What does '0' do in Java?

Question

I'm trying to find the product of all digits in a number, which I have stored as a string (due to int and long length limits).

So the number looks like this:

final String number = "1234567898765432123......etc"

If I use this code, it works:

product *= number.charAt(i + j) - '0';

It doesn't if I remove the '0'.

I got this code from another online resource. Can someone explain what the '0' does?

Answer 1

Ascii characters are actually numbers. And 0 .. 9 digits are numbers starting from decimal 48 (0x30 hexadecimal).

'0' is 48
'1' is 49
...
'9' is 57

So to get the value of any character digit, you can just remove the '0', ie 48.

'1' - '0' => 1
 49 -  48 => 1

If you don't remove '0' (48) the total sum will be over by 48 * numberOfDigits.

See an ascii table to locate digits in it.

Note that '0' is the character 0, not the string "0" containing the character '0'.