteArrayOutputStream;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.Socket;
import java.net.UnknownHostException;

import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends ActionBarActivity {
TextView textResponse;
TextView textIn;
EditText editTextAddress, editTextPort;
Button buttonConnect, buttonStopTest, buttonDisconnect;
Socket socket = null;
DataInputStream incomingString = null;
DataOutputStream terminalLetter = null;

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

editTextAddress = (EditText)findViewById(R.id.address);
editTextPort = (EditText)findViewById(R.id.port);
buttonConnect = (Button)findViewById(R.id.connect);
buttonDisconnect = (Button)findViewById(R.id.disconnect);
buttonStopTest = (Button)findViewById(R.id.stop_test);
textResponse = (TextView)findViewById(R.id.response);
textIn = (TextView)findViewById(R.id.text_in);

buttonConnect.setOnClickListener(buttonConnectOnClickListener);

buttonStopTest.setOnClickListener(buttonStopTestOnClickListener);

buttonDisconnect.setOnClickListener(buttonDisconnectOnClickListener);
}

OnClickListener buttonConnectOnClickListener = new OnClickListener(){
//setOnClickListener sets a callback to be invoked when the button is clicked
@Override
public void onClick(View arg0) {
//Clicking button (Connect) calls a MyClientTask defined below.
MyClientTask myClientTask = new
MyClientTask(editTextAddress.getText().toString(),
Integer.parseInt(editTextPort.getText().toString()));
myClientTask.execute();
}
};

OnClickListener buttonStopTestOnClickListener = new OnClickListener(){
//setOnClickListener sets a callback to be invoked when the button is clicked
@Override
public void onClick(View arg0) {
try {
terminalLetter = new DataOutputStream(socket.getOutputStream());
terminalLetter.writeByte('b');
terminalLetter.writeByte('\n');
terminalLetter.close();
socket.close();
}
catch (IOException e) {
System.err.println("Couldn't get I/O for the connection.");
}
}
};

OnClickListener buttonDisconnectOnClickListener = new OnClickListener(){
//setOnClickListener sets a callback to be invoked when the button is clicked
@Override
public void onClick(View arg0) {
try {
socket.close();
}
catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
};

public class MyClientTask extends AsyncTask<Void, Void, Void> {

String IPaddress;
int Port;
String response = "";

MyClientTask(String addr, int port){
IPaddress = addr;
Port = port;
}

@Override
protected Void doInBackground(Void... arg0) {
//Took socket declaration from here.

try {
//Taking relevant parameters and applying them to socket
socket = new Socket(IPaddress, Port);
ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream(1024);
byte[] buffer = new byte[1024];
int bytesRead;

InputStream inputStream = socket.getInputStream();

incomingString = new DataInputStream(socket.getInputStream());
textIn.setText(incomingString.readUTF());

/*
* notice:
* inputStream.read() will block if no data return
*/
while ((bytesRead = inputStream.read(buffer)) != -1){
byteArrayOutputStream.write(buffer, 0, bytesRead);
response += byteArrayOutputStream.toString("UTF-8");
}
}
catch (UnknownHostException e) {
// TODO Auto-generated catch block
e.printStackTrace();
response = "UnknownHostException: " + e.toString();
}
catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
response = "IOException: " + e.toString();
}
finally{
if(socket != null){
try {
socket.close();
}
catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
return null;
}

@Override
protected void onPostExecute(Void result) {
textResponse.setText(response);
super.onPostExecute(result);
}

}

}

I put the commands for reading data from the server and writing it to the TextView with the code that sets up the connection to the socket. The idea was that it would be the next step after the connection is established but I realize that it looks like I am only reading once. I tried adding a while loop and bringing it out of the doInBackground. In each case, all I got was the whole app crashing on me. It looks fairly straight forward in the example but there the connection is automatic, not triggered and I have not been able to modify the code successfully.

I am still new to Android and this feels like it is an important part of app development so I am hoping someone in the community can help me understand where the section of code relevant to reading data (continuously) should be placed in relation to the rest.

Best,

Yusif Nurizade

P.S. I was going to include the fragment but, since the post is long enough, I chose to omit it. I can share upon request and the logcat.