It's just becoming ridiculous...
public static Number convertNumberToOtherNumberSystem(Number number, NumberSystem expectedNumberSystem) {
checkDataIsValid(number);
BigInteger result = to10NumberSystem(number);
return from10TOExpectedNumberSystem(result, expectedNumberSystem);
}

private static Number from10TOExpectedNumberSystem(BigInteger number, NumberSystem expectedNumberSystem) {
if (NumberSystemType._10.equals(expectedNumberSystem)) {
return new Number(expectedNumberSystem, number.toString());
} else {
BigInteger divider = new BigInteger("" + expectedNumberSystem.getNumberSystemIntValue());
BigInteger dividend = new BigInteger(number.toString());
String result = "";
//number is current
while (dividend.compareTo(BigInteger.ZERO) > 0) {
BigInteger integer = dividend.divide(divider);
String s = dividend.subtract(integer.multiply(divider)).toString();
result = (s.length() == 1 ? s : getChar(s)) + result;
dividend = integer;
}
return new Number(expectedNumberSystem, result);
}
}

private static String getChar(String s) {
return "" + (char) (Integer.parseInt(s) + 87);
}

private static void checkDataIsValid(Number number) {
if (number.getNumberSystem().getNumberSystemIntValue() <= 10) {
checkDigitsLimited(number.getDigit(), number.getNumberSystem().getNumberSystemIntValue() + 48);
} else {
checkCharsLimited(number.getDigit(), number.getNumberSystem().getNumberSystemIntValue() + 87);
}
}

private static void checkCharsLimited(String digit, int value) {
for (char c : digit.toCharArray()) {
if (!(c >= 48 && c <= 57) && !(c >= 97 && c < value)) {
throw new NumberFormatException();
}
}

}

private static void checkDigitsLimited(String digit, int value) {
for (char c : digit.toCharArray()) {
if (!(c >= 48 && c < value)) {
throw new NumberFormatException();
}
}
}

private static BigInteger to10NumberSystem(Number number) {
BigInteger result;
if (NumberSystemType._10.equals(number.getNumberSystem())) {
result = new BigInteger(number.getDigit());
} else {
result = BigInteger.ZERO;
char[] digits = number.getDigit().toLowerCase().toCharArray();

//to _10 number system
BigInteger power = new BigInteger("" + number.getNumberSystem().getNumberSystemIntValue());
BigInteger multiplier = BigInteger.ONE;

for (int i = digits.length - 1; i >= 0; i--) {