Find missing letter of a sentence - Pangram checker

Here I'm going to show simple way to find the missing letter of a sentence using a console application. The sentence "A quick brown fox jumps over the lazy dog" contains every single letter in the alphabet. Such sentences are called pangrams. I am to write a method getMissingLetters, which takes a String, sentence, and returns all the letters it is missing (which prevent it from being a pangram).this is the interface of the application:

This class is responsible for logic:

That is all