ASCII { public static boolean isDigit(char ch) { return ch >= '0' && ch <= '9'; }  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 isDigit() { assertThat(ASCII.isDigit('0')).isTrue(); assertThat(ASCII.isDigit('9')).isTrue(); assertThat(ASCII.isDigit('!')).isFalse(); assertThat(ASCII.isDigit('a')).isFalse(); assertThat(ASCII.isDigit('z')).isFalse(); assertThat(ASCII.isDigit('A')).isFalse(); assertThat(ASCII.isDigit('Z')).isFalse(); }