Java:从列表< B>到列表< A>当B实现A? [英] Java: Casting from List<B> to List<A> when B implements A?
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问题描述
我有以下课程&接口定义:
I have the following class & interface defined:
public interface A {
}
public class B implements A {
}
我有一个列表
>
B
对象,我需要转换为 $ c> objects:
I have a List
of B
objects that I need to cast to a List
of A
objects:
List<B> listB = new List<B>();
listB.add(new B()); // dummy data
listB.add(new B()); // dummy data
listB.add(new B()); // dummy data
List<A> listA = (List<A>) listB;
上面的最后一行导致编译错误无法从列表< B>转换到列表< A& 。我试图解决这个与下面的代替:
The last line above results in compile error "Cannot cast from List<B> to List<A>". I attempted to work around this with this following instead:
List<A> listA = Arrays.asList((A[]) listB.toArray());
$ b <有人知道我该如何解决这个问题吗?
Unfortunately, that throws a ClassCastException
. Does anyone know how I can resolve this?
推荐答案
你不能这样。创建一个新的:
You cannot cast it like that. Create a new one:
List<A> listA = new ArrayList<A>(listB);
构造函数需要 Collection< ;?扩展A>
。它将指向相同的引用。
The constructor takes Collection<? extends A>
. It will point to the same references anyway.
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