Can be declare enum inside the interface?

It's perfectly legal to have an enum declared inside an interface.

In this situation the interface is just used as a namespace for the enum and nothing more. The interface is used normally wherever we use it.

public interface IService {

     public enum Status { // enum

           OK(200), INTERNAL_ERROR(500), KO(0);

           private int errorCode;

           private Status(int errorCode) {

                this.errorCode = errorCode;

           }

           public int getErrorCode(){

                return errorCode;

           }

     }

}