How to get drop down for possible values in swagger

Hi I have Rest API and used swagger for the test this API.

below is one of my API.

@RequestMapping(value = "/api/test", method = RequestMethod.POST)
public void test(String string){
    // body
}

The Possible value for the arguments are "Database" or "Cache".

So i want drop down in swagger view.

I have gone through the google search , i can not found how to implement with java.