java.lang.Object; cannot be cast to model class: error in Spring Boot

Option 1) Make sure the RoomCPCMapping is a projection interface:

public interface RoomCPCMappingResult {

    String getNCCPCode();
    String getNPercent();

    ...
}

Option 2) Use the result class legacy option:

SELECT new com.my.package.RoomCPCMappingResult(u.nCCPCode,u.nPercent)
FROM RoomCPCMapping u JOIN u.clientCPC ur 
where u.nRoomAllocationId=:nRoomAllocationId

just make sure you have an adequate constructor present there.