取得原则的解释 [英] Explanation of the get-put principle

问题描述

我读过O'Reilly的书籍，因为我开始知道这个 get-put原则 。

< blockquote>

• 仅当获得 扩展 通配符>值的结构。


• 仅当将值值放入结构中时，使用 super 通配符。


/ blockquote>



例外情况：







扩展 通配符除了值 null ，它属于每个引用类型。 超级 通配符，但<这是每种引用类型的超类型。





任何人都可以帮助我深入探索这条规则？如果可能的话，请把它们按层次分开。 解决方案

考虑一堆香蕉。这是一个集合<延伸Fruit> ，因为它是一种特定种类的水果的集合 - 但你不知道（来自该声明）它是一个集合的水果。你可以从它获取一个项目并知道它肯定会成为一个水果，但是你不能向它添加 - 你可能试图向一堆苹果添加一个苹果香蕉，这肯定是错的。您可以将 null 添加到它，因为这将是任何种水果的有效值。

>

现在考虑一个水果篮。这是一个集合<超级香蕉> ，因为它是某种类型的集合大于香蕉（例如， Collection< Fruit> ; 或集合< TropicalFruit> ）。您可以 为此添加一根香蕉，但是如果您从碗中拿取一件物品，您不知道会得到什么 - 它可能不是香蕉。所有你知道的肯定是它将是一个有效的（可能 null ） Object 引用。

（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有关的东西的答案，您可能会抛出它。）

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

• Use an extends wildcard when you only get values out of a structure.
• Use a super wildcard when you only put values into a structure.
• And don't use a wildcard when you both want to get and put from/to a structure.

Exceptions are:

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

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

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

解决方案

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 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.

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 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.

(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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