md5 strips arbitrary 0

Hi guys, I've been working with some people and they needed a basic md5 encryption of a number (plus a salt) to authenticate our users with their system.

So I quickly Googled md5 encryptions in java and I got the below code

java Code:

String sessionid = valueSentIn + "saltvalue";
 
byte[] defaultBytes = sessionid.getBytes();
            MessageDigest algorithm = MessageDigest.getInstance("MD5");
            algorithm.reset();
            algorithm.update(defaultBytes);
            byte messageDigest[] = algorithm.digest();
                    
            StringBuffer hexString = new StringBuffer();
            for (int i=0;i<messageDigest.length;i++) {
                hexString.append(Integer.toHexString(0xFF & messageDigest[i]));
            }           
            
            return hexString.toString();

The above code works about 50% of the time. The other 50% of the time, it trims out a random 0 somewhere in the output. The 0 can be anywhere and I wasn't able to find anything on Googling the error.

The representative I talked to (I had assumed the error was on their end, and the information was missing from their system) had mentioned that Java had a flaw with md5 that they knew about. The code they proposed I use is below.

java Code:

String sessionid = valueSentIn + "saltvalue";
MessageDigest md;
md = MessageDigest.getInstance("MD5");
md.update(sessionid.getBytes());
return convertToHex(md.digest());

java Code:

public static String convertToHex(byte[] data) {
        StringBuffer buf = new StringBuffer();
        for (int i = 0; i < data.length; i++) {
            int halfbyte = (data[i] >>> 4) & 0x0F;
            int two_halfs = 0;
            do {
                if ((0 <= halfbyte) && (halfbyte <= 9))
                    buf.append((char) ('0' + halfbyte));
                else
                    buf.append((char) ('a' + (halfbyte - 10)));
                halfbyte = data[i] & 0x0F;
            } while(two_halfs++ < 1);
        }
        return buf.toString();
    }

The only thing I can find that is different is that the 2nd set of code seems to be looking for a "halfbyte."

I was hoping someone here could explain to me, in a little more detail:

#1 why the first code stripped out the random value
and
#2 what the major difference between the code is

Any information at all would be extremely helpful.