Simple code for finding missing alphabet letters in a sentence

I consider no code is trash even if it's too trivial. And I also hate the sensation of knowing I code something similar before, but have to repeat the coding because I didn't save it. Some time ago I had to code a solution for a simple problem as part of the recruiting process in a company. The problem to solve is to find the missing letters of the alphabet in a given sentence.

import java.util.HashSet;
import java.util.Set;
 
/**
* Test code to find missing letters of the alphabet from a String sentence.
* @author gabriel.solano
*
*/
public class MissingLetters {
 
private final int ASCII_CODE_FOR_LETTER_A = 97;
private final int ASCII_CODE_FOR_LETTER_Z = 122;
 
/**
* Gets the missing letters of a sentence in lower case.
* @param sentence
* @return String having all the letters that the sentence is missing from the alphabet.
*/
public String getMissingLetters(String sentence){
/*
 * 1. Let's populate a set with the unique characters of the sentence.
 *    This approach avoids having two nested for's in the code (better performance).
 */
Set<Integer> uniqueASCIICodes = new HashSet<Integer>();
 
for (char character : sentence.toLowerCase().toCharArray() ) {
 if (character >= ASCII_CODE_FOR_LETTER_A
   && character <= ASCII_CODE_FOR_LETTER_Z) { // Range of lower case letters.
  uniqueASCIICodes.add((int)character);
 
  if (uniqueASCIICodes.size() == 26) {
   break; // Sentence already covered all letter from the alphabet.
  }
 }
}
/*
 * 2. Move in the range of ascii codes of lower case alphabet
 * and check if letter was present in sentence.
 */
StringBuilder misingLettersBuilder = new StringBuilder();
 
for (int i=ASCII_CODE_FOR_LETTER_A; i <= ASCII_CODE_FOR_LETTER_Z; i++) {
 if (!uniqueASCIICodes.contains(i)) {
  misingLettersBuilder.append((char)i);
 }
}
   return misingLettersBuilder.toString();
}
 
public static void main(String[] args) {

   String case1 = "A quick brown fox jumps over the lazy dog";
   String case2 = "bjkmqz";
   String case3 = "cfjkpquvwxz";
   String case4 = "";
 
   MissingLetters missingLetters = new MissingLetters();
 
  System.out.println("Missing letters for[" + case1 + "]: " +
  missingLetters.getMissingLetters(case1));
  System.out.println("Missing letters for[" + case2 + "]: " +
  missingLetters.getMissingLetters(case2));
  System.out.println("Missing letters for[" + case3 + "]: " +
  missingLetters.getMissingLetters(case3));
  System.out.println("Missing letters for[" + case4 + "]: " +
  missingLetters.getMissingLetters(case4));
   }
}

This will be the program output:

Missing letters for[A quick brown fox jumps over the lazy dog]: 
Missing letters for[bjkmqz]: acdefghilnoprstuvwxy
Missing letters for[cfjkpquvwxz]: abdeghilmnorsty
Missing letters for[]: abcdefghijklmnopqrstuvwxyz