In this step we will create the serial connection that attaches the android app and the arduino together. In addition there will be buttons that will call functions to initiate specific tasks like sending, closing, and opening the connection.

MainActivity.java

public class MainActivity extends AppCompatActivity {






    //--------------------------------
    //      Serial Connection
    //-------------------------------

    public final String ACTION_USB_PERMISSION = "com.hariharan.arduinousb.USB_PERMISSION";
    Button startButton, sendButton, orderButton, clearButton, stopButton;
    TextView textView;
    EditText editText;
    UsbManager usbManager;
    UsbDevice device;
    UsbSerialDevice serialPort;
    UsbDeviceConnection connection;

    UsbSerialInterface.UsbReadCallback mCallback = new UsbSerialInterface.UsbReadCallback() { //Defining a Callback which triggers whenever data is read.
        @Override
        public void onReceivedData(byte[] arg0) {
            String data = null;
            try {
                data = new String(arg0, "UTF-8");
                data.concat("/n");
                tvAppend(textView, data);
            } catch (UnsupportedEncodingException e) {
                e.printStackTrace();
            }


        }
    };
    private final BroadcastReceiver broadcastReceiver = new BroadcastReceiver() { //Broadcast Receiver to automatically start and stop the Serial connection.
        @Override
        public void onReceive(Context context, Intent intent) {
            if (intent.getAction().equals(ACTION_USB_PERMISSION)) {
                boolean granted = intent.getExtras().getBoolean(UsbManager.EXTRA_PERMISSION_GRANTED);
                if (granted) {
                    connection = usbManager.openDevice(device);
                    serialPort = UsbSerialDevice.createUsbSerialDevice(device, connection);
                    if (serialPort != null) {
                        if (serialPort.open()) { //Set Serial Connection Parameters.
                            setUiEnabled(true);
                            serialPort.setBaudRate(9600);
                            serialPort.setDataBits(UsbSerialInterface.DATA_BITS_8);
                            serialPort.setStopBits(UsbSerialInterface.STOP_BITS_1);
                            serialPort.setParity(UsbSerialInterface.PARITY_NONE);
                            serialPort.setFlowControl(UsbSerialInterface.FLOW_CONTROL_OFF);
                            serialPort.read(mCallback);
                            tvAppend(textView,"Serial Connection Opened!\n");

                        } else {
                            Log.d("SERIAL", "PORT NOT OPEN");
                        }
                    } else {
                        Log.d("SERIAL", "PORT IS NULL");
                    }
                } else {
                    Log.d("SERIAL", "PERM NOT GRANTED");
                }
            } else if (intent.getAction().equals(UsbManager.ACTION_USB_DEVICE_ATTACHED)) {
                onClickStart(startButton);
            } else if (intent.getAction().equals(UsbManager.ACTION_USB_DEVICE_DETACHED)) {
                onClickStop(stopButton);

            }
        }

        ;
    };