如何显示方法是否可以返回null [英] How to show if a method may return null
问题描述
发布这个问题和阅读那个我意识到知道一个方法是否应该返回null是非常重要的，或者如果这被认为是错误条件并且应该抛出异常。还有一个很好的讨论，何时返回'null'或抛出异常。
After posting this question and reading that one I realized that it is very important to know if a method is supposed to return null, or if this is considered an error condition and an exceptions should be thrown. There also is a nice discussion when to return ‘null’ or throw exception .
我正在编写一个方法，我已经知道是否要返回null或抛出异常，表达我的决定的最佳方式是什么，换句话说，记录我的合同？
我能想到的一些方法：
- 在规格/文档中写下来（有人会阅读吗？）
- 将它作为方法名称的一部分（正如我建议的那样这里）
- 假设抛出异常的每个方法都不返回null，而每个不抛出的方法可能返回null。 / li>
- Write it down in the specs / the documentation (will anyone read it?)
- Make it part of the method name (as I suggested here)
- assume that every method that throws an exception will not return null, and every one that does 'not' throw might return null.
我主要谈论java，但它可能适用于其他la nguages：为什么有一种正式的方式来表达是否会抛出异常（抛出关键字）但是没有正式的方式来表达是否可能返回null？
I'm mainly talking about java, but it might apply to other languages, too: Why is there a formal way to express if exceptions will be thrown (the throws keywords) but no formal way to express if null might be returned?
为什么没有这样的东西：
Why isn't there something like that:
public notnull Object methodWhichCannotReturnNull(int i) throws Exception
{
return null; // this would lead to a compiler error!
}
摘要和结论
有很多方法来表达合同：
Summary and Conclusion
There are many ways to express the contract:
- 如果您的IDE支持它（作为IntelliJ），最好使用类似的注释
@NotNull因为它对程序员可见，可用于自动编译时检查。有一个 Eclipse插件来添加对这些的支持，但它没有为我工作。 - 如果这些不是一个选项，请使用自定义类型，如
选项< T>或NotNull< T>，这增加了清晰度，至少增加了运行时检查。 - 无论如何，在JavaDoc中记录合同永远不会伤害，有时甚至会有帮助。
- 使用方法名称来记录返回值的 nullability 除了我之外没有人提出过，虽然它可能非常详细而且并不总是有用，但我仍然相信它有时也有它的优点。
- If your IDE supports it (as IntelliJ), it's best to use an annotation like
@NotNullbecause it is visible to the programmer and can be used for automated compile time checking. There's a plugin for Eclipse to add support for these, but it didn't work for me. - If these are not an option, use custom Types like
Option<T>orNotNull<T>, which add clarity and at least runtime checking. - In any way, documenting the contract in the JavaDoc never hurts and sometimes even helps.
- Using method names to document the nullability of the return value was not proposed by anyone but me, and though it might be very verbose und not always useful, I still believe sometimes it has its advantages, too.
推荐答案
一个非常好的跟进问题。我认为 null 是一个真正特殊的值，如果一个方法可能返回 null 它必须在Javadoc中清楚地记录它的时候是（ @return some value ...，如果...... ，则为null）。在编码时我是防御性的，并假设一个方法可能会返回 null ，除非我确信它不能（例如，因为Javadoc这样说。）
A very good follow up question. I consider null a truly special value, and if a method may return null it must clearly document in the Javadoc when it does (@return some value ..., or null if ...). When coding I'm defensive, and assume a method may return null unless I'm convinced it can't (e.g., because the Javadoc said so.)
人们意识到这是一个问题，建议的解决方案是使用注释以一种可以自动检查的方式陈述意图。请参阅 JSR 305：软件缺陷检测注释， JSR 308：Java类型的注释和 JetBrain的Nullable方法。
People realized that this is an issue, and a proposed solution is to use annotations to state the intention in a way it can be checked automatically. See JSR 305: Annotations for Software Defect Detection, JSR 308: Annotations on Java Types and JetBrain's Nullable How-To.
您的示例可能如下所示，并被IDE拒绝，编译器或其他代码分析工具。
Your example might look like this, and refused by the IDE, the compiler or other code analysis tools.
@NotNull
public Object methodWhichCannotReturnNull(int i) throws Exception
{
return null; // this would lead to a compiler error!
}
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