Question

我有以下代码。我已经开始学习scala所以可能会有更好的方法来做这些东西，但我想学习它的每一点。如果代码看起来很幼稚，请耐心等待。

  class ColaProduct() extends Product{
    override def productName = "Cola"
    override def productDetails = "Chilled Cola"
    override def toString(): String = super.toString()
  }

  class MilkProduct() extends Product{
    override def productName = "Milk"
    override def productDetails = "Healthy Milk"
    override def toString(): String = super.toString()
  }

  trait Machine {
    private val productMap = scala.collection.mutable.Map[String, Product]()
    def addProduct(product: Product): Unit ={
      productMap += product.productName.toString -> product
    }
    def checkAvl(name :String): Product ={
      if(productMap contains(name)){
        return productMap.get(name).asInstanceOf[Product]
      } else null
    }
    def process(name :String)
  }

  class VendingMachineImpl() extends Machine{
    override def process(name : String): Unit ={
      val product = checkAvl(name)
      if(null !=product){
        print("Got you :"+product.toString())
      }
    }
  }

  trait Product {

    private val defaultString: String = "Default"
    def productName = defaultString
    def productDetails = defaultString

    override def toString(): String = {
      return productName + " || " + productDetails
    }
  }

  def main(args : Array[String]): Unit ={
    val vendingMachineImpl = new VendingMachineImpl()
    vendingMachineImpl.addProduct(new ColaProduct)
    vendingMachineImpl.addProduct(new MilkProduct)

    vendingMachineImpl.process("Cola")
  }

例外：

Exception in thread "main" java.lang.ClassCastException: scala.Some cannot be cast to Product
    at vendingMachine$Machine$class.checkAvl(vendingMachine.scala:27)
    at vendingMachine$vendingMachineImpl.checkAvl(vendingMachine.scala:33)
    at vendingMachine$vendingMachineImpl.process(vendingMachine.scala:35)
    at vendingMachine$.main(vendingMachine.scala:47)

我认为，一旦我用指定的类型定义map，我就不必匹配一次从map中检索值。这种理解是否正确如果不是，请告诉我这里出了什么问题。

Answer 1

如果我们查看scala.collection.mutable.Map.get()的scaladoc，方法签名将被描述为：

 abstract def get(key: A): Option[B]

该方法返回类型Option的值;如果您尝试将其强制转换为某种不相关的类型，则会获得ClassCastException，与任何其他不兼容的类型一样。如果您需要产品，则需要：

检查Option是否为空
展开

这是一种方法（不改变其余代码）：

trait Machine {
  // Notes:
  // - no need to use 'return' keyword
  // - It's good practice in Scala not to return null: use Option for optional values
  def checkAvl(name :String): Option[Product] =
    productMap.get(name)
}

class VendingMachineImpl() extends Machine{
  override def process(name : String): Unit ={
    val product = checkAvl(name)
    // Think of Option as a collection containing 0 or 1 element: you can use
    // foreach, map, etc.
    // Also, string interpolation (with the s"" syntax) is cool. :-)
    product.foreach(print(p => s"Got you : $p"))
  }
}

编辑：此外，在Scala中，由于模式匹配，通常可以避免显式强制转换。例如，如果您想要明确地从Product打开Option[Product]，您可以使用：

val opt: Option[Product] = ???
opt match {
  // type-safe cast to type Some and deconstruction of the object (we get out the value
  // wich was originally passed to the Some constructor) :
  case Some(product) => print(s"Got you : $product")
  // type-safe cast to type None :
  case None => // do nothing
}

编辑2：您还可以查看这两种方法，以便从Map中检索值，具体取决于您要执行的操作：

val map: Map[String, String] = Map("k1" -> "v1", "k2" -> "v2")

// getOrElse (using a default value)
map.getOrElse("k1", "default") // returns "v1"
map.getOrElse("foobar", "default") // returns "default"

// apply (implementation-dependent, but fails fast by default)
map("k1") // returns "v1"
map("foobar") // throws a NoSuchElementException (it can be different for other Map
              // implementations, but it's the default behavior)

Answer 2

尝试productMap.get（name）.get（）。asInstanceOf [产品] 或者只是productMap（name）.asInstanceOf [Product]

scala.Some无法强制转换为自定义对象

2 个答案: