how to write a method that computes how often each letter A-Z or a-z occurs in a string

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Java

I am trying to write a method that computes how often each letter A-Z or a-z occurs in a string. Don't distinguish between upper- and lowercase letters. Return an array of length 26. The ith element contains the number of times that the letter 'a' + i or 'A' + i occurs in the string. The string may contain other characters that you should simply skip.

I have the following code:

public class Letters
{
   /**
      Counts the frequencies of letters A-Za-z in a string
      @param str a string
      @return an array of 26 counts. The i-th count is the number of occurrences
      of 'A' + i or 'a' + i.
   */
   public int[] letterFrequencies(String str)
   {
      ........

   }
}