93-convert-15digit-id-to-18digit-id.cls

public String convert15to18DigitId(String fifteenDigit)
{
    if (fifteenDigit == Null || fifteenDigit.length() != 15) {
        return '-1';
    }
    Id eightteenDigit = fifteenDigit;
    return eightteenDigit;
}