Question

I have this Java class in a Jar file included as a dependency of an Scala program (like the Axis jar):

class MyClass {
    private String[] someStrings;
    public String[] getSomeStrings() { return someStrings; }
}

In my Scala program I have a Java API that return an instance of MyClass instance of MyClass to my program in Scala:

val myInstance = JavaAPI.getInstanceOfMyClass()

Then I try to use the someStrings array in my Scala program but it’s null (let say that it wasn’t properly initialized)

for(str <- myInstance.getSomeStrings()) ...

So this throws a NullPointerException.

I’ve found that in order to use it in the for comprehension I can wrap it into an Option so that it handles the NPE properly.

for(str <- Option[Array[String]](myInstance.getSomeStrings).getOrElse(Array[String]())

But that doesn’t look OK to me.

Is there a way to create like an implicit method that takes that value even if it’s null and wrap it into the Option, like:

implicit def wrapNull(a: Null): Option[Nothing] = None
implicit def wrapArray(a: Array[String]): Option[Array[String]] = Some(a)

So that when I do:

for(str <- myInstance.getSomeStrings())

I don’t get the NPE

Answer 1

I don’t think your version with getOrElse is that bad (you can make it a little shorter by removing the [Array[String]] after Option, since that can be inferred). If you want something even more concise, the following works:

for (str <- Option(myInstance.getSomeStrings).flatten) ...

You could also use the fact that Option has a foreach:

for {
  strings <- Option(myInstance.getSomeStrings)
  str <- strings
} ...

Note that you can’t use yield here, for the reason that drexin highlights in a comment below.

Or you could pimp MyClass:

implicit def withNullWrapper(c: MyClass) = new {
  def getSomeStringsOrNot() = Option(c.getSomeStrings).getOrElse(Array[String]())
}

for (str <- myInstance.getSomeStringsOrNot) ...