Q:

How to handle errors in a Spring-MVC project without if else?

I can’t understand how to handle the following error:
In the class CustomerService I delete the customer by id, and if such an id does not exist, then an error must be thrown! How can you do without an if else  construct?
CustomerService:
// Delete customer
    public void deleteCustomer(Long id){
            Customer customer = customerRepository.getByIdAndUserRole(id, "customer");
            customerRepository.delete(customer);
        }

CustomerController:
// DELETE MAPPING
    //
    // Delete customer with ID
    @DeleteMapping("/customers/{id}")
    void deleteCustomer(@PathVariable Long id) {
        customerService.deleteCustomer(id);
    }

A:

Try to use Controller Advice. Whenever a exception occur it will directly handled by the handler. No if/else or try/catch blocks will be required.
1) Create a class CustomerControllerHandler, annotate with @ControllerAdvice.
2) Now create methods with arguments having the type of Exception.
3) The methods will return the JSON/POJO/void you want.
4) Annotate the methods with @ExceptionHandler(Exception.class) and 
      @ResponseStatus(HttpStatus.BAD_REQUEST),
@ControllerAdvice
public class CustomerControllerHandler {
     @ExceptionHandler(Exception.class)
     @ResponseStatus(HttpStatus.BAD_REQUEST)
     public void processException(Exception ex) {
     }
}

