Java泛型，Unbound通配符<？> vs< Object> [英] Java generics, Unbound wildcards <?> vs <Object>

问题描述

我已阅读了几个主题，其中涵盖了有关泛型的某些问题，例如他们的与原始类型的关系。但是，我希望在 Java SE中找到的某一行上另行说明根据一句话：

printList的目标是打印任何类型的列表，但是它无法实现该目标 - 它只打印一个Object实例列表;它不能打印List< Integer>，List< String>，List< Double>等等，因为它们不是List< Object>的子类型。 b $ b

如果我对这句话理解的很好; List<？> 和 List< Object> 之间的区别在于，我们可以使用类型参数<$通过实现前者，c $ c> List< String> 或 List< Integer> 而如果我们实现了后者，我们只能使用类型参数 List< Object> 。就像 List<？> 是 Object的上界，即 List <？对象> 。

但是接下来的句子让我感到困惑，因为根据我以前的理解， List< Object> 应该只包含类 Object 的实例，而不是别的。

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请注意 List< Object> 和 List<？> 不一样。您可以将Object或Object的任何子类型插入到 List< Object> 中。但是您只能将 null 插入到 List<？> 中。




解决方案

这里有两个单独的问题。按照你的说法， List< Object> 实际上可以采用任何对象。 List< Number> 至少可以带 Number 对象，当然也可以是任何子类，像整数。

然而，像这样的方法：

  public void print（List< Number> list）; 
  

实际上仅 取一个列表 code>，它完全是 > List< Number> 。它不会接受任何声明为 List< Integer> 的列表。

所以区别在于 List<> 将使用带有任何声明的任何List，但 List< Object> 仅 标记为 List< Object> ，没有别的。

引用简单地指出， List<？> 是一个列表，您完全不知道它的项目是什么类型。因此，除了 null 以外，您不能添加任何内容。

I've read a few topics which cover certain questions about generics, such as their relationship with raw types. But I'd like an additional explanation on a certain line found in the Java SE tutorial on unbound generics .

According to a sentence :

The goal of printList is to print a list of any type, but it fails to achieve that goal — it prints only a list of Object instances; it cannot print List<Integer>, List<String>, List<Double>, and so on, because they are not subtypes of List<Object>.

If I understand well this sentence; the difference between List<?> and List<Object>, is that we can use the type argument List<String> or List<Integer> by implementing the former. While if we implement the later, we can only use the type argument List<Object>. As if List<?> is an upper bound to Object namely List<? Object>.

But then the following sentence confuses me, in the sense that according to what I previously understood, List<Object> should only contain instances of the class Object and not something else.

It's important to note that List<Object> and List<?> are not the same. You can insert an Object, or any subtype of Object, into a List<Object>. But you can only insert null into a List<?>.

解决方案

There are two separate issues here. A List<Object> can in fact take any object as you say. A List<Number> can take at least Number objects, or of course any subclasses, like Integer.

However a method like this:

public void print(List<Number> list);

will actually only take a List which is exactly List<Number>. It will not take any list which is declared List<Integer>.

So the difference is List<?> will take any List with whatever declaration, but List<Object> will only take something that was declared as List<Object>, nothing else.

The last quote simply states, that List<?> is a list for which you literally don't know what type its items are. Because of that, you can not add anything to it other than null.

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