Long starting with zero has wrong value

Question

I'm trying calculate the number of numbers in the long variable. However, if i declare
long number = 0123456 -> 5.
But when I declare
long number = 123456 -> 6.
Sorry my English isn't good. Thank you. This is my code:

public static void main(String[] args) {
  long number = 0123456;
  System.out.println(getSize(number));
}

public static int getSize(long d){
        String number = Long.toString(d);
        return number.length();
    }

Can anyone explain for me please, thank you.