toString() is a method which should be reference via it's class. If you want an int to be returned as a string you us the Integer class to reference the toString method in this class. Integer.toString(someNumber).
toString() is static which is why you don't need a instance variable to refer to the method. All that is needed is the class.
P.S. if you want to do more toString()'s for different primitive data types look at the documentation for java.lang.