Scala初始化空栈

Question

我在 Scala 中有以下结构：

var pos = new HashMap[Char, Stack[Int]] withDefaultValue Stack[Int].empty

我正在尝试将所有堆栈初始化为空。

但是，我收到以下错误：

Solution.scala:11: error: missing arguments for method apply in class GenericCompanion;
follow this method with `_' if you want to treat it as a partially applied function
        var pos = new HashMap[Char, Stack[Int]] withDefaultValue Stack[Int].empty

如果我尝试删除元素类型信息：

var pos = new HashMap[Char, Stack[Int]] withDefaultValue Stack.empty

它也失败了：

Solution.scala:11: error: type mismatch;
 found   : scala.collection.mutable.Stack[Nothing]
 required: scala.collection.mutable.Stack[Int]
Note: Nothing <: Int, but class Stack is invariant in type A.
You may wish to investigate a wildcard type such as `_ <: Int`. (SLS 3.2.10)
        var pos = new HashMap[Char, Stack[Int]] withDefaultValue Stack.empty

我正在使用可变集合：

import collection.mutable._

如何将堆栈初始化为空？

Answer 1

empty 是伴侣 Stack 对象上的通用方法，因此您需要为其提供类型参数，否则您会收到 Stack[Nothing]。

使用Stack.empty[Int]：

var pos = new HashMap[Char, Stack[Int]] withDefaultValue Stack.empty[Int]