Nearest multiple of 10

This code implements a function called roundToNearest that rounds a given number (represented as a string) to the nearest multiple of 10. Here's a summary and step-by-step explanation:

Summary:
The function takes a string representation of a number and rounds it to the nearest multiple of 10. If the last digit is 5 or less, it rounds down; otherwise, it rounds up. The function handles cases where rounding up causes carry-over to higher digits.

Step-by-step explanation:

1. Input validation:

   • Check if the input string is null or empty. If so, throw an IllegalArgumentException.

2. Rounding down:

   • If the last digit is '5' or less, replace it with '0' and return the result.

3. Rounding up:

   • If the last digit is greater than '5', the function proceeds to round up.

4. Initialization:

   • Convert the input string to a character array.
   • Set the last digit to '0'.
   • Initialize a boolean toAdd to true, which indicates if we need to carry over to the next digit.

5. Rounding process:

   • Iterate through the digits from right to left (excluding the last digit).
   • If toAdd is false, break the loop (no more carry-over needed).
   • If the current digit is '9':
     • Set it to '0'.
     • Keep toAdd as true (continue carry-over).
   • If the current digit is not '9':
     • Increment the digit.
     • Set toAdd to false (no more carry-over needed).

6. Handling overflow:

   • After the loop, if toAdd is still true, it means we need to add a new leading digit '1'.

7. Result formation:

   • Combine the prefix (if any) with the modified character array.
   • Return the result as a string.

The code handles various cases, including numbers that require multiple carry-overs (like 999 rounding to 1000) and ensures correct rounding for all input values.