在 Scala 中初始化一个 var
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initialise a var in scala
提问by user unknown
I have a class where I like to initialize my var by reading a configfile, which produces intermediate objects/vals, which I would like to group and hide in a method. Here is the bare minimum of the problem - I call the ctor with a param i, in reality a File to parse, and the init-method generates the String s, in reality more complicated than here, with a lot of intermediate objects being created:
我有一个类，我喜欢通过读取配置文件来初始化我的 var，该文件生成中间对象/val，我想将它们分组并隐藏在方法中。这是问题的最低限度 - 我用参数 i 调用 ctor，实际上是要解析的文件，而 init 方法生成 String s，实际上比这里更复杂，创建了许多中间对象：
class Foo (val i: Int) {
var s : String;
def init () {
s = "" + i
}
init ()
}
This will produce the error: class Foo needs to be abstract, since variable s is not defined. In this example it is easy to solve by setting the String to "": var s = "";, but in reality the object is more complex than String, without an apropriate Null-implementation.
这将产生错误：class Foo needs to be abstract, since variable s is not defined。在这个例子中，通过将 String 设置为 "": 很容易解决var s = "";，但实际上对象比 String 更复杂，没有适当的 Null 实现。
I know, I can use an Option, which works for more complicated things than String too:
我知道，我可以使用 Option，它也适用于比 String 更复杂的事情：
var s : Option [String] = None
def init () {
s = Some ("" + i)
}
or I can dispense with my methodcall. Using an Option will force me to write Some over and over again, without much benefit, since there is no need for a None else than to initialize it that way I thought I could.
或者我可以省去我的方法调用。使用 Option 将迫使我一遍又一遍地编写 Some ，没有太大好处，因为除了以我认为可以的方式初始化它之外，不需要 None 。
Is there another way to achieve my goal?
还有其他方法可以实现我的目标吗？
回答by sepp2k
var s : Whatever = _will initialize s to the default value for Whatever (null for reference types, 0 for numbers, false for bools etc.)
var s : Whatever = _将 s 初始化为 What 的默认值（引用类型为 null，数字为 0，bool 为 false 等）
回答by missingfaktor
Instead of creating separate methods for initialization, you should perform the initialization using the following way :
您应该使用以下方式执行初始化，而不是创建单独的初始化方法：
class Foo(val i: Int) {
var s: String = {
var s0 = " "
s0 += i
// do some more stuff with s0
s0
}
var dashedDate = {
val dashed = new SimpleDateFormat("yy-MM-dd")
dashed.format(updated)
}
// Initializing more than one field:
var (x, y, z) = {
var x0, y0, z0 = 0
// some calculations
(x0, y0, z0)
}
}
回答by egervari
Honestly, why are you using var? Why not just do:
老实说，你为什么使用var？为什么不这样做：
val rootObject = readFile(filename)
This would make the most sense to me.
这对我来说是最有意义的。
