问题
I have a Scala method that takes 2 parameters:
def test(x:Long,y:Int){}
On some occasion I need to pass null instead of long ... something like that:
test(null,x)
The result:
scala> test(null,2) :7: error: type mismatch; found : Null(null) required: Long test(null,2)
Why do I need to pass null? Actually ,for some reason,I can't pass any default values. Thus, I need such a null.
*Note:*I know that the solution would be making it Option. However let's say I have no control over this method signature,can I do any work around?
Any ideas!
Thanks.
回答1:
Since you can't change the signature, consider the mistake of Thinking Option[Foo] is the only/most natural way to express a missing function argument.
If the param to your function is a lower bound, then Long.MinValue might be a natural default.
If by "for some reason,I can't pass any default values" (whatever that could possibly mean) you mean you can't add defaults to the signature, and you're going the route suggested in another answer of adapting the method, you might as well change f(a,b) to g(b, a=Long.MinValue) or whatever before forwarding.
Instead of making clients of your adaptor method call g(b, None), let them call g(b). You're not passing the Option to the underlying f(a,b) anyway.
回答2:
Null is a subtype of types which inherit from AnyRef, not from value types which inherit from AnyVal. This is why you are not able to pass null in. This corresponds to how, in java, you cant have a null of type long. (ignoring the boxed Long type).
However, this is an indication that the signature of the method should be changed to:
def test(x: Option[Long], y: Int)
which indicates that sometimes it goes no value for x. Since we have this nice Option class to deal with just this instance, there is little if any valid reasons to use null values, where you are relying on developers remembering to check for null values. Instead, with Option, the compiler will force you to take care of the fact that the value might not be there.
回答3:
The way to convert scala primitives to Java wrapper classes, is to use the static valueOf members on the Java Primitive wrappers. I had this issue where I needed to convert an Option[Double] to a java.lang.Double or null. This is what I did:
val value: Option[Double]
val orNull = value.map(java.lang.Double.valueOf(_)).orNull
Just passing literal null should work if you are calling a method that accepts java.lang.Long/Double/Integer
来源：https://stackoverflow.com/questions/13637827/pass-null-to-a-method-expects-long