NullPointerException for Bluetooth mConnectedThread

Question

Why is there a NullPointerException error for mConnectedThread? The Bluetooth connection between the phone and remote device is established.

Here is the main part of the code where the error is:

public void write(byte[] out) {
        // Create temporary object
        ConnectedThread r;
        // Synchronize a copy of the ConnectedThread
        synchronized (this) {
            r = mConnectedThread;
            System.out.println(out);
        }
        // Perform the write unsynchronized
        r.write(out);
        System.out.println(out);
    }

ConnectedThread.java code:

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

        public ConnectedThread(BluetoothSocket socket) {
            mmSocket = socket;
            InputStream tmpIn = null;
            OutputStream tmpOut = null;

            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() {
            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);

                    // Send the obtained bytes to the UI Activity
                    mHandler.obtainMessage(MenuActivity.MESSAGE_READ, bytes, -1, buffer).sendToTarget();
                } catch (IOException e) {
                    Log.e(TAG, "disconnected", e);
                    connectionLost();
                    break;
                }
            }
        }

        /**
         * Write to the connected OutStream.
         * @param buffer  The bytes to write
         */
        public void write(byte[] buffer) {
            try {
                mmOutStream.write(buffer);
                System.out.println("Sent");
                // Share the sent message back to the UI Activity
                //mHandler.obtainMessage(MenuActivity.MESSAGE_WRITE, -1, -1, buffer).sendToTarget();
            } catch (IOException e) {
                Log.e(TAG, "Exception during write", e);
            }
        }

Answer 1

Theres the problem shannon.

You have only declared mConnectedThread by doing,

private ConnectedThread mConnectedThread;

Its yet not initialized and by default its value is null which you have assigned to

 r = mConnectedThread; 

hence r is also null. Thus doing,

r.write(out);

generated the NullPinterException.

Make sure you have initiazed the mConnectedThread variable.