import string

for testSentence in [
    'A quick brown fox jumps over the lazy dog',
    'A slow yellow fox crawls under the proactive dog',
    'Lions, and tigers, and bears, oh my!',
    'abcdefghijklmnopqrstuvwxyz']:
    print(''.join(set(string.ascii_lowercase) - set(testSentence)))

private static String getMissingLetters(String sentence) {
	Set<Character> missingCharSet = new TreeSet<Character>();
	Set<Character> foundCharSet = new TreeSet<Character>();
	StringBuffer missingCharsSB = new StringBuffer();

	for (char c='a'; c<='z'; ++c)
		missingCharSet.add(c);
	for (char c: sentence.toLowerCase().toCharArray())
		foundCharSet.add(c);

	missingCharSet.removeAll(foundCharSet);

	for (char c : missingCharSet)
		missingCharsSB.append(c);

	return missingCharsSB.toString();
}