Java get-put 原理的解释

Explanation of the get-put principle

提问by JavaResp

I have read O'Reilly book, in that I came to know this get-put principle.

我读过 O'Reilly 的书，因为我知道了这个get-put 原则。

• Use an extendswildcard when you only getvalues out of a structure.
• Use a superwildcard when you only putvalues into a structure.
• And don't use a wildcard when you both want to get and put from/to a structure.

• extends当您只从结构中获取值时使用通配符。
• super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。

Exceptionsare:

例外情况是：

• You cannot put anything into a type declared with an extendswildcard except for the value null, which belongs to every reference type.

• You cannot get anything out from a type declared with an superwildcard except for a value of type Object, which is a super type of every reference type.

• extends除了null属于每个引用类型的 value 之外，您不能将任何内容放入使用通配符声明的类型中。

• super除了 type 的值（Object它是每个引用类型的超类型）之外，您无法从使用通配符声明的类型中获取任何内容。

Can anyone help me to explore this rule at depth? If possible, please put them hierarchical manner.

谁能帮我深入探索这条规则？如果可能，请把它们分层放置。

采纳答案by Jon Skeet

Consider a bunch of bananas. This is a Collection<? extends Fruit>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 getan item from it and know it will definitely be a fruit, but you can't addto it - you might be trying to add an apple to a bunch of bananas, which would definitely be wrong. You canadd nullto it, as that will be a valid value for anykind of fruit.

考虑一堆香蕉。这是Collection<? extends Fruit>因为它是一种特定种类的水果的集合 - 但你不知道（从那个声明中）它是一种什么样的水果。你可以从中得到一个物品并且知道它肯定是一个水果，但你不能添加它——你可能试图在一堆香蕉中添加一个苹果，这肯定是错误的。您可以添加null它，因为这对于任何种类的水果都是有效的值。

Now consider a fruitbowl. This is a Collection<? super Banana>, in that it's a collection of some type "greater than" Banana(for instance, Collection<Fruit>or Collection<TropicalFruit>). You can definitelyadd a banana to this, but if you fetch an item from the bowl you don't know what you'll get - it may well notbe a banana. All you know for sure is that it will be a valid (possibly null) Objectreference.

现在考虑一个水果碗。这是Collection<? super Banana>，因为它是某种“大于”类型的集合Banana（例如，Collection<Fruit>or Collection<TropicalFruit>）。您绝对可以在其中添加香蕉，但是如果您从碗中取出物品，您不知道会得到什么 - 它很可能不是香蕉。您所知道的只是，它将是一个有效的（可能null）Object参考。

(In general, for Java generics questions, the Java Generics FAQis an excellent resource which contains the answer to almost anything generics-related you're likely to throw at it.)

（一般来说，对于 Java 泛型问题，Java 泛型常见问题是一个很好的资源，其中包含几乎所有您可能会抛出的与泛型相关的答案。）