Scala 中如何使用 continue 和 break 跳出循环

梦里梦外; 2023-02-25 14:24 17阅读 0赞

**示例代码:**

object Test {
      def main(args: Array[String]): Unit = {
        import scala.util.control.Breaks._
        println("=============== Continue ===================")
        for (i <- 1 to 10) {
          breakable {
            if (i % 2 == 0) {
              break
            }
            println(i)
          }
        }
        println("=============== Continue ===================")
        for (i <- 1 to 10 if i % 2 != 0) {
            println(i)
        }
    
        println("=============== Break ===================")
        breakable {
          for (i <- 1 to 10) {
            if (i % 2 == 0) {
              break
            }
            println(i)
          }
        }
    
        println("=============== Break ===================")
        var loop = true
        for (i <- 1 to 10 if loop == true) {
          if( i % 2 == 0){
            loop = false
          }
          println(i)
        }
      }
    }

查看Breaks源码得知，breakable 与 break 方法组合用于控制循环的原理就是利用 break 方法抛出一个异常，然后 breakable 方法再捕获这个异常，从而结束整个 breakable 方法块内代码的执行，但是不影响 breakable 方法体外代码的执行，从而实现控制。  
**源码如下:**

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