java Cast from List< B>到列表< A>其中B延伸A [英] java cast from List<B> to List<A> where B extends A

问题描述

这是可能吗？

 接口B扩展A {} 
 public List< B> ; getList（）; 
 List< A> = getList（）; // Type mismatch：can not convert from List< B>到列表< A> 
  

我认为我正在寻找的主题是协变类型为此处和

解决方案

p>这是一个直观的例子，如何这可以使事情变得可怕的错误：

 接口B扩展A {} 
 List< B> blist = new List< B>（）; 
 List< A> alist = blist 
 alist.add（new A（））; //应该是好的，对吧？ 
 B b = b1ist.get（0）; // fail：即使blist是List< B> ;,它现在有一个A in 
  

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.

解决方案

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

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