java从List转换<B>为List <A>，其中B扩展为A.</a>

Question

is this possible? if not, why isn't this possible in Java?

interface B extends A {}
public List<B> getList();
List<A> = getList(); // Type mismatch: cannot convert from List<B> to List<A>

I think the topic I'm looking for is "covariant types" as here and here , but its murky and it doesn't solve my problem.

Answer 1

Here is an intuitive example of how this can make things go horribly wrong:

interface B extends A {}
List<B> blist=new List<B>();
List<A> alist=blist;
alist.add(new A()); //should be ok, right?
B b = blist.get(0); //fail: even though blist is a List<B>, it now has an A in it

Answer 2

尝试

List<? extends A> = getList()

Answer 3

你不能这样做的原因是A和B不一样，你指定getList返回一个B的List（不是超类或子类）