scala - 关于Scala变量可变性的问题

Question

我明白 val 关键字确定底层变量是不可变类型(以后不能重新分配)。现在我在 scala 编程中遇到了一个段落(第 3 章，scala 的下一步 - 使用类型参数化数组)，它指出

val greetStrings: Array[String] = new Array[String](3)
greetStrings(0) = "Hello"
greetStrings(1) = ", "
greetStrings(2) = "world!\n"

These three lines of code illustrate an important concept to understand about Scala concerning the meaning of val. When you define a variable with val, the variable can’t be reassigned, but the object to which it refers could potentially still be changed. So in this case, you couldn’t reassign greetStrings to a different array; greetStrings will always point to the same Array[String] instance with which it was initialized. But you can change the elements of that Array[String] over time, so the array itself is mutable.

所以改变数组的元素是有效的。如果我们这样定义它是无效的

greetStrings = Array("a","b","c")

它满足下面的陈述

When you define a variable with val, the variable can’t be reassigned, but the object to which it refers could potentially still be changed.

但如果我声明这样的事情

val str = "immutable string"

根据书中给出的定义

这是什么意思它所指的对象仍有可能被更改 在上面的代码行中？？

Answer 1

声明 val 不保证甚至暗示不可变类型。它只声明您可能在 Java 中调用的内容 最后 多变的。标识符不能重新分配，但值可能是可变类型。

在您的字符串值示例中，您同时拥有 val 和一个不可变类型，String。所以这个标识符既不可重新分配也不可修改(不可变)。