Compile Error - Cannot Cast From Integer To Byte

I have a msg object that contains an ArrayList<Integer> collection. However, in order to send the elements in the array over the udp socket, it needs to be sent as a byte[] array. So why am I using ArrayList<Integer> over byte array in first place? Well when I receive data from socket from embedded c program, I need to get an unsigned representation of the data, and thus I need to store it in integers, since bytes in Java are unsigned and unsigned chars in c that are greater than 127 will yield incorrect values in java. But when I send an ack back over the socket, I need to send the data back as bytes. So I convert the ArrayList<Integer> to a byte array:

Java Code: byte[] data = msg.toByteArray();
DatagramPacket response = new DatagramPacket(data, data.length,
packet.getAddress(), packet.getPort());
public class Gprs {
...
public byte[] toByteArray(){

[Code] ....

The problem is I get an "Cannot cast from Integer to byte" when trying to cast the integer to byte: data[i] = (byte)m_data.get(i);

How can I resolve this?