In Java, how do I convert a byte array to a string of hex digits while keeping leading zeros?

Check out Hex.encodeHexString from Apache Commons Codec.

import org.apache.commons.codec.binary.Hex;

String hex = Hex.encodeHexString(bytes);

You can use the one below. I tested this with leading zero bytes and with initial negative bytes as well

public static String toHex(byte[] bytes) {
    BigInteger bi = new BigInteger(1, bytes);
    return String.format("%0" + (bytes.length << 1) + "X", bi);
}

If you want lowercase hex digits, use "x" in the format String.

A simple approach would be to check how many digits are output by Integer.toHexString() and add a leading zero to each byte if needed. Something like this:

public static String toHexString(byte[] bytes) {
    StringBuilder hexString = new StringBuilder();

    for (int i = 0; i < bytes.length; i++) {
        String hex = Integer.toHexString(0xFF & bytes[i]);
        if (hex.length() == 1) {
            hexString.append('0');
        }
        hexString.append(hex);
    }

    return hexString.toString();
}