Java泛型 - 将子类列表分配给超类列表 [英] Java Generics -- Assigning a list of subclass to a list of superclass

问题描述

所以我有类似下面的内容：

  B类扩展A; 
 
列表< B> bList = new ArrayList< B>（）; 
列表< A> aList = bList; 
  

为什么最后一项任务失败？对于新手问题抱歉

解决方案

为了解释这一点，让我用Integer替换B，用Number替换A。这只是为了使它更容易解释。

 类Integer extends Number; 
 
列表<整数> iList = new ArrayList< Integer>（）; 
列表<号码> nList = iList //会失败
  

这会失败的原因是因为nList可以取任何数字 - 它可以采用Integer，它可以采用Double，或者Number的任何子类。但是，这不适用于iList。您不能将Double添加到iList，因为它仅接受Integer及其子类。希望这有助于向你解释。

I have a basic question regarding assignment of a list of subclass to a list of superclass.

So I have something like the following:

Class B extends A;

List <B> bList = new ArrayList<B>();
List <A> aList = bList; 

Why does this last assignment fail? Sorry for the newbie question

解决方案

To explain this, let me substitute "B" with Integer and "A" with Number. This is just to make it a little easier to explain.

Class Integer extends Number;

List <Integer> iList = new ArrayList<Integer>();
List <Number> nList = iList // will fail

The reason this would fail is because nList can take any Number -- it can take Integer, it can take Double, or for that matter any subclass of Number. However, this is not true for iList. You cannot add a Double to iList because it accepts only Integer and its subclasses. Hope this helps explain it to you.

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