Why does the same bitwise calculation yield different results in Java and JavaScript?

Question

Problem:

I need to calculate a CRC16 for hex codes like this one:

08010000016B40D8EA30010000000000000000000000000000000105021503010101425E0F01F10000601A014E000000000000000001

For this, I have a working solution in JavaScript. Due to new requirements, I have to translate this code into Java.

Expected result:

For above hex string: 0000C7CF ( 51151 ).

My approach:

I have the following, working JavaScript code, which I am trying to translate into Java:

var hex = "08010000016B40D8EA30010000000000000000000000000000000105021503010101425E0F01F10000601A014E000000000000000001";

var str = '';
for (var i = 0; i < hex.length; i += 2){ 
  str += String.fromCharCode( parseInt(hex.substr(i, 2), 16) );
}

var crc = 0x0000;
var poly = 0xA001;

for (var pos = 0; pos < str.length; pos++) {
  crc ^= str.charCodeAt(pos);
  for (var i = 8; i !== 0; i--) {
    if ((crc & 0x0001) !== 0) {
      crc >>= 1;
      crc ^= poly;
    } else
      crc >>= 1;
  }
}
console.log( crc ); // 51151
console.log( crc.toString(16) ); // c7cf

My current implementation in Java looks like this:

String hex = "08010000016B40D8EA30010000000000000000000000000000000105021503010101425E0F01F10000601A014E000000000000000001";
byte[] arr = Hex.decodeHex( hex );
long polynomial = 0xA001;
long crc = 0x0000;
for (byte b : arr) {
    crc ^= b;
    for (int i = 8; i != 0; i--) {
        if ((crc & 0x01) != 0) {
            crc >>= 1;
            crc ^= polynomial;
        } else {
            crc >>= 1;
        }
    }
}
System.out.println( crc ); // -37776

As you can see, the translated Java code does not calculate the expected result.

Question:

Why does this code yield different results in Java and JavaScript?

Answer 1

Change the following line from

crc ^= b

to

crc ^= (b & 0xff)

In java, byte ranges from -128 to 127, so you have to make it unsigned.