Print the letters of an input string in alphabetic order alongside their frequency

Letters.java

package com.example;

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Letters {
    
    public static void main(String[] args) {
        String input = args[0].toLowerCase();
        Map<Character, Integer> letterFrequency = new HashMap<Character, Integer>();
        for (int i = 0; i < input.length(); i++) {
            char c = input.charAt(i);
            if (!Character.isLetter(c))
                continue;
            if (letterFrequency.containsKey(c)) {
                int frequency = letterFrequency.get(c);
                letterFrequency.put(c, ++frequency);
            } else
                letterFrequency.put(c, 1);
        }
        List<Character> letters =
                new ArrayList<Character>(letterFrequency.keySet());
        Collections.sort(letters);
        for (Character letter : letters)
            System.out.printf("%s: %2d%n", letter, letterFrequency.get(letter));
    }
    
}