在可以将类型分配为“null”之前，必须将其限制为什么？
trait Link[This] {
var next:This = null
}
给出“类型不匹配；发现：Null(null) required: This”
所以大概我需要告诉类型检查器这将是一个可以分配 null 的类型。我该怎么做？
（如果在问这样的问题之前我应该先阅读一个网站，请指出它。我目前正在阅读《Scala 编程》第二版的预印本）
trait Link[This] {
var next:This = null
}
gives "type mismatch; found: Null(null) required: This"
So presumably I need to tell the type checker that This is going to be a type that can be assigned null. How do I do this?
(If there's a site I should be reading first before asking questions like this, please point me at it. I'm currently part-way through the preprint of the 2nd Ed of Programming In Scala)
如果你对这篇内容有疑问，欢迎到本站社区发帖提问 参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。
绑定邮箱获取回复消息
由于您还没有绑定你的真实邮箱，如果其他用户或者作者回复了您的评论，将不能在第一时间通知您！
发布评论
评论（3）
您必须将
This限制为Null的超类 - 这是告诉编译器null是该类型的有效值的方式。 （事实上，考虑Any、AnyRef和AnyVal只会让问题变得混乱 - 只需询问编译器你想要什么！）但是，我建议您避免使用
null，您可以使用Option[This]并影响None- 这样的构造将允许您使用模式匹配，这是一个非常强烈的声明，即使用该字段的客户应该预期它可能没有任何价值。You have to constrain
Thisto be a superclass ofNull- which is the way of telling the compiler thatnullis a valid value for that type. (In fact, thinking aboutAny,AnyRefandAnyValonly muddles the problem - just ask the compiler for what you want!)However, I would suggest that you avoid using
null, you could useOption[This]and affectNone- such a construction will allow you to use pattern matching, and is a very strong statement that clients using this field should expect it to maybe have no value.这应该有效。您是否有特定原因需要使用类型参数化特征？
This should work. Is there a specific reason that you want to need to parameterize the trait with a type?
我的第一个想法是行不通的。我不知道为什么。
当最坏的情况发生时，总会有强制转换：
通过强制转换，您不再需要编译此特征的类型绑定，尽管您可能出于其他原因需要它。
My first thought, which didn't work. I'm not sure why.
When worse comes to worst, there's always casting:
With the cast, you no longer need the type bound for this trait to compile, although you might want it there for other reasons.