Convert unsigned long to BigInteger in Java

From the OpenJDK implementation of Long:

/**
 * Return a BigInteger equal to the unsigned value of the argument.
 */
private static BigInteger toUnsignedBigInteger(long i) {
    if (i >= 0L) {
        return BigInteger.valueOf(i);
    } else {
        int upper = (int) (i >>> 32);
        int lower = (int) i;
         // return (upper << 32) + lower
        return BigInteger.valueOf(Integer.toUnsignedLong(upper))
                .shiftLeft(32)
                .add(BigInteger.valueOf(Integer.toUnsignedLong(lower)));
    }
}