Question

How can I use a for-comprehension that returns something I can assign to an ordered Map? This is a simplification of the code I have:

class Bar
class Foo(val name: String, val bar: Bar)
val myList: java.util.List[Foo] = ...
val result: ListMap[String, Bar] =
    for {
        foo <- myList
    } yield (foo.name, foo.bar)

I need to make sure my result is an ordered Map, in the order tuples are returned from the for-comprehension.

With the above, I get the error:

error: type mismatch;
found   : scala.collection.mutable.Buffer[(String,Bar)]
required: scala.collection.immutable.ListMap[String,Bar]
foo <- myList

This compiles:

class Bar
class Foo(val name: String, val bar: Bar)
val myList: java.util.List[Foo] = ...
val result: Predef.Map[String, Bar] =
    {
        for {
            foo <- myList
        } yield (foo.name, foo.bar)
    } toMap

but then I assume the map won’t be ordered, and I need an explicit toMap call.

How can I achieve this?

Answer 1

You can achieve do it by using the companion object of ListMap class as followings:

class Bar
class Foo(val name: String, val bar: Bar)
val myList: java.util.List[Foo] = ...
val result = ListMap((for(foo <- myList) yield (foo.name, foo.bar)):_*)