Scala for-comprehension 返回有序映射 [英] Scala for-comprehension returning an ordered map

问题描述

如何使用 for-comprehension 返回我可以分配给有序 Map 的内容?这是我拥有的代码的简化:

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)

我需要确保我的结果是一个有序的 Map，按照从 for-comprehension 返回的顺序元组.

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

上面的，我得到错误:

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

编译:

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

但是我假设地图不会被订购，我需要一个显式的 toMap 调用.

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

我怎样才能做到这一点?

How can I achieve this?

推荐答案

可以通过使用 ListMap 类的伴生对象来实现，如下所示:

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)):_*)

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