### Question:
StringSimpleFieldActions implements AtlasFieldAction { @AtlasActionProcessor public static String fileExtension(FileExtension action, String input) { if (input == null) { return null; } int ndx = input.lastIndexOf('.'); return ndx < 0 ? null : input.substring(ndx + 1); }  @AtlasActionProcessor static String capitalize(Capitalize action, String input); @AtlasActionProcessor static String fileExtension(FileExtension action, String input); @AtlasActionProcessor static String lowercase(Lowercase action, String input); @AtlasActionProcessor static Character lowercaseChar(LowercaseChar action, Character input); @AtlasActionProcessor static String normalize(Normalize action, String input); @AtlasActionProcessor static String removeFileExtension(RemoveFileExtension action, String input); @AtlasActionProcessor static String separateByDash(SeparateByDash action, String input); @AtlasActionProcessor static String separateByUnderscore(SeparateByUnderscore action, String input); @AtlasActionProcessor static String trim(Trim action, String input); @AtlasActionProcessor static String trimLeft(TrimLeft action, String input); @AtlasActionProcessor static String trimRight(TrimRight action, String input); @AtlasActionProcessor static String uppercase(Uppercase action, String input); @AtlasActionProcessor static Character uppercaseChar(UppercaseChar action, Character input); static final String STRING_SEPARATOR_REGEX; static final Pattern STRING_SEPARATOR_PATTERN; }### Answer:
@Test public void testFileExtension() { assertNull(StringSimpleFieldActions.fileExtension(null, null)); assertNull(StringSimpleFieldActions.fileExtension(null, "")); assertNull(StringSimpleFieldActions.fileExtension(null, "foo")); assertEquals("", StringSimpleFieldActions.fileExtension(null, ".")); assertEquals("", StringSimpleFieldActions.fileExtension(null, "foo.")); assertEquals("bar", StringSimpleFieldActions.fileExtension(null, "foo.bar")); assertEquals("bar", StringSimpleFieldActions.fileExtension(null, "foo.foo.bar")); }