问题描述
我读过 O'Reilly 的书，因为我知道了这个get-put 原则.
I have read O'Reilly book, in that I came to know this get-put principle.
- 当您只从结构中获取值时，使用
通配符. - 当您只将值放入结构中时，请使用
通配符. - 当你们都想从/向结构中获取和放入时，不要使用通配符.
例外是:
除了值
null通配符声明的类型中>，属于每个引用类型.
除了 Object 类型的值之外，您无法从使用 通配符声明的类型中获取任何内容，它是每个引用类型的超类型.
You cannot get anything out from a type declared with an wildcard except for a value of type Object, which is a
谁能帮我深入探索这条规则?如果可能，请把它们分层放置.
Can anyone help me to explore this rule at depth? If possible, please put them hierarchical manner.
推荐答案
考虑一堆香蕉.这是一个 Collection得到一个物品并且知道它肯定是一个水果，但你不能添加到它——你可能试图在一堆苹果中添加一个香蕉，这肯定是错误的.您可以向它添加null，因为这对于任何种类的水果来说都是一个有效值.
Consider a bunch of bananas. This is a Collection<? in that it's a collection of a particular kind of fruit - but you don't know (from that declaration) what kind of fruit it's a collection of. You can get an item from it and know it will definitely be a fruit, but you can't add to it - you might be trying to add an apple to a bunch of bananas, which would definitely be wrong. You can add null to it, as that will be a valid value for any kind of fruit.
现在考虑一个水果碗.这是一个 Collection，因为它是某种大于"Banana 类型的集合(例如，Collection 或 Collection;).你可以绝对在上面加一根香蕉，但是如果你从碗里拿出一个东西，你不知道会得到什么——它很可能不是香蕉.您所知道的只是它是一个有效的(可能是 null)Object 引用.
Now consider a fruitbowl. This is a Collection<? , in that it's a collection of some type "greater than" Banana (for instance, Collection<Fruit> or Collection<TropicalFruit>). You can definitely add a banana to this, but if you fetch an item from the bowl you don't know what you'll get - it may well not be a banana. All you know for sure is that it will be a valid (possibly null) Object reference.
(一般来说，对于 Java 泛型问题，Java 泛型常见问题解答是一个很好的资源，其中包含几乎所有与泛型相关的问题的答案.)
(In general, for Java generics questions, the Java Generics FAQ is an excellent resource which contains the answer to almost anything generics-related you're likely to throw at it.)
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