### Question:
CharacterUtils { public static List<Character> filterLetters(List<Character> characters) { return characters.stream().filter(Character::isLetter).collect(toList()); } private  CharacterUtils(); static List<Character> collectPrintableCharactersOf(Charset charset); static List<Character> filterLetters(List<Character> characters);  }### Answer:
@Test void testFilterLetters() { List<Character> characters = filterLetters(asList('a', 'b', '1')); assertThat(characters).containsExactly('a', 'b'); }