### Question:
StringComplexFieldActions implements AtlasFieldAction { @AtlasActionProcessor public static String subString(SubString subString, String input) { if (input == null || input.length() == 0) { return input; } if (subString == null || subString.getStartIndex() == null || subString.getStartIndex() < 0) { throw new IllegalArgumentException("SubString action must be specified with a positive startIndex"); } return doSubString(input, subString.getStartIndex(), subString.getEndIndex()); }  @AtlasActionProcessor static String append(Append append, String input); @AtlasActionProcessor(sourceType = FieldType.ANY) static String concatenate(Concatenate concat, List<String> inputs); @AtlasActionProcessor static Boolean endsWith(EndsWith endsWith, String input); @AtlasActionProcessor static String format(Format format, List<Object> input); @AtlasActionProcessor static String genareteUUID(GenerateUUID action); @AtlasActionProcessor static Number indexOf(IndexOf indexOf, String input); @AtlasActionProcessor static Number lastIndexOf(LastIndexOf lastIndexOf, String input); @AtlasActionProcessor static String padStringRight(PadStringRight padStringRight, String input); @AtlasActionProcessor static String padStringLeft(PadStringLeft padStringLeft, String input); @AtlasActionProcessor static String prepend(Prepend action, String input); @AtlasActionProcessor static String replaceAll(ReplaceAll replaceAll, String input); @AtlasActionProcessor static String replaceFirst(ReplaceFirst replaceFirst, String input); @AtlasActionProcessor(sourceType = FieldType.ANY) static String[] split(Split split, String input); @AtlasActionProcessor(sourceType = FieldType.ANY) static String[] repeat(Repeat repeat, String input); @AtlasActionProcessor static Boolean startsWith(StartsWith startsWith, String input); @AtlasActionProcessor static String subString(SubString subString, String input); @AtlasActionProcessor static String subStringAfter(SubStringAfter subStringAfter, String input); @AtlasActionProcessor static String subStringBefore(SubStringBefore subStringBefore, String input); static final String STRING_SEPARATOR_REGEX; static final Pattern STRING_SEPARATOR_PATTERN; }### Answer:
@Test public void testSubString() { SubString action = new SubString(); action.setStartIndex(2); action.setEndIndex(4); assertNull(StringComplexFieldActions.subString(action, null)); assertEquals("", StringComplexFieldActions.subString(action, "")); try { StringComplexFieldActions.subString(null, "aa"); fail("IllegalArgumentException expected"); } catch (IllegalArgumentException e) { assertTrue(true); } }