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scala中下划线的含义

admin 11月 12, 2020 0

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Placeholder syntax

这个用法比较多，表示某一个参数。比如对collection调用方法map、filter、sortWith、foreach等等表示对每一个元素进行处理，甚至可以使用_.XXX方式

Scala represents anonymous functions with a elegant syntax. The _acts as a placeholder for parameters in the anonymous function. The _ should be used only once, But we can use two or more underscores to refer different parameters.

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List(1,2,3,4,5).foreach(print(_))
 

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List(1,2,3,4,5).foreach( a => print(a))

Here the _ refers to the parameter. The first one is a short form of the second one. Lets look at another example which take two parameters.

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val sum = List(1,2,3,4,5).reduceLeft(_+_)
 
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val sum = List(1,2,3,4,5).reduceLeft((a, b) => a + b)

There is a good post which explains the inner details of the above example.

Wildcard patterns

In Scala, pattern matching is somewhat similar to java switch statement. But it is more powerful.

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def (x: Int): String = x match {
   case 1 => "one"
   case 2 => "two"
   case _ => "anything other than one and two"
 }

In Scala, each selector will be matched with the patterns in the order they appear and the first match will be executed. _acts like a wildcard. It will match anything. Scala allows nested patterns, so we can nest the _also.Lets see another example that uses_ in nested pattern.

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expr match {
  case List(1,_,_) => " a list with three element and the first element is 1"
  case List(_*)  => " a list with zero or more elements "
  case Map[_,_] => " matches a map with any key type and any value type "
  case _ =>
  }

Wildcard imports

In scala, _acts similar to * in java while importing packages.

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import scala.util.matching._
// imports all the members of the object Fun. (static import in java)
import com.test.Fun._
// imports all the members of the object Fun but renames Foo to Bar
import com.test.Fun.{ Foo => Bar , _ }
// imports all the members except Foo. To exclude a member rename it to _
import com.test.Fun.{ Foo => _ , _ }

类的setter方法

getters-and-setters-in-scala

In scala, a getter and setter will be implicitly defined for all non-private var in a object. The getter name is same as the variable name and _= is added for setter name. We can define our own getters and setters. Ok lets see an example which uses the getter and setters.

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class Test {
   private var a = 0
   def age = a
   def age_=(n:Int) = {
      require(n>0)
      a = n
   }
}
 

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val t = new Test
t.age = 5
println(t.age)

Converting call-by-name parameters to functions

Scala is a functional language. So we can treat function as a normal variable. If you try to assign a function to a new variable, the function will be invoked and the result will be assigned to the variable. This confusion occurs due to the optional braces for method invocation. We should use _ after the function name to assign it to another variable.

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class Test {
  def fun = {
    // some code
  }
  val funLike = fun _
}

用于将方法转换成函数

参考资料:

  1. ananthakumaran.in
  2. zhihu
  3. stackoverflow
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