Question

I am trying to adapt the Android BluetoothChat example to save the InputStream to file using openFileOutput from within the BluetoothChatService class. When using the Environment.getExternalStorageDirectory and saving to the sdcard using filewriter there is no problem, but using the openFileOutput method with MODE_PRIVATE returns a nullPointerException as mentioned in several other questions/answers. I cant for the life of me figure out the correct way to get the correct context.

   private class ConnectedThread extends Thread {
        private final BluetoothSocket mmSocket;
        private final InputStream mmInStream;
        private final OutputStream mmOutStream;
        private Context mContext;

        public ConnectedThread(BluetoothSocket socket) {
            Log.d(TAG, "create ConnectedThread");
            mmSocket = socket;
            InputStream tmpIn = null;
            OutputStream tmpOut = null;

            // Get the BluetoothSocket input and output streams
            try {
                tmpIn = socket.getInputStream();
                tmpOut = socket.getOutputStream();
            } catch (IOException e) {
                Log.e(TAG, "temp sockets not created", e);
            }

            mmInStream = tmpIn;
            mmOutStream = tmpOut;
        }

        public void run() {
            Log.i(TAG, "BEGIN mConnectedThread");
            byte[] buffer = new byte[1024];
            int bytes;

            // Keep listening to the InputStream while connected
            while (true) {
                try {
                    // Read from the InputStream
                    bytes = mmInStream.read(buffer);
                    // Save to file
                    String FILENAME = "chat.txt";

                    FileOutputStream fos = mContext.openFileOutput(FILENAME, Context.MODE_PRIVATE);
                    fos.write(bytes);
                    fos.close();
                    // Send the obtained bytes to the UI Activity
                    mHandler.obtainMessage(BluetoothChat.MESSAGE_READ, bytes, -1, buffer)
                            .sendToTarget();
                } catch (IOException e) {
                    Log.e(TAG, "disconnected", e);
                    connectionLost();
                    break;
                }
            }
        }

Answer 1

Well, right now mContext is null. You’re not assigning anything to it anywhere. You could change your constructor signature to pass it in:

public ConnectedThread(Context context, BluetoothSocket socket) {
    this.mContext = context;
    ...
}

If you’re actually following the example, you could do the same type of thing in the BluetoothChatService… note that they don’t do anything with the context passed in. Store that context off in a field and it would also be accessible to the ConnectedThread.