Como contar letras de uma string

/**
 *
 * @author javadb.com
 */
public class Main {
    
    /**
     * This method counts the number of letters in a String
     */
    public void countLetters(String str) {
        
        if (str == null)
            return;
        
        int counter = 0;
        
        for (int i = 0; i < str.length(); i++) {

            if (Character.isLetter(str.charAt(i)))
                counter++;
        }
        
        System.out.println("The input parameter contained " + counter + " letters.");
    }

    
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        new Main().countLetters("abcde123");
    }
}