java - Spring Dependency injection using interface with more than one implementation -

my question has been asked here in following link.

spring: why autowire interface , not implemented class?

what want know if use @qualifier inject bean purpose of autowiring interface ?? why not auto-wire same implementation class ??

by autowiring interface want take advantage of run-time polymorphism that's not achieved if follow approach of @qualifier. please suggest me standard way.

following simple code if without spring. wonder how spring inject prepaidpaymentservice instance , postpaidpaymentservice instance??

 public interface paymentservice{         public void processpayment();     }    public class prepaidpaymentservice implements paymentservice{      public void processpayment(){          system.out.println("its prepaid payment service");      } }   public class postpaidpaymentservice implements paymentservice{      public void processpayment(){           system.out.println("its postpaid payment service");      } }   public class test {       public paymentservice service;      public static void main(string[] args) {          test test = new test();           int = 1;         if(i ==1 ){             test.setservice(new prepaidpaymentservice());             test.service.processpayment();         }         = 2;         if(i == 2){             test.setservice(new postpaidpaymentservice());              test.service.processpayment();         }       }       public void setservice(paymentservice service){         this.service = service;     }  } 

one reason autowiring interface (even @qualifier) can inject different implementation testing purposes.

for example, if have service uses dao access database, might want replace dao in order unit-test service. autowiring on interfa