Scala类型不匹配问题（预期是Map，发现是scala.collection.mutable.HashMap）

发布于 2024-11-30 08:05:50 字数 849 浏览 0 评论 0原文

我仍然是一个新手 Scala 程序员，如果这个问题看起来很幼稚，我很抱歉，但我搜索了一段时间并没有找到解决方案。我正在使用 Scala 2.8，并且我有一个类 PXGivenZ 定义为：

class PXGivenZ (val x:Int, val z:Seq[Int], val values: Map[Seq[Int], Map[Int, Double]] ){...}

当我尝试将该类的一个元素实例化到另一个程序块中时，如下所示：

// x is an Int
// z is a LinkedList of Int
...
var zMap = new HashMap[Seq[Int], HashMap[Int, Double]]
...
val pxgivenz = new PXGivenZ(x, z, zMap)

我收到以下错误：

found   : scala.collection.mutable.HashMap[Seq[Int],scala.collection.mutable.HashMap[Int,Double]]
 required: Map[Seq[Int],Map[Int,Double]]
           val pxgivenz = new PXGivenZ(x, z, zMap) 
                                             ^

显然有一些我不明白的内容：如何Map[Seq[Int],Map[Int,Double]] 与 HashMap[Seq[Int], HashMap[Int,Double]] 不同吗？或者“可变”类出了什么问题？

预先感谢任何愿意帮助我的人！

原文

I am still a newbie Scala programmer, so sorry if this question may look naive, but I searched for a while and found no solutions. I am using Scala 2.8, and I have a class PXGivenZ defined as:

class PXGivenZ (val x:Int, val z:Seq[Int], val values: Map[Seq[Int], Map[Int, Double]] ){...}

When I try to instantiate an element of that class into another block of program like this:

// x is an Int
// z is a LinkedList of Int
...
var zMap = new HashMap[Seq[Int], HashMap[Int, Double]]
...
val pxgivenz = new PXGivenZ(x, z, zMap)

I get the following error:

found   : scala.collection.mutable.HashMap[Seq[Int],scala.collection.mutable.HashMap[Int,Double]]
 required: Map[Seq[Int],Map[Int,Double]]
           val pxgivenz = new PXGivenZ(x, z, zMap) 
                                             ^

There is clearly something I don't get: how is a Map[Seq[Int],Map[Int,Double]] different from a HashMap[Seq[Int], HashMap[Int,Double]]? Or is something wrong with the "mutable" classes?

Thanks in advance to anyone who will help me!

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熟人话多 2024-12-07 08:05:50

默认情况下，在 scala 文件中导入的 Map 是 scala.collection.immutable.Map 而不是 scala.collection.Map。当然，就您而言，HashMap 是一个可变映射，而不是不可变映射。

因此，如果您希望 Map 在文件中引用 scala.collection.Map，则必须显式导入它：

import scala.collection.Map

这种选择的原因是您不会操作不可变结构和可变结构以同样的方式。因此，scala 默认情况下推断您将使用“最安全”的不可变结构。如果您不想这样做，则必须明确更改它。

By default, the Map that is imported in a scala file is scala.collection.immutable.Map and not scala.collection.Map. And of course, in your case, HashMap is a mutable map, not an immutable one.

Thus if you want that Map refers to scala.collection.Map in your file, you have to import it explicitely:

import scala.collection.Map

The reason of this choice is that you will not manipulate an immutable and a mutable structure in the same way. Thus, scala infers by default that you will use immutable structure which are "most secure". If you don't want to do so, you must change it explicitly.

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