我正在实现一个类，它充当依赖于同一数组的三个堆栈（来自 Gayle Laakmann 的破解编码采访）。
我无法理解为什么此代码会导致类型不匹配：
class ThreeStacks[A, B, C ](val stackSize:Int = 1000) {
/**
* Push a value on to the first stack
* @param value value of type A */
def push1[A](value :A): Unit = stack1.push(value)
/**
* Push a value on to the second stack
* @param value value of type B */
def push2[B](value: B): Unit = stack2.push(value)
/**
* Push a value on to the third stack
* @param value value of type C */
def push3[C](value: C): Unit = stack3.push(value)
/**
* Pops the top value off the first stack
* @return Some(value) at the top of the stack, None if the stack is empty */
def pop1[A](): Option[A] = stack1.pop()
/**
* Pops the top value off the second stack
* @return Some(value) at the top of the stack, None if the stack is empty */
def pop2[B](): Option[B] = stack2.pop()
/**
* Pops the top value off the second stack
* @return Some(value) at the top of the stack, None if the stack is empty */
def pop3[C](): Option[C] = stack3.pop()
// the shared stacks
private val stack1 = SharedStack[A]()
private val stack2 = SharedStack[B]()
private val stack3 = SharedStack[C]()
// the shared stack class is simply a container for push and pop functions
class SharedStack[T](val push: T => Unit, val pop: () => Option[T])
// the sharedstack object is a factory setups a shared stack object with its proper push/pop functions that are received
// from the stackmemory instance
private object SharedStack{
val stackMemory = new StackMemory(stackSize)
def apply[T](): SharedStack[T] = {
val stackTuple = stackMemory.registerStack[T]()
new SharedStack(stackTuple._1, stackTuple._2)
}
}
// the memory object responsible for handling the sharing of the array
private class StackMemory(val stackSize: Int = 1000){
// helps prevent the conflict warnings with immutable and mutable collections
private val MutableMap = scala.collection.mutable.Map
// the array that contains the stack nodes
private val array = new Array[Option[StackNode[Any]]](stackSize)
// keeps track of the heads of all the stacks
private val registeredStackHeads = MutableMap[Int, Int]()
// an index to help assign every new stack a unique key
private var nextRegistrationIndex = 1
// a list of indices in the array that have been freed
private var freedIndices = List[Int]()
// the next available free index, increments till it reaches the end of the array, different from the freeIndices list
// in that it only is concerned with free cells while they are available at the end of the array
private var nextFreeIndex = 0
/**
* Registers a stack within the memory object and returns the push/pop functions
* @return the push/pop functions relevant for that stack */
def registerStack[T](): (T => Unit, () => Option[T]) = {
val index = nextRegistrationIndex
val noPreviousValueIndex = -1
// register the index in the stack heads with an empty value
registeredStackHeads(index) = noPreviousValueIndex
nextRegistrationIndex += 1
(push[T](index), pop[T](index) )
}
/**
* Returns the push function for a stack at the given index
* @param stackIndex the index of the requesting stack
* @return the push function for the stack */
def push[T](stackIndex: Int) = {
(value: T) => {
// get the indices and initialize a stack node
val headIndex = registeredStackHeads(stackIndex)
val index = nextAvailableIndex()
val stackNode = new StackNode[T](headIndex, value)
// store the new stack head and place it in to the array
registeredStackHeads(stackIndex) = index
array(index) = Some(stackNode)
}
}
/**
* The pop function for the stack at a given index
* @param stackIndex the index of the requesting stack
* @return the pop function for the stack */
def pop[T](stackIndex: Int): () => Option[T] = {
() => {
val headIndex = registeredStackHeads(stackIndex)
if(headIndex < 0) None else {
array(headIndex) match {
case Some(s : StackNode[T]) => {
registeredStackHeads(stackIndex) = s.previousIndex
freeIndex(headIndex)
Some(s.value)
}
case _ => None
}
}
}
}
// retrieve the next available index, throws an exception if there are no available indices
private def nextAvailableIndex():Int = {
// return a new index if the array isn't full otherwise throw an exception
def newIndex() : Int = {
if(nextFreeIndex >= stackSize){
throw new Exception("No more room in stack array available")
}
else{
val index = nextFreeIndex
nextFreeIndex += 1
index
}
}
// first check if any of the old indices have been freed
freedIndices match {
case Nil => newIndex()
case head::tail => freedIndices = tail; head
}
}
// frees an index and adds it to the free indices list
private def freeIndex(i: Int): Unit = {
if (i >= array.length) throw new ArrayIndexOutOfBoundsException("The index to be freed does not exist in the array")
array(i) = None
freedIndices = i::freedIndices
}
// covariant node that helps keep track of the previous index of the stack
private class StackNode[+S](val previousIndex: Int, val value: S)
}
}
如果我调用 stack1.push 或 stack1.pop，代码的工作方式与假设相同，但它会导致包装器 push1 和 pop1 上的类型不匹配，为什么会发生这种情况？
example for push1:
type mismatch;
found : value.type (with underlying type A)
required: A
def push1[A](value :A): Unit = stack1.push(value)
example for pop1:
type mismatch;
found : Option[A(in class ThreeStacks)]
required: Option[A(in method pop1)]
def pop1[A](): Option[A] = stack1.pop()