爲什麼我的不可變對象突變Scala

Question

我在下面的代碼中遇到了一個最令人不安的錯誤。即使我將它作爲不可變的Map傳遞，按鈕Map也會發生變異。按鍵保持不變，並且地圖指向兩個不可變的Ints，但在下方，您可以看到地圖在運行過程中顯然具有不同的值。我絕對難住，不知道發生了什麼。

def makeTrace(trace : List[(String)], buttons : Map[String, (Int,Int)], 
    outputScreen : ScreenRegion, hashMap : Map[Array[Byte], String]) 
    : (List[(String,String)], Map[Array[Byte], String]) = { 

println(buttons.toString) 
//clearing the device 
val clear = buttons.getOrElse("clear", throw new Exception("Clear Not Found")) 
//clear.circle(3000) 
val thisButton = new ScreenLocation(clear._1, clear._2) 
click(thisButton) 

//updates the map and returns a list of (transition, state) 
trace.foldLeft((Nil : List[(String,String)], hashMap))((list, trace) => { 
    println(buttons.toString) 
    val transition : String = trace 
    val location = buttons.getOrElse(transition, throw new Exception("whatever")) 
    val button = new ScreenLocation(location._1, location._2) 
    button.circle(500) 
    button.label(transition, 500) 
    click(button) 

    //reading and hashing 
    pause(500) 
    val capturedImage : BufferedImage = outputScreen.capture() 
    val outputStream : ByteArrayOutputStream = new ByteArrayOutputStream(); 
    ImageIO.write(capturedImage, "png", outputStream) 
    val byte : Array[Byte] = outputStream.toByteArray(); 
    //end hash 

    //if our state exists for the hash 
    if (hashMap.contains(byte)){ list match { 
    case (accumulator, map) => ((transition , hashMap.getOrElse(byte, throw new Exception("Our map broke if"))):: accumulator, map) 
    } 
    //if we need to update the map 
    }else list match { 
    case (accumulator, map) => { 
     //adding a new state based on the maps size 
     val newMap : Map[Array[Byte], String] = map + ((byte , "State" + map.size.toString)) 
     val imageFile : File = new File("State" + map.size.toString + ".png"); 
     ImageIO.write(capturedImage, "png", imageFile); 
     ((transition, newMap.getOrElse(byte, throw new Exception("Our map broke else"))) :: accumulator, newMap) 
    }   
    } 
}) 

}

之前，我調用這個函數初始化我的地圖指向不可變對象的不可變映射。

val buttons = makeImmutable(MutButtons) 
    val traceAndMap = TraceFinder.makeTrace(("clear" ::"five"::"five"::"minus"::"five"::"equals":: Nil), buttons, outputScreen, Map.empty) 

哪裏makeImmutable是

def makeImmutable(buttons : Map[String, (Int,Int)]) : Map[String, (Int,Int)] = { 
    buttons.mapValues(button => button match { 
    case (x, y) => 
     val newX = x 
     val newY = y 
     (newX,newY) 
    }) 
} 

這裏是輸出，你可以看到清晰的狀態變化，減，五

Map(equals -> (959,425), clear -> (842,313), minus -> (920,409), five -> (842,377)) 
Map(equals -> (959,425), clear -> (842,313), minus -> (920,409), five -> (842,377)) 
Map(equals -> (959,425), clear -> (959,345), minus -> (920,409), five -> (842,377)) 
Map(equals -> (959,425), clear -> (842,313), minus -> (920,409), five -> (842,377)) 
Map(equals -> (959,425), clear -> (842,313), minus -> (920,409), five -> (881,377)) 
Map(equals -> (959,425), clear -> (842,313), minus -> (920,409), five -> (842,377)) 
Map(equals -> (959,425), clear -> (842,313), minus -> (920,441), five -> (842,377)) 
Map(equals -> (959,425), clear -> (842,313), minus -> (920,409), five -> (881,377)) 
Map(equals -> (959,425), clear -> (842,313), minus -> (920,409), five -> (842,377)) 
Map(equals -> (959,425), clear -> (842,313), minus -> (920,409), five -> (842,377)) 

Answer 1

首先，嘗試在你的代碼周圍噴灑println(map.getClass)。確保它確實是你認爲它的地圖。它應該有immutable在其包名，例如：

scala> println(Map(1->1, 2->2, 3->3, 4->4).getClass) 
class scala.collection.immutable.Map$Map4 

第二，確保你真的是打印出你以爲你是完全相同的地圖;使用參考身份哈希碼是一個很好的方法來做到這一點：println(System.identityHashCode(map))。

機會非常好，其中一件事情不會給你預期的結果。那麼你只需要弄清楚問題出在哪裏。如果沒有完整的可運行示例，如果沒有過多的代碼凝視，很難提供更好的建議。

Answer 2

我懷疑你有斯卡拉進口.collection.Map在makeImmutable函數的作用域中，它允許您將MutButtons（推測是一個可變的Map）傳遞給makeImmutable函數。 Map上的mapValues方法返回底層映射的視圖，而不是不可變的副本。例如：

import scala.collection.Map 
import scala.collection.mutable.{Map => MutableMap} 

def doubleValues(m: Map[Int, Int]) = m mapValues { _ * 2 } 

val m = MutableMap(1 -> 1, 2 -> 2) 
val n = doubleValues(m) 
println("m = " + m) 
println("n = " + n) 
m += (3 -> 3) 
println("n = " + n) 

運行此程序產生以下輸出：

m = Map(2 -> 2, 1 -> 1) 
n = Map(2 -> 4, 1 -> 2) 
n = Map(2 -> 4, 1 -> 2, 3 -> 6) 

要從makeImmutable返回一個真正不可變的映射，在所述映射的值後調用.toMap。