Java泛型和addAll方法 [英] java generics and the addAll method

问题描述

Java集合中addAll（..）方法的正确参数类型是什么？如果我这样做：

What is the correct type of argument to the addAll(..) method in Java collections? If I do something like this:

List<? extends Map<String, Object[]>> currentList = new ArrayList<Map<String, Object[]>>();
Collection<HashMap<String, Object[]>> addAll = new ArrayList<HashMap<String, Object[]>>();
// add some hashmaps to the list..
currentList.addAll(addAll); 

我知道我需要初始化两个变量。但是，我得到了一个编译错误（来自eclipse）：

I understand I need to initialize both variables. However, I get a compilation error (from eclipse):

Multiple markers at this line
    - The method addAll(Collection<? extends capture#1-of ? extends Map<String,Object[]>>) in the type List<capture#1-of ? extends Map<String,Object[]>> is not applicable for the arguments (List<capture#2-of ? extends 
     Map<String,Object[]>>)
    - The method addAll(Collection<? extends capture#1-of ? extends Map<String,Object[]>>) in the type List<capture#1-of ? extends Map<String,Object[]>> is not applicable for the arguments 
     (Collection<HashMap<String,Object[]>>)

我在做什么错？

推荐答案

您只能将 T 的实例插入列表< T> 。

You can only insert instances of T into a List<T>.

列表类型？扩展Map< X，Y>> 表示 unknown 类型 T 的列表，其中扩展Map< X，Y> 。例如，它可以代表 List< LinkedHashMap< X，Y>> 。显然，您可能不会在这样的列表中插入普通的 HashMap< X，Y> 。

The type List<? extends Map<X,Y>> stands for a list of an unknown type T which extends Map<X,Y>. For instance, it could stand for a List<LinkedHashMap<X,Y>>. Obviously, you may not insert an ordinary HashMap<X,Y> into such a list.

：

List<Map<String, Object[]>> currentList;

或者，如果您想变得真正灵活，可以这样做：

Or if you want to be really flexible you could do:

List<? super Map<String, Object[]>> currentList;

这会让您做疯狂的事情，例如：

Which would allow you to do crazy things like:

currentList = new ArrayList<Map<? super String, ? extends Object[]>>();

您可能还希望阅读Angelika Langer的泛型常见问题解答，尤其是有关通配符的部分。

You might also wish to read Angelika Langer's Generics FAQ, particularly the section about wildcards.

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