在java中跳出循環的時候,我們可以直接break就行了,但是在scala裏面沒有break,那怎麼跳出循環呢?
直接看下面的demo:
package test
import scala.util.control.Breaks
object ListDemo {
def main(args: Array[String]): Unit = {
var loop = Breaks
var i = 0
loop.breakable {
while (i < 10) {
println(i)
i += 1
if (i == 5) {
loop.break()
}
}
}
}
}
這個地方需要用loop.breakable包括一下,不然會報一個錯
Exception in thread "main" scala.util.control.BreakControl
下面看一下Breaks的源碼:
/* __ *\
** ________ ___ / / ___ Scala API **
** / __/ __// _ | / / / _ | (c) 2003-2013, LAMP/EPFL **
** __\ \/ /__/ __ |/ /__/ __ | http://scala-lang.org/ **
** /____/\___/_/ |_/____/_/ | | **
** |/ **
\* */
package scala
package util.control
/** A class that can be instantiated for the break control abstraction.
* Example usage:
* {{{
* val mybreaks = new Breaks
* import mybreaks.{break, breakable}
*
* breakable {
* for (...) {
* if (...) break()
* }
* }
* }}}
* Calls to break from one instantiation of `Breaks` will never
* target breakable objects of some other instantiation.
*/
class Breaks {
private val breakException = new BreakControl
/**
* A block from which one can exit with a `break`. The `break` may be
* executed further down in the call stack provided that it is called on the
* exact same instance of `Breaks`.
*/
def breakable(op: => Unit) {
try {
op
} catch {
case ex: BreakControl =>
if (ex ne breakException) throw ex
}
}
sealed trait TryBlock[T] {
def catchBreak(onBreak: =>T): T
}
/**
* This variant enables the execution of a code block in case of a `break()`:
* {{{
* tryBreakable {
* for (...) {
* if (...) break()
* }
* } catchBreak {
* doCleanup()
* }
* }}}
*/
def tryBreakable[T](op: =>T) = new TryBlock[T] {
def catchBreak(onBreak: =>T) = try {
op
} catch {
case ex: BreakControl =>
if (ex ne breakException) throw ex
onBreak
}
}
/**
* Break from dynamically closest enclosing breakable block using this exact
* `Breaks` instance.
*
* @note This might be different than the statically closest enclosing block!
*/
def break(): Nothing = { throw breakException }
}
/** An object that can be used for the break control abstraction.
* Example usage:
* {{{
* import Breaks.{break, breakable}
*
* breakable {
* for (...) {
* if (...) break
* }
* }
* }}}
*/
object Breaks extends Breaks
private class BreakControl extends ControlThrowable
從上面的源碼可以看出,breakable與break方法組合用於控制循環的原理就是利用break方法拋出一個異常,然後breakable方法再捕獲這個異常,從而結束整個breakable方法塊內代碼的執行,但是不影響breakable方法體外代碼的執行,從而實現控制。
如果有寫的不對的地方,歡迎大家指正,如果有什麼疑問,可以加QQ羣:340297350 謝謝