### Question:
HexUtils { public static boolean containsLetters(String parameter) { if (parameter == null) throw new NullPointerException("Parameter cannot be null."); byte[] byteArray = parameter.getBytes(); for (int i = 0; i < byteArray.length; i++){ if (!((byteArray[i] >= '0') && (byteArray[i] <= '9'))) return true; } return false; }  static String byteArrayToHexString(byte[] value); static String byteToHexString(byte value); static byte[] hexStringToByteArray(String value); static boolean containsLetters(String parameter); static String integerToHexString(int value, int minBytes); static String prettyHexString(String hexString); static String prettyHexString(byte[] value);  }### Answer:
@Test public final void testContainsLettersNullParameter() { String s = null; exception.expect(NullPointerException.class); exception.expectMessage(is(equalTo("Parameter cannot be null."))); HexUtils.containsLetters(s); }
@Test public final void testContainsLettersEmptyParameter() { String s = ""; boolean expectedResult = false; boolean result = HexUtils.containsLetters(s); assertThat("The given string does not contain letters", result, is(equalTo(expectedResult))); }
@Test public final void testContainsLettersTrue() { String s = "0123456789abc"; boolean expectedResult = true; boolean result = HexUtils.containsLetters(s); assertThat("The given string contains letters", result, is(equalTo(expectedResult))); }
@Test public final void testContainsLettersFalse() { String s = "013456789"; boolean expectedResult = false; boolean result = HexUtils.containsLetters(s); assertThat("The given string does not contain letters", result, is(equalTo(expectedResult))); }