为什么下面的java代码导致编译错误 [英] Why is the following java code leads to compilation error

问题描述

我目前正致力于让我的代码包含更多泛型。我遇到了一个编译错误，看起来相当复杂，但我可以减少到以下代码中的等效错误：

  List< ;列表与LT;？>> a = new ArrayList< List< Integer>>（）; 
  

为什么会发生这种情况？我可以做些什么来解决这个问题？

解决方案

具有不同类型参数的泛型类的实例不相关，即使 String 是 Object的子类型， List< String> 是不是 List< Object> 的子类型，即使 List< Integer> 是<$ c $的子类型c> List<> ， List< List< Integer>> 不是的子类型List< List<

$ p $也许你正在寻找



 列表与LT ;?扩展List<>>> a = new ArrayList< List< Integer>>（）; 
  




I am currently working on making my code contain more generics. I encountered a compilation error which looks quite complicated but which I was able to reduce to an equivalent error in the following code:

List<List<?>> a = new ArrayList<List<Integer>>();

Why this happens? What can I do to fix it?

解决方案

Instances of a generic class with different type parameters are not related, i.e. even though String is a subtype of Object, List<String> is not a subtype of List<Object>, and even though List<Integer> is a subtype of List<?>, List<List<Integer>> is not a subtype of List<List<?>>.

Perhaps you are looking for

List<? extends List<?>> a = new ArrayList<List<Integer>>();

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