ASCII { public static boolean isLetter(char ch) { return isLowerCase(ch) || isUpperCase(ch); }  static String toUpperCase(@Nullable String text); static char toUpperCase(char ch); static String toLowerCase(@Nullable String text); static char toLowerCase(char ch); static boolean isLowerCase(char ch); static boolean isUpperCase(char ch); static boolean isDigit(char ch); static boolean isLetter(char ch);  }
@Test void isLetter() { assertThat(ASCII.isLetter('0')).isFalse(); assertThat(ASCII.isLetter('9')).isFalse(); assertThat(ASCII.isLetter('@')).isFalse(); assertThat(ASCII.isLetter('a')).isTrue(); assertThat(ASCII.isLetter('z')).isTrue(); assertThat(ASCII.isLetter('A')).isTrue(); assertThat(ASCII.isLetter('Z')).isTrue(); }