When I start my first spring boot application, I was very happy about after first Hello World program

just one hello method and return Hello world 

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class HelloWorld {

    @GetMapping(path = "/hello-world")
    public String Hello(){
        return "Hello World !!!";
    }
}

and i run with this with 

http://localhost:8080/hello-world

Guess what !!!

got a big fat error page 

Whitelabel Error Page
This application has no explicit mapping for /error, so you are seeing this as a fallback.
Thu Apr 07 07:17:58 AEST 2022
There was an unexpected error (type=Not Found, status=404).
No message available

Solution for this funny message 
This is clearly not an error with my code. code or syntax are all correct. 
the problem lies on my folder stucture 

com
   +- APP
         +- Application.java  <--- your main class should be here, above your controller classes
         |
         +- model
         |   +- user.java
         +- controller
             +- UserController.java
my conroller package was out side the main package. 
more technical perspective 
it should be