[comment]: # Scala underscore的用途
_ 的用途
// import all
import scala.io._
// import all, but hide Codec
import scala.io.{Codec => _, _}
// import all, but rename Codec as CodeA
import scala.io.{Codec => CodeA, _}
// import all from an object
import scala.io.Codec._
object Main {
// Higher kinded type parameter
import scala.language.higherKinds
class CollectionAdapter[T[_]] {
def getCollectionFromValue[A](a: A, f: A => T[A]): T[A] = {
f(a)
}
def intToList(a: Int): List[Int] = {
List(a)
}
}
def main(args: Array[String]): Unit = {
// Higher kinded type parameter
val adapter = new CollectionAdapter[List]()
println(adapter.getCollectionFromValue(1, adapter.intToList))
// output: List(1)
// Initialize a variable with default value
var i:Int = 0 // _ cannot be used in a function
println(i)
// output: 0
// Partial function application, Assign a function rather than run it.
def fun1(): Unit = {println("fun1 is invoked.")}
def obj1 = fun1 // run fun1 and return the result to obj1
// output: fun1 is invoked.
// output: ()
def fun2 = fun1 _ // assign fun1 to fun2
println(fun2)
// output: <function0>
// Anonymouse function, scala will infer it base on the context.
val list1 = (1 to 10)
println(list1.filter(_ > 5))
// output: Vector(6, 7, 8, 9, 10)
// equivalent to:
println(list1.filter(a => a > 5))
// output: Vector(6, 7, 8, 9, 10)
// setter function
class A {
private var _count = 0
// getter
def count = {_count}
// setter
def count_= (n: Int) = {_count = n}
// bang ??
def bang_! (n: Boolean) = {5}
}
val a = new A
a.count = 5
println(a.count)
// output: 5
// Pattern match
def matchTest(x: Any): String = x match {
case 1 => "one"
case 2 => "two"
case List(0, _, _) => "a list with three elements and the first element is 0"
case List(_*) => "a list with zero or more elements"
case _: Map[_, _] => "matches a map with any key type and any value type"
case _ => "anything else"
}
// anonymous variable. (or we can say ignored variable)
for (_ <- 1 to 2) {
println("hi")
}
// Sequence list2 is passed as multiple parameters to f(a: T*)
def fun3(a: Int*) = { a.map(println(_)) }
fun3(1 to 3: _*)
// output:
// 1
// 2
// 3
// Access tuple
var t = new Tuple3(1, "Two", "Three")
println(t._2)
// output: Two
}
}