Question

I need to prepend the string “00” or byte 0x00 to the beginning of a byte array? I tried to do it with a for loop but when I convert it to hex it doesn’t show up in the front.

Answer 1

The string “00” is different than the number 0x00 when converted to Bytes. What is the data type you are trying to prepend to your byte array? Assuming it’s a Byte representation of the string “00”, try the following:

bytes[] orig = <your byte array>;  
String prepend = "00";  
bytes[] prependBytes = prepend.getBytes();  
bytes[] output = new Bytes[prependBytes.length + orig.length];

for(i=0;i<prependBytes.length;i++){
    output[i] = prependBytes[i];
}

for(i=prependBytes.length;i<(orig.length+prepend.lenth);i++){
  output[i] = orig[i];
}

or you can use Arrays.copy(…) instead of the two for loops as mentioned before to do the prepending. See How to combine two byte arrays

Alternativly, if you are trying to literally prepend 0 to your byte array, decalare prependBytes in the following way and use the same algorithm

byte[] prependBytes = new byte[]{0,0};

Also you say that you’re converting your byte array to hex, and that may truncate leading zeros. To test this, try prepending the follwoing and converting to hex and see if there is a different output:

byte[] prependBytes = new byte[]{1,1};

If it is removing the leading zeros that you want, you may wish to convert your hex number to a string and format it.