Byte Array Sign Extend

I have a checksum function that is suppose to read IPV4 packet and return a short integer value. The IPV4 packets are stored in a byte array. I am having trouble storing the first 8 bits and second 8 bits of the short integer into the byte arrays especially when they have leading 1s. For example, if my checksum returns 5571 (binary = 0001 0101 1100 0011) The first 8 bits is suppose to represent 195 but when I try to assign a larger integer type to a btye the information gets sign extended. The 195 turns into -61. I tried using bit addition like array[10] = array[10] & 0xff, but the result remains the same.

public static short checksum(byte [] a, int length) {
short sum = 0;
long data;
int i = 0;
while(length > 1) {
data = (((a[i] << 8) & 0xff00) | ((a[i + 1]) & 0xff));
sum += data;

[code]....