Scala按类型收集过滤器 [英] Scala collection filter by type

问题描述

我想获取只包含特定类型元素的现有集合的子集合。下面的工作：

pre code $ class C（val name：String）
class D（name：String）extends C新的C（C1），新的D（D1），新的C（C2），新的D（D2 ））
collection.collect {case d：D => d} .size必须是=== 2 //工程

但是当我尝试扩展集合带有方法onlyInstancesOf [Type]的类不起作用。首先我的实现：

pre code $ object隐藏def extendScalaCollection [E]（coll：Traversable [E]）= new CollectionExtension [E]（coll）
}

class CollectionExtension [E]（coll：Traversable [E]）{

def onlyInstancesOf [SpecialE< ;: E：Traversable [SpecialE] = {
coll.collect（{case special：SpecialE => special}）。asInstanceOf [Traversable [SpecialE]]
}
}

所以当我使用这个扩展名并执行时：

  collection.onlyInstancesOf [D] .size必须是=== 2 
  

我得到一个错误，.size返回4而不是2.另外，我检查，结果实际上包含C1和C2，虽然它不应该。

当我这样做：

  collection.onlyInstancesOf [D] .foreach（e => println（e.name））
  code> 

我得到这个例外：



  java.lang.ClassCastException：Collec tionsSpec $$ anonfun $ 1 $ C $ 1不能转换为CollectionsSpec $$ anonfun $ 1 $ D $ 1 
  

显然结果集仍然包含应该被过滤掉的元素。



我不明白为什么发生这种情况，任何人都可以解释这一点吗？

编辑：
Scala：Scala代码运行器版本2.8.0.final


解决方案

注意编译器警告，并添加-unchecked你的scala命令行选项。



  M：\> scala -unchecked 
欢迎使用Scala 2.8.0.final（Java HotSpot（TM）客户机VM，Java 1.6.0_21）
。 
键入表达式让他们评估。 
输入：help获取更多信息。 
 
 scala> class CollectionExtension [E]（coll：Traversable [E]）{
 | 
 | def onlyInstancesOf [SpecialE<：E]：Traversable [SpecialE] = {
 | coll.collect（{case special：SpecialE => special}）。asInstanceOf [Traversable [SpecialE]] 
 | } 
 | } 
< console>：8：warning：抽象类型SpecialE在类型模式下SpecialE被取消选中，因为它被擦除
 coll.collect（{case special：SpecialE => special}）。asInstanceOf [Traversable [SpecialE]] 
 ^ 
定义的类CollectionExtension 
  

编译器可以做的最好的是相当于：

$ p code> coll.collect（{case special：AnyRef => special}） .asInstanceOf [Traversable [_]]


有关类型擦除的更详细的解释，在Manifests附近解决它，请参阅：

https：/ /stackoverflow.com/questions/tagged/type-erasure+scala

im new to scala and ran into the following problem:

I want to get a subcollection of an existing collection that only contains elements of a specific type. The following works:

class C(val name : String)
class D(name : String) extends C(name) { }

val collection = Set[C](new C("C1"),new D("D1"),new C("C2"),new D("D2"))
collection.collect{case d : D => d}.size must be === 2 // works

But when i try to extend the collection classes with a method "onlyInstancesOf[Type]" this does not work. First my implementation:

object Collection {
    implicit def extendScalaCollection[E](coll : Traversable[E]) = new CollectionExtension[E](coll)
}

class CollectionExtension[E](coll : Traversable[E]) {

    def onlyInstancesOf[SpecialE <: E] : Traversable[SpecialE] = {
        coll.collect({case special : SpecialE => special}).asInstanceOf[Traversable[SpecialE]]
    }
}

So when i use this extension and execute:

collection.onlyInstancesOf[D].size must be === 2

I get an error that .size returned 4 and not 2. Also, i checked, the result actually contains C1 and C2 though it should not.

When i do:

collection.onlyInstancesOf[D].foreach(e => println(e.name))

I get the exception:

java.lang.ClassCastException: CollectionsSpec$$anonfun$1$C$1 cannot be cast to CollectionsSpec$$anonfun$1$D$1

So obviously the resulting set still contains the elements that should have been filtered out.

I dont get why this happens, can anyone explain this?

Edit: Scala: Scala code runner version 2.8.0.final

解决方案

Pay attention to the compiler warnings, and add -unchecked your scala command line options.

M:\>scala -unchecked
Welcome to Scala version 2.8.0.final (Java HotSpot(TM) Client VM, Java 1.6.0_21)
.
Type in expressions to have them evaluated.
Type :help for more information.

scala> class CollectionExtension[E](coll : Traversable[E]) {
     |
     |     def onlyInstancesOf[SpecialE <: E] : Traversable[SpecialE] = {
     |         coll.collect({case special : SpecialE => special}).asInstanceOf[Traversable[SpecialE]]
     |     }
     | }
<console>:8: warning: abstract type SpecialE in type pattern SpecialE is unchecked since it is eliminated by erasure
               coll.collect({case special : SpecialE => special}).asInstanceOf[Traversable[SpecialE]]
                                            ^
defined class CollectionExtension

The warning means that the best the compiler can do is equivalent to:

coll.collect({case special : AnyRef => special}).asInstanceOf[Traversable[_]]

For a more detailed explanation of type erasure, and ways you can work around it with Manifests, see:

https://stackoverflow.com/questions/tagged/type-erasure+scala

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