public class Letters
{
   /**
      Counts the frequencies of letters A-Za-z in a string
      @param str a string
      @return an array of 26 percentages. The i-th count is the percentage of occurrences
      of 'A' + i or 'a' + i among all letters (a number between 0.0 and 1.0).
   */
   public double[] letterFrequencies(String str)
   {
      str.toLowerCase();
      double[] result = new double[26];
      char[] arrStr = str.toCharArray();
      int numberOfLetters = 0;

      for(char a : arrStr)
      {
         if(Character.isLetter(a))
         {
            result[Character.getNumericValue(a) - 10] += 1;
            numberOfLetters++;
         }
      }

      for(int i = 0; i < result.length; i++)
      {
         result[i] /= numberOfLetters;
      }

      return result;
   }
}