blocks|key|3179539|text|正如您所说的，这与gwt编译器有关。编辑:然而，正如Daniel在下面的评论中指出的那样，这通常不适用于GWT编译器，只有在使用GWT-rpc.gwt-rpc.gwt-时才适用。|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|3179540|如果你声明List而不是ArrayList作为返回类型，gwt编译器将在编译后的代码中包含完整的列表层次结构(即所有实现List的类型)。如果使用ArrayList，编译器将只需要包括ArrayList层次结构(即实现ArrayList的所有类型--通常就是ArrayList本身)。使用接口而不是具体的类，您将在编译时间和生成的代码大小(以及每个用户在运行应用程序时必须下载的代码量)方面付出代价。|3179541|您还询问了原因:如果使用接口(而不是具体的类)，编译器在编译时不知道将使用这些接口的哪些实现。因此，它包含了所有可能的实现。|3179542|关于你的最后一个问题:所有的方法都可以声明为返回接口(这就是你想要的，对吧？)然而，上述处罚适用。|3179543|顺便说一句:据我所知，这个问题并不局限于方法。它适用于所有类型声明:变量、参数。无论何时使用接口声明某些东西，编译器都会包含完整的子接口和实现类的层次结构。(因此，很明显，如果您声明自己的接口只有一个或两个实现类，那么您不会招致很大的损失。这就是我在GWT中使用接口的方式。)|3179544|简而言之:尽可能使用具体的类。|3179545|(小建议:如果你在参考视频时给出时间戳，会有所帮助。)|3179546|entityMap^0|I|1U|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|T|8|@$9|U|A|V|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|W|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|X|8|@]|D|@]|E|$]]|$1|J|3|K|5|6|7|Y|8|@]|D|@]|E|$]]|$1|L|3|M|5|6|7|Z|8|@]|D|@]|E|$]]|$1|N|3|O|5|6|7|10|8|@]|D|@]|E|$]]|$1|P|3|Q|5|6|7|11|8|@]|D|@]|E|$]]|$1|R|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|S|$]]

This has to do with the gwt-compiler, as you say correctly. EDIT: However, as Daniel noted in a comment below, this does not apply to the gwt-compiler in general but only when using GWT-RPC.

If you declare List instead of ArrayList as the return type, the gwt-compiler will include the complete List-hierarchy (i.e. all types implementing List) in your compiled code. If you use ArrayList, the compiler will only need to include the ArrayList hierarchy (i.e. all types implementing ArrayList -- which usually is just ArrayList itself). Using an interface instead of a concrete class you will pay a penalty in terms of compile time and in the size of your generated code (and thus the amount of code each user has to download when running your app).

You were also asking for the reason: If you use the interface (instead of a concrete class) the compiler does not know at compile time which implementations of these interfaces are going to be used. Thus, it includes all possible implementations.

Regarding your last question: all methods CAN be declared to return interface (that is what you ment, right?). However, the above penalty applies.

And by the way: As I understand it, this problem is not restricted to methods. It applies to all type declarations: variables, parameters. Whenever you use an interface to declare something, the compiler will include the complete hierarchy of sub-interfaces and implementing classes. (So obviously if you declare your own interface with only one or two implementing classes then you are not incurring a big penalty. That is how I use interfaces in GWT.)

In short: use concrete classes whenever possible.

(Small suggestion: it would help if you gave the time stamp when you refer to a video.)

blocks|key|268322|text|在Google+IO+2011+-+High-performance+GWT上展示了这一点和其他性能技巧。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|268323|在大约7分钟的时候，演讲者说到了“RPC类型爆炸”：|268324|📷|atomic|268325|​|268326|出于某种原因，我以为GWT编译器会再次优化它，但似乎我错了。|268327|entityMap|0|LINK|mutability|MUTABLE|url|http://www.youtube.com/watch?v=0F5zc1UAt2Y|1|IMAGE|IMMUTABLE|imageUrl|https://ask.qcloudimg.com/http-save/yehe-900000/24ffdbb6edf87d303d93256fafb6972f.png|imageAlt^0|I|K|0|0|0|0|1|1|0|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@$A|11|B|12|1|13]]|C|$]]|$1|D|3|E|5|6|7|14|8|@]|9|@]|C|$]]|$1|F|3|G|5|H|7|15|8|@]|9|@$A|16|B|17|1|18]]|C|$]]|$1|I|3|J|5|6|7|19|8|@]|9|@]|C|$]]|$1|K|3|L|5|6|7|1A|8|@]|9|@]|C|$]]|$1|M|3|-4|5|6|7|1B|8|@]|9|@]|C|$]]]|N|$O|$5|P|Q|R|C|$S|T]]|U|$5|V|Q|W|C|$X|Y|Z|-4]]]]

This and other performance tips were presented at Google IO 2011 - <a href="http://www.youtube.com/watch?v=0F5zc1UAt2Y" rel="nofollow noreferrer">High-performance GWT</a>.

At about the 7 min point the speak addresses 'RPC Type Explosion':
<img src="https://i.stack.imgur.com/3C6Uv.png" alt="#4 Watch out for RPC type explosion">

For some reason I thought the GWT compiler would optimize it away again but it appears I was mistaken.

In <a href="http://www.google.com/events/io/2009/sessions/GoogleWebToolkitBestPractices.html" rel="noreferrer">this video from Google IO 2009</a>, the presenter very quickly says that signatures of methods should return concrete types instead of interfaces.

From what I heard in the video, this has something to do with the GWT Java-to-Javascript compiler.

<ul>
<li>What's the reason behind this choice ?</li>
<li>What does the interface in the method signature do to the compiler ?</li>
<li>What methods can return interfaces instead of concrete types, and which are better off returning concrete instances ?</li>
</ul>

<img src="https://i.stack.imgur.com/S2sYr.png" alt="Snippet of code from Google IO presentation showing it&#39;s better that your method returns ArrayList than List">

In GWT, why shouldn't a method return an interface?

编译器

Java

在中，演示者很快就会说方法的签名应该返回具体的类型，而不是接口。根据我在视频中听到的，这与GWT Java- to -Javascript编译器有关。此选择背后的原因是什么？方法签名中的接口对编译器有什么作用？哪些方法可以返回接口而不是具体类型，哪些方法最好返回具体实例?​​

问在GWT中，为什么方法不应该返回接口？
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