为什么你不能在java的名单列表？ [英] Why can't you have a list of lists in java?

问题描述

在Java中，为什么不code以下的工作线？

In Java, why doesn't the following line of code work?

List<List<String>> myList = new ArrayList<ArrayList<String>>();

它的工作原理，如果我将其更改为

It works if I change it to

List<ArrayList<String>> myList = new ArrayList<ArrayList<String>>();

起初，我想也许你不能有一个接口的名单，但我可以创建一个列表＆LT; Runnable的方式＆gt; 就好了

想法？

推荐答案

泛型类型都比较迂腐。

列表办法列表或任何子类型，但＆LT;列表＆gt; < /只code>表示列表。如果你想要一个子类，你需要有＆LT ;?扩展列表＆gt;

List means List or any sub-type, but <List> means only List. If you want a sub-type you need to have <? extends List>

我怀疑你可以用

List<List<String>> myList = new ArrayList<List<String>>();

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你不能这样做的原因是，你可以使用一个参考和额外的间接水平，你必须要小心的参考。

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The reason you can't do this is that you can be using a reference to a reference and with an extra level of indirection you have to be careful.

// with one level of indirection its simple.
ArrayList alist = new ArrayList();
List list = aList; // all good
list = new LinkedList(); // alist is still good.

使用仿制药可以有两个级别的间接，可以给你的问题​​让他们更迂腐，以避免这些问题。

With generics you can have two level of indirection which can give you problems so they are more pedantic to avoid these issues.

// with two levels of indirection
List<ArrayList> alist = new ArrayList<ArrayList>();
List<List> list = (List) alist; // gives you a warning.
list.add(new LinkedList()); // adding a LinkedList into a list of ArrayList!!
System.out.println(alist.get(0)); // runtime error

打印

Exception in thread "main" java.lang.ClassCastException: java.util.LinkedList
     cannot be cast to java.util.ArrayList

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