Unsafe and safe concurrent threads for experimenting in the Scala REPL

ThreadSafety.scala

var shared = 0
val lock = new java.lang.Object

def unsafe = { 
  val local = shared
  Thread.sleep(0)
  shared = local + 1
}

def safe = { 
  lock.synchronized { 
    val local = shared
    Thread.sleep(0)
    shared = local + 1
  } 
}

def runConcurrently(block: => Unit)(howMany: Int) = {
  shared = 0
  val ps = Seq.fill(howMany)(new java.lang.Thread {
    override def run() = block
  })
  ps foreach { _.start() }
  ps foreach { _.join() }
  shared
}

val vu = Iterator.continually(runConcurrently(unsafe)(4)) // infinite stream of results of running runUnsafe
vu take 1000 count { _ != 4 }          // see how many of the first 1000 are not 4

val vs = Iterator.continually(runConcurrently(safe)(4)) // infinite stream of results of running runSafe
vs take 1000 count { _ != 4 }          // see how many of the first 1000 are not 4