Example: number of occurrences of a character in a string

Recursive characterization of the operation of counting the occurrences of the character c in the string s:

• if s is the empty string "", then return 0;
• otherwise, if the first character of s is equal to c, then return 1 plus the number of occurrences of c in the string equal to s without the first character;
• otherwise (i.e., if the first character of s is different from c), return the number of occurrences of c in the string equal to s without the first character.

Implementation:

public static int countChars(String s, char c) {
  if (s.length() == 0)
    return 0;
  else if (s.charAt(0) == c)
    return 1 + countChars(s.substring(1), c);
  else
    return countChars(s.substring(1), c);
}