Urgent please

Your task is to provide a suite of tests for an invert method of Inverter class, using JUnit 4 framework, covering all the requirements below. Description of the invert method invert method accepts a string and returns a string. - When the string is empty it returns empty string. - When the argument passed to the method is null it returns an empty string - When the string has exactly one character the same string is returned - When the string is longer then 1 character its inverted version is returned. Examples of invert method usage Inverter.invert("a"); // returns "a" Inverter.invert(nu11); // returns " " Inverter.invert("abcd"); // return "dcba