Generic:ArrayList of？在Java中扩展ISomeInterface

Question

我在以下代码中遇到了一些麻烦.

public ArrayList<? extends IEvent> getEventsByDateRange(DateTime minStartTime,  DateTime minEndTime) 
{
    ArrayList<? extends IEvent> returnedEvents = new ArrayList<GoogleEvent>();
    returnedEvents.add(new GoogleEvent());
    return (returnedEvents);
}

这将为"returnedEvents.add(new GoogleEvent());代码行"返回以下编译错误:

方法add(捕获#1-of？extends IEvent)在ArrayList类型中不适用于参数(GoogleEvent)

该课程的声明GoogleEvent如下:

public class GoogleEvent implements IEvent {...}

我知道在Java中使用泛型有一些棘手的部分,因此是通配符,但我似乎无法弄清楚这一点.

谢谢.

Answer 1

你为什么不写:

public List<IEvent> getEventsByDateRange(DateTime minStartTime, DateTime minEndTime)
{
    List<IEvent> returnedEvents = new ArrayList<IEvent>();
    returnedEvents.add(new GoogleEvent());
    return returnedEvents;
}