### Question:
StringComplexFieldActions implements AtlasFieldAction { @AtlasActionProcessor public static String append(Append append, String input) { if (append == null) { throw new IllegalArgumentException("Append must be specified with a string"); } String string = append.getString(); if (input == null && string == null) { return null; } if (string == null) { return input.toString(); } return input == null ? string : input.toString().concat(string); }  @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 testAppend() { Append action = new Append(); assertEquals(null, StringComplexFieldActions.append(action, null)); assertEquals("foo", StringComplexFieldActions.append(action, "foo")); assertEquals("1", StringComplexFieldActions.append(action, "1")); action.setString(""); assertEquals("", StringComplexFieldActions.append(action, null)); assertEquals("foo", StringComplexFieldActions.append(action, "foo")); action.setString("bar"); assertEquals("bar", StringComplexFieldActions.append(action, null)); assertEquals("foobar", StringComplexFieldActions.append(action, "foo")); assertEquals("1bar", StringComplexFieldActions.append(action, "1")); }
@Test(expected=IllegalArgumentException.class) public void testAppendNullAction() { StringComplexFieldActions.append(null, null); }