[英]Subtyping or Generics in Java?
This is a Java Assignment question, possibly concerning sub-typing and generics. 这是一个Java分配问题,可能涉及子类型和泛型。 I have a class that extends ArrayList
called Rows 我有一个扩展ArrayList
的类,称为Rows
public class Rows extends ArrayList<List<Thing>> implements List<List<Thing>>{}
I need to return it in another class which has a method that returns a List<List<Thing>>
, thus I need to do this in order to get the desired return type: 我需要在另一个类中返回它,该类具有一个返回List<List<Thing>>
,因此我需要执行此操作以获得所需的返回类型:
private List<List<Thing>> list;
public List<List<Thing>> rows() {
Rows r = (Rows) list;// But this cast does not work at runtime
return (List<List<Thing>>) r;
}
The error Eclipse returns is java.lang.ClassCastException: java.util.ArrayList
cannot be cast to package.Rows. Eclipse返回的错误是java.lang.ClassCastException: java.util.ArrayList
无法转换为package.Rows。 I am quite confused as my Rows class extends ArrayList
and I thought that it should be able to be cast to it. 当我的Rows类扩展ArrayList
,我感到很困惑,我认为应该可以将其强制转换为它。
Rows extends ArrayList<List<Thing>>
The above definition says that Rows
is a subclass of ArrayList<List<Thing>>
(which means it's a specialized version), not that Rows
is the same thing as an ArrayList<List<Thing>>
. 上面的定义说Rows
是ArrayList<List<Thing>>
的子类 (这意味着它是一个专门的版本), 而不是 Rows
与ArrayList<List<Thing>>
。
This means that a Rows
is a kind of ArrayList<List<Thing>>
(so upcasting works) but an ArrayList<List<Thing>>
is not necessarily a Rows
(so downcasting doesn't work). 这意味着Rows
是ArrayList<List<Thing>>
(因此,向上转换有效),但是ArrayList<List<Thing>>
并不一定是Rows
(因此,向下转换不起作用)。
If you have an object that you've created as a new ArrayList<List<Thing>>
, then this is its type and you can't downcast it further. 如果您有一个已创建为new ArrayList<List<Thing>>
,则这是它的类型,您不能进一步对其进行向下转换。 If you want to be able to use it as a Rows
, you simply need to create it it as a new Rows
. 如果您希望能够将其用作Rows
,则只需将其创建为new Rows
。
(By the way, plural is unconventional for class names.) (顺便说一下,对于类名,复数是非常规的。)
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