java - How do I Override a Iterable<interface> return type method into Iterable<? extends interface> return type method -

i have interface need implement. looks this:

public interface simulationstate {     ...     public iterable<vehiclestatus> getvehiclestatuses(); } 

i trying extend interface sort of decorator interface following:

public interface simulationstatedec<v extends vehiclestatus> extends simulationstate {     ...     @override     public iterable<v> getvehiclestatuses(); } 

or

public interface simulationstatedec<v extends vehiclestatus> extends simulationstate {     ...     @override     public iterable<? extends vehiclestatus> getvehiclestatuses(); } 

here's concrete class i'm implementing interface at:

public class warehousestate implements simulationstate {     ...            @override     public iterable<vehiclestatus> getvehiclestatuses() {     return this.vstatuses;    } } 

i implementing vehiclestatus interface @ class named vehiclestate return implemented vehiclestate though iterable in concrete class

so @ class in project calling getvehiclestatuses() method , obligated cast every element vehiclestate following:

public class gate {     ...     private arraylist<vehiclethread> vehicles;      public gate(warehouse warehouse, gatepriority priority) {         ...         this.vehicles = new arraylist<>();         warehousestate.getvehiclestatuses().foreach(v -> vehicles.add(new vehiclethread((vehiclestate) v, warehouse)));         sortsametimearrivalby(priority);      } } 

i know doing wrong here, wondering if way of implementation allowed. compiler tells me change method in implemented interface , it's not applicable application has work on different framework.

i need return variety of concrete types implements same interface without having cast every element.

this how should use it, since x< a> not substitutable x< b > though extends b, put iterable< vehiclestatus> not replaceable iterable< v> v extends vehiclestatus.hence, error changing method return type while overriding

   public class vehiclestatus{     }     public interface simulationstate {       iterable<? extends vehiclestatus> getvehiclestatuses();    }     public interface simulationstatedec<v extends vehiclestatus> extends simulationstate {      @override      public iterable<v> getvehiclestatuses();    }