问题
Notice: As of Scala 2.11, NotNull is deprecated.
As far as I understand, if you want a reference type to be non-nullable you have to mixin the magic NotNull trait, and the compiler will automatically prevent you from putting null-able values in it. See this mailing-list thread for instance.
What lacking is, a decent library support for non-nullable types. If I would like to write a package that don't need to interface java code directly, and I want to prevent all types in this package from using null by default, I have no choice but to redefine all builting variables like so
//can't actually do that, but just to give the general idea
class NString extends String with NotNull
class NMap[X,Y] extends Map[X,Y] with NotNull
...
I expect scala to have (as a compiler plugin, or library) option for me to write
import collections.notnull._
in order to easily disallow null usage in a specific scala file.
Is there an option to easily force many usefull types in the standard library to be not-nullable?
回答1:
I don't really know what the deal is with NotNull, but I get the impression that Scala hasn't fully worked out how it wants to deal with NotNull/Nullable concepts. My own policy is to never use null in Scala, and if you call a Java API that may return null, immediately convert it to an Option.
This utility method is my best friend:
def ?[A <: AnyRef](nullable: A): Option[A] = if (nullable eq null) None else Some(nullable)
Then you do stuff like this:
val foo: Option[Foo] = ?(getFooFromJavaAPIThatMightReturnNull())
I find this far simplier than trying to track what may or may not be null.
So I didn't answer your question at all, but I pass this on in case it's useful...
Update: more recent Scala versions now support this in the standard API:
val foo: Option[Foo] = Option(getFooFromJavaAPIThatMightReturnNull())
来源：https://stackoverflow.com/questions/1522367/library-support-for-scalas-notnull-trait