How to define trait for adding to Seq?

Question

I have a beginners Scala question. I have a class, Sample which extends the trait SampleAPI. Now I'm trying to build a sequence of Sample instances using seq. I will look something like this:

var samples: Seq[SampleAPI] = Seq()
for(...) {
    samples :+= new Sample(...))
}

This gives me the following compiler error: "type mismatch; found : Seq[java.lang.Object] required: Seq[se.uu.medsci.queue.setup.SampleAPI]"

So I tried:

samples :+= (new Sample(sampleName, this, illuminaXMLReportReader)).asInstanceOf[SampleAPI]

Which instead throws a runtime exception, saying that Sample cannot be bast to SampleAPI. I guess that this comes down to a problem in my understanding of the use of traits in Scala. Any help in figuring this out would be much appreciated.

Answer 1

Is the compiler error coming up on this line?

samples :+= new Sample(...))

If so, I think the problem is that your Sample class doesn't actually extend SampleAPI .

What's happening has to do with the contravariant type parameter of the List type in Scala. If you start with a List[SampleAPI] , then add a Sample to that list, it needs to find the least-upper-bound on the types included in the list to use as the new type parameter. If Sample is a SampleAPI , then the least-upper-bound is just SampleAPI and you get a List[SampleAPI] as the result of the :+= operation. However, if Sample is not a SampleAPI then the least-upper-bound on the two types is just Object , hence your compiler error saying that it was expecting a Seq[SampleAPI] but found a Seq[Object] .