传递列表<子类>到需要 List的方法
我有一个需要 List 作为参数的方法：
public void myMethod(List<SuperClass> list) {}
我想使用 List 调用该方法，例如：
List<SubClass> subList = new ArrayList<>();
// ...
myMethod(subList); // Got an argument mismatch error on this line.
我不应该能够当SubClass扩展SuperClass时执行此操作？
I have a method that is expecting a List<SuperClass> as argument:
public void myMethod(List<SuperClass> list) {}
I want to call that method with a List<Subclass> something like:
List<SubClass> subList = new ArrayList<>();
// ...
myMethod(subList); // Got an argument mismatch error on this line.
Shouldn't I be able to do this when SubClass extends SuperClass?
如果你对这篇内容有疑问，欢迎到本站社区发帖提问 参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。
绑定邮箱获取回复消息
由于您还没有绑定你的真实邮箱，如果其他用户或者作者回复了您的评论，将不能在第一时间通知您！
发布评论
评论（2）
不，泛型不是这样工作的。您可以做的是将您的方法定义为
MyMethod(List list)（按照惯例，它应该命名为myMethod(...)顺便说一句）。List与List的问题在于，您可以向此类列表添加新元素，而编译器不允许您向列表添加某些内容。列表- 这是有原因的：请考虑以下事项：
如果您现在有一个
List，您可以添加A、的实例>B和C。但是，如果将List作为List参数，编译器不知道是否允许将A或C的实例添加到该列表中（不允许，但如果您传递的是List）。因此编译器限制你不要这样做。将参数定义为
List告诉编译器可以将所有三个类的实例放入该列表中。现在，如果您被允许传递List作为这样的参数，您最终可能会得到一个包含A< 实例的List/code> 和/或C。这显然不是您想要的，并且可能会导致运行时错误，而这些错误应该在编译时通过使用泛型来预防。这就是为什么你的方法不起作用。No, generics don't work like that. What you could do is define your method as
MyMethod(List<? extends SuperClass> list)(by convention it should be namedmyMethod(...)btw).The problem with
List<SuperClass>vs.List<SubClass>is that you could add new elements to such lists whereas the compiler wouldn't allow you to add something to aList<? extends SuperClass>- and this has a reason:Consider the following:
If you now have a
List<A>you could add instances ofA,BandC. However, if you pass aList<B>to a method as aList<? extends A>parameter, the compiler doesn't know whether it is allowed to add instances ofAorCto that list (it wouldn't be allowed, but in case you'd pass aList<A>it would be). Thus the compiler restricts you not to do so.Defining a parameter as
List<A>tells the compiler that is is ok to put instances of all three classes to that list. Now if you would be allowed to pass aList<B>as such a parameter you could end up with aList<B>that contains instances ofAand/orC. And this is clearly not what you want and could result in runtime bugs that should be prevented at compile time already - by using generics. That's why your approach doesn't work.值得注意的是，您还可以从子类列表创建超类列表，如下所示：
Worth noting, you can also create the list of your superClass from a list of subClass as such: