Coding Challenge: Java Fundamentals

✒ Reference: String incrementer

Challenge Description

Your job is to write a function which increments a string, to create a new string.

• If the string already ends with a number, the number should be incremented by 1.
• If the string does not end with a number. the number 1 should be appended to the new string.

Initial approach

I made a mistake by only checking the last character of the string to see if it is a number.

  public static String incrementString(String str) {
    int len = str.length() - 1;
    char lastChar = str.charAt(len);
    
    if (Character.isDigit(lastChar)) {
      int nextInt = Character.getNumericValue(lastChar) + 1;
      return (str.substring(0, len)).concat(Integer.toString(nextInt));
    } else {
      return str + "1";
    }
  }

But this only increments the last character, and becomes a problem if the number is more than two digits long and ends with 9. (i.e. foo119 becomes foo1110)

Next approach

So I checked for the leading 0s, then padded the string with the appropriate number of 0s.

public class Kata {
  public static String incrementString(String str) {
    
    for (int i=0; i<str.length(); i++) {
      // Check if character contains digit
      if (Character.isDigit(str.charAt(i))) {
        
        // Create a number 1 bigger
        String strInt = str.substring(i, str.length());
        int nextInt = Integer.valueOf(str.substring(i, str.length())) + 1;
        
        // If there is a leading zero, add it to string
        if (Integer.toString(nextInt).length() < strInt.length()) {
          String result = String.format("%0" + strInt.length() + "d", nextInt);
          return (str.substring(0, i)).concat(result);
        } 
        
        // If not, just return without leading 0s
        return (str.substring(0, i).concat(Integer.toString(nextInt)));
      }
    }
    return str + "1";
  }
}

But this does not take into account the strings with numbers in the middle, for example 99obar99. I should consider using Regex to check for trailing numbers!