为什么在 Scala 中使用 def 和 val，反之亦然 [英] why use def and val in Scala or vice versa

问题描述

我知道这之前已经在其他帖子中讨论过，并且我理解使用 def 和 val 之间的基本区别.def 用于定义方法，val 用于定义不可变引用.通过提出这个问题，我试图完成的是了解 def 是否还有其他内容.它可以与 val 互换使用吗?

I know this has been discussed on SO in other posts before and I understand the basic difference between the use of def and val. def is used for defining a method and val for an immutable reference. What I am trying to accomplish by asking this question is to understand if there is something more to def. Can it be used interchangeably with a val?

最近我尝试了以下代码，如果我目前对 def 的理解足够，我无法说服自己:

Recently I tried the following code and cannot convince myself if my present understanding of def is sufficient:

scala> def i: Int = 3
i: Int

scala> i
res2: Int = 3

所以我很好奇，这等价于val i = 3吗?

So I am curious, is this equivalent to val i = 3?

然后我尝试了这个:

scala> i()
<console>:9: error: Int does not take parameters
i()

我这样做只是为了测试我对 def 语义的理解.现在我想知道，当 i 是一个方法时，为什么 Scala 会抱怨...不带参数"?

I did this just to test my understanding of the semantics of def. Now I want to know, when i is a method, why Scala complains with "...does not take parameters"?

接下来我尝试了以下操作:

Next I tried the following:

scala> def i(): Int = 3
i: ()Int

scala> i()
res4: Int = 3

这一次 Scala 似乎同意 i 是一种方法.那么我可以使用 def 代替 val 来声明和初始化变量吗?

This time Scala seems to agree that i is a method. So can I use def in place of val interchangeable to declare and initialize a variable?

推荐答案

Both

def i = 3

和

def i() = 3

声明方法.唯一的区别是，第一个是没有参数列表的方法，第二个是带有空参数列表的方法.前者通常用于没有副作用的方法，后者用于有副作用的方法.如果值永远不会改变并且您想避免重新计算它，您应该使用 val 而不是 def.def 每次被调用时都会重新计算，而 val 只被赋值一次.

declare methods. The only difference is, that the first one is a method without a parameter list and the second is a method with an empty parameter list. The former is usually used for methods without side effects and the latter for methods with side effects. You should use a val instead of a def if the value never changes and you want to avoid recomputing it. A def gets recomputed every time it is called, while a val is assigned a value only once.

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