構造函數調用期間和創建對象後的參數驗證

Question

我想檢查構造函數參數並拒絕構造拋出IllegalArgumentException（如果參數設置無效（值不符合預期約束））。同時檢查應該在修改對象時設置相同的參數集的同時工作。如何在Scala中編寫這個代碼？

scala> class Rectangle (var Length:Double, var Width:Double){ 
    | if (Length <0) 
    | throw new IllegalArgumentException("The Length must be Non-negative") 
    | if (Width <0) 
    | throw new IllegalArgumentException("The Width must be Non-negative") 
    | def setLength(l:Double) = Length = l 
    | } 
    defined class Rectangle 

    scala> var R = new Rectangle (-9, -9) 
    java.lang.IllegalArgumentException: The Length must be Non-negative 
at Rectangle.<init>(<console>:9) 

    scala> var R = new Rectangle (0, -9) 
    java.lang.IllegalArgumentException: The Width must be Non-negative 
at Rectangle.<init>(<console>:11) 



    scala> var R = new Rectangle(9, 9) 
    R: Rectangle = Rectangle@1590164 


    scala> R.Length 
    res7: Double = 9.0 

    scala> R.Width 
    res8: Double = 9.0 

    scala> R.setLength(18) 

    scala> R.Length 
    res10: Double = 18.0 

    scala> R.setLength(-9) 
    // R.setLength should not the set the Length to -9. ************************** 
    scala> R.Length 
    res12: Double = -9.0 

Answer 1

它必須是可變的嗎？

case class Rectangle(length:Double, width:Double) { 
    if (length < 0) 
    throw new IllegalArgumentException("The Length must be Non-negative") 
    if (width < 0) 
    throw new IllegalArgumentException("The Width must be Non-negative") 
    def setLength(l: Double) = copy(length=l) 
} 

Answer 2

你需要一個屬性setter/getter方法。

class Rectangle(private var l: Int, private var w: Int) { 
    require(l > 0, "Invalid length: " + l + " <0 ") 
    require(w > 0, "Invalid width: " + w + " <0 ") 
    def length = l 
    def width = w 
    def length_=(len: Int) { require(len > 0, "Invalid length: " + len + " < 0"); l = len } 
    def width_=(wid: Int) { require(wid > 0, "Invalid width: " + wid + " < 0"); w = wid } 
    override def toString = length + ", " + width 
} 

Answer 3

可變狀態是一個壞主意，但如果你需要它，不希望驗證的兩個點，你可以使用王子約翰·韋斯利的略微修改的方案：

class Rectangle(l: Int, w: Int) { 

    private[this] var _length: Int = _ 
    private[this] var _width: Int = _ 
    def length: Int = _length 
    def width: Int = _width 
    def length_=(len: Int) { require(len > 0, "Invalid length: " + len + " < 0"); _length = len } 
    def width_=(wid: Int) { require(wid > 0, "Invalid width: " + wid + " < 0"); _width = wid } 

    length = l 
    width = w 

    override def toString = length + ", " + width 
} 

Answer 4

我不喜歡到目前爲止提到的答案。你的問題是參數必須有效才能創建一個對象。其他解決方案中止對象創建，但在我看來，你真正想要的不是墮胎，而是在創建對象之前進行一些檢查。爲了實現這一點，你需要的工廠：

object Rectangle { 
    def apply(length:Double, width:Double): Rectangle = { 
    require(length >= 0, "The Length must be Non-negative") 
    require(width >= 0, "The Width must be Non-negative") 
    new Rectangle(length, width) 
    } 
} 
class Rectangle private(val length:Double, val width:Double) { 
    def length_=(l: Double) = Rectangle(l, width) 
} 

這裏唯一的問題是，我們是不允許創建一個案例類的時候，我們要定義我們自己的應用，方法（有作爲類的構造相同的方法簽名） 。