scala.Some不能轉換爲自定義對象

Question

我有以下代碼。我已經開始學習Scala，所以可能會有更好的方法來做這些事情，但我想學習它的每一點。如果代碼看起來很幼稚，請耐心等待。

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) 

我相信是有一次我與指定類型定義map我沒有一次匹配從地圖檢索值。如果不是，請讓我知道這裏出了什麼問題，這種理解是否正確？

Answer 1

如果我們看一下scaladoc爲scala.collection.mutable.Map.get()，方法簽名描述如：

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

該方法返回類型爲Option的值;如果您嘗試將其轉換爲某種不相關的類型，您將得到ClassCastException，與其他任何不兼容的類型一樣。如果你想有一個產品，你需要：

• 檢查Option不是空
• 拆開包裝內部包含

產品下面是做這件事的一種方式（不改變其他地區的代碼）：

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")) 
    } 
} 

---

編輯： 另外，在Scala中，由於模式匹配，通常可以避免顯式強制轉換。例如，如果你想明確地從Option[Product]解開的Product，你可以使用：

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 
} 

---

編輯2： 你還可以檢查出其他兩種方法用於從Map檢索值，取決於你正在嘗試做什麼：

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

嘗試productMap.get（名）獲得（）。asInstanceOf [產品] 或只是productMap（名稱）.asInstanceOf [產品]