blocks|key|243982|text|可以在类声明中使用有界类型参数吗？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|243983|class+EnglishReaderOfPublications<E+extends+English>+{+...|code-block|syntax|javascript|243984|然后，您可以在具有通配符参数的任何地方使用此类型参数。|243985|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Can you use a bounded type parameter in the class declaration?

<pre><code>class EnglishReaderOfPublications&lt;E extends English&gt; { ...
</code></pre>

Then you can use this type parameter everywhere you have the wildcard parameter.

blocks|key|242611|text|有界通配符是有感染力的，这是您所链接的页面似乎在感叹的。好吧，不可否认的是...但我想我不认为这是一个大问题。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|242612|在很多情况下，我返回有界通配符正是因为它具有传染性。JDK，不管是好是坏(我说是好是坏，但这是一个不同的soap盒子:)没有用于只读集合的接口。如果我返回一个不想让人修改的List<Foo>+(也许它甚至被安全地包装在Collections.unmodifiableList中)，就没有办法在我的返回签名中声明它。作为穷人的变通方法，我经常返回List<?+extends+Foo>。仍然可以尝试通过删除元素或插入null来修改该列表，但至少不能使用add(new+Foo())这一事实提醒我们，该列表可能是只读的。|offset|length|style|CODE|242613|更广泛地说，如果您真的希望调用站点限制对对象的访问，我认为使用有界返回类型返回某些内容是完全合理的。|242614|另一个示例是线程安全队列，您将其传递给不同的线程，其中一个线程是生产者，另一个线程是消费者。如果你给生产者一个Queue<?+super+Foo>，很明显你打算让他们把东西放进去(而不是把东西拿出来)。同样，如果你给消费者一个Queue<?+extends+Foo>，很明显你想让他们把东西拿出来(而不是放进去)。|242615|entityMap^0|0|2E|9|31|S|4S|J|5R|4|69|E|0|0|1J|I|35|K|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@$D|P|E|Q|F|G]|$D|R|E|S|F|G]|$D|T|E|U|F|G]|$D|V|E|W|F|G]|$D|X|E|Y|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|Z|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|10|8|@$D|11|E|12|F|G]|$D|13|E|14|F|G]]|9|@]|A|$]]|$1|L|3|-4|5|6|7|15|8|@]|9|@]|A|$]]]|M|$]]

Bounded wildcards are contagious, which is what the page you link to seems to lament. Well, there's no denying that... but I guess I don't see it as such a big problem.

There are many cases where I return a bounded wildcard exactly because it's contagious. The JDK, for better or worse (I say for worse, but that's a different soap box :) ) doesn't have interfaces for read-only collections. If I return a <code>List&lt;Foo&gt;</code> that I don't want people modifying (maybe it's even safely wrapped in <code>Collections.unmodifiableList</code>), there's no way to declare that in my return signature. As a poor-man's workaround, I'll often return <code>List&lt;? extends Foo&gt;</code>. It's still possible to try to modify that list by removing elements or inserting <code>null</code>, but at least the fact that you can't <code>add(new Foo())</code> serves as a bit of a reminder that this list is probably read-only.

More generally, I think returning something with a bounded return type is perfectly reasonable if you really want the call site to have restricted access to the object.

Another example would be a thread-safe queue that you hand off to different threads, where one thread is a producer and the other is a consumer. If you give the producer a <code>Queue&lt;? super Foo&gt;</code>, it's clear that you intend them to put items into it (and not take items out). Likewise, if you give the consumer a <code>Queue&lt;? extends Foo&gt;</code>, it's clear you intend them to take items out (and not put items in).

blocks|key|2911792|text|“理想情况下，getPublication的调用者不想要有界通配符形式的结果。他们会对Publication<English>感到满意。”|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2911793|为什么他们“不”想要呢？他们会对Publication<?+extends+English>感到“满意”吗？问题是这些调用者需要对这个发布对象做什么。如果他们所做的就是从中获取东西，那么Publication<?+extends+English>就足够了，而且它更好，因为它更通用。但是，如果他们需要把东西放进去，那么你就不能使用Publication<?+extends+English>。|2911794|entityMap^0|17|K|0|G|U|2L|U|4L|U|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]|$9|R|A|S|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|I|$]]

"ideally, the callers of getPublication wouldn't want the result as a bounded wildcard. They would be happy with <code>Publication&lt;English&gt;</code>."

Why "wouldn't" they want it? Would they "be happy" with <code>Publication&lt;? extends English&gt;</code>? The question is really what these callers need to do with this Publication object. If all they do is get things out of it, then <code>Publication&lt;? extends English&gt;</code> is sufficient, and it is better because it is more general. However, if they need to put things into it, then you can't use <code>Publication&lt;? extends English&gt;</code>.

I've read in various places including <a href="http://www.angelikalanger.com/GenericsFAQ/FAQSections/ProgrammingIdioms.html#Should%20I%20use%20wildcards%20in%20the%20return%20type%20of%20a%20method?" rel="noreferrer">here</a> that having a bounded wildcard in a method return type is a bad idea. However, I can't find a way to avoid it with my class. Am I missing something?

The situation looks something like this:

<pre><code>class EnglishReaderOfPublications {

 private final Publication&lt;? extends English&gt; publication;

 EnglishReaderOfPublications(Publication&lt;? extends English&gt; publication) {
 this.publication = publication;
 }

 void readPublication() {
 publication.omNomNom();
 }

 Publication&lt;? extends English&gt; getPublication() {
 return publication;
 }
}
</code></pre>

In summary, a class that I want to be able to consume any publication that is in some variant to English. The class needs to allow access to the publication from outside but, ideally, the callers of <code>getPublication</code> wouldn't want the result as a bounded wildcard. They would be happy with <code>Publication&lt;English&gt;</code>.

Is there a way round this?

Java bounded wildcard in return type

我在很多地方都读到过，包括，在方法返回类型中使用有界通配符不是一个好主意。然而，我找不到一种方法来避免它与我的类。我是不是遗漏了什么？情况看起来像这样：class EnglishReaderOfPublications { private final Publication<? extends English> ...

问返回类型中的Java边界通配符
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