When jumping out of the loop in java, we can just break directly, but there is no break in scala, so how to break out of the loop?
Look directly at the demo below:
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()
}
}
}
}
}
This place needs to be included with loop.breakable, otherwise an error will be reported
Exception in thread "main" scala.util.control.BreakControl
Let's take a look at the source code of Breaks:
/* __ *\
** ________ ___ / / ___ 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
As can be seen from the above source code, the principle of combining breakable and break methods to control the loop is to use the break method to throw an exception, and then the breakable method catches the exception, thereby ending the execution of the code in the entire breakable method block, but does not affect Breakable method in vitro code execution, so as to achieve control.
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