带有通配符的列表会导致通用伏都教错误 [英] Lists with wildcards cause Generic voodoo error

问题描述

有谁知道为什么下面的代码不能编译?add() 和 addAll() 都没有按预期工作.删除?扩展"部分使一切正常，但是我将无法添加 Foo 的子类.

Does anyone know why the following code does not compile? Neither add() nor addAll() works as expected. Removing the "? extends" part makes everything work, but then I would not be able to add subclasses of Foo.

 List<? extends Foo> list1 = new ArrayList<Foo>();
 List<? extends Foo> list2 = new ArrayList<Foo>();

 /* Won't compile */
 list2.add( new Foo() ); //error 1
 list1.addAll(list2);    //error 2 

错误 1:

IntelliJ 说:

IntelliJ says:

add(capture<? extends Foo>) in List cannot be applied to add(Foo)

编译器说:

cannot find symbol
symbol  : method addAll(java.util.List<capture#692 of ? extends Foo>)
location: interface java.util.List<capture#128 of ? extends Foo>

错误 2:

IntelliJ 给了我

IntelliJ gives me

addAll(java.util.Collection<? extends capture<? extends Foo>>) in List cannot be applied to addAll(java.util.List<capture<? extends Foo>>)

而编译器只是说

cannot find symbol
symbol  : method addAll(java.util.List<capture#692 of ? extends Foo>)
location: interface java.util.List<capture#128 of ? extends Foo>
        list1.addAll(list2);

推荐答案

(这里假设 Bar 和 Baz 都是 Foo 的子类型>.)

(I assume here that Bar and Baz are both subtypes of Foo.)

列表 表示某种类型的元素列表，它是 Foo 的子类型，但我们不知道是哪种类型.此类列表的示例包括 ArrayList、LinkedList 和 ArrayList.

List<? extends Foo> means a list of elements of some type, which is a subtype of Foo, but we don't know which type. Examples of such lists would be a ArrayList<Foo>, a LinkedList<Bar> and a ArrayList<Baz>.

由于我们不知道哪个子类型是类型参数，所以我们不能将 Foo 对象放入其中，Bar 或 Baz> 对象.但是我们仍然知道类型参数是 Foo 的子类型，因此列表中的每个元素(以及我们可以从列表中获取的)都必须是一个 Foo 对象，所以我们可以使用 Foo f = list.get(0); 和类似的东西.

As we don't know which subtype is the type parameter, we can't put Foo objects into it, neither Bar or Baz objects. But we still know that the type parameter is a subtype of Foo, so every element already in the list (and which we can get from the list) must be a Foo object, so we can use Foo f = list.get(0); and similar things.

这样的列表只能用于从列表中取出元素，根本不能添加元素(除了null，但我不知道编译器是否真的允许这样做).

Such a list can only be used for taking elements out of the list, not to adding elements at all (apart from null, but I don't know if the compiler actually allows this).

A List 另一方面允许添加任何是 Foo 对象的对象 - 以及 Bar 和 Baz 是 Foo 的子类型，所有的 Bar 和 Baz 对象都是 Foo 对象，所以它们可以也添加.

A List<Foo> on the other hand allows adding any object which is a Foo object - and as Bar and Baz are subtypes of Foo, all Bar and Baz objects are Foo objects, so they can be added, too.

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