Add first lecture 6 demos

src/main/scala/lecture6/01-ExcutorCreate.scala

+package lecture6
+
+import java.util.concurrent.ForkJoinPool
+
+// Run using `running lecture6.ExecutorsCreate`.
+@main def ExecutorsCreate =
+  val executor = new ForkJoinPool
+  executor.execute(new Runnable:
+    override def run() = log("This task is run asynchronously.")
+  )
+  Thread.sleep(500)

src/main/scala/lecture6/02-ExecutionContextCreate.scala

+package lecture6
+
+import scala.concurrent.ExecutionContext
+@main def ExecutionContextCreate =
+  val ectx = ExecutionContext.global
+  ectx.execute(new Runnable:
+    override def run() = log("This task is run asynchronously.")
+  )
+  Thread.sleep(500)

src/main/scala/lecture6/03-SimpleExecute.scala

+package lecture6
+
+import scala.concurrent.ExecutionContext
+def execute(body: => Unit) =
+  ExecutionContext.global.execute(new Runnable:
+    override def run() = body
+  )
+
+@main def SimpleExecute =
+  execute { log("This task is run asynchronously.") }
+  Thread.sleep(500)

src/main/scala/lecture6/04-OnePlaceBufferDemo.scala

+package lecture6
+
+import scala.concurrent.ExecutionContext
+import lecture1.thread
+
+class OnePlaceBuffer[Elem]:
+  var elem: Elem = _
+  var full = false
+  def put(e: Elem): Unit = this.synchronized {
+    while full do wait()
+    elem = e
+    full = true
+    log(f"Put $e in the buffer")
+    notifyAll()
+  }
+  def get(): Elem = this.synchronized {
+    while !full do wait()
+    full = false
+    notifyAll()
+    log(f"Got $elem from the buffer")
+    elem
+  }
+
+@main def OnePlaceBufferDemo =
+  val buffer = OnePlaceBuffer[Int]()
+  val getThreads = for i <- 0 until 10 yield thread { buffer.get() }
+  val putThreads = for i <- 0 until 10 yield thread { buffer.put(i) }
+  putThreads.foreach(_.join())
+  getThreads.foreach(_.join())

src/main/scala/lecture6/common.scala

+package lecture6
+
+val log = println(_)