scala 语法糖：_* 用于将 Seq 视为方法参数
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Syntax sugar: _* for treating Seq as method parameters
提问by user7865221
I just noticed this construct somewhere on web:
我刚刚在网络上的某个地方注意到了这个结构：
val list = List(someCollection: _*)
What does _*mean? Is this a syntax sugar for some method call? What constraints should my custom class satisfy so that it can take advantage of this syntax sugar?
什么_*意思？这是某些方法调用的语法糖吗？我的自定义类应该满足哪些约束才能利用这种语法糖？
回答by Kevin Wright
Generally, the :notation is used for type ascription, forcing the compiler to see a value as some particular type. This is not quitethe same as casting.
通常，该:符号用于类型归属，强制编译器将值视为某种特定类型。这不是很一样的铸件。
val b = 1 : Byte
val f = 1 : Float
val d = 1 : Double
In this case, you're ascribing the special varargstype. This mirrors the asterisk notation used for declaring a varargs parameter and can be used on a variable of any type that subclasses Seq[T]:
在这种情况下，您将归因于特殊的varargs类型。这反映了用于声明 varargs 参数的星号符号，可用于任何类型的子类变量Seq[T]：
def f(args: String*) = ... //varargs parameter, use as an Array[String]
val list = List("a", "b", "c")
f(list : _*)
回答by David K.
That's scala syntax for exploding an array. Some functions take a variable number of arguments and to pass in an array you need to append : _*to the array argument.
这是用于爆炸数组的 Scala 语法。某些函数采用可变数量的参数，要传入数组，您需要附加: _*到数组参数。
回答by GraceMeng
Variable (number of) Arguments are defined using *. For example,
变量（数量）参数使用 * 定义。例如，
def wordcount(words: String*) = println(words.size)
def wordcount(words: String*) = println(words.size)
wordcount expects a string as parameter,
wordcount 需要一个字符串作为参数，
scala> wordcount("I")
1
but accepts more Strings as its input parameter (_* is needed for Type Ascription)
但接受更多字符串作为其输入参数（类型归属需要 _*）
scala> val wordList = List("I", "love", "Scala")
scala> wordcount(wordList: _*)
3
