ButtonApp.scala

import scala.swing._
import scala.swing.event._

object ButtonApp extends SimpleSwingApplication {
  def top = new MainFrame {
    title = "My Frame"
    contents = new GridPanel(2, 2) {
      hGap = 3
      vGap = 3
      contents += new Button {
        text = "Press Me!"
        reactions += {
          case ButtonClicked(_) => text = "Hello Scala"
        }
      }
    }
    size = new Dimension(300, 80)
  }
}

https://lampsvn.epfl.ch/trac/scala/browser/scala/trunk/src/swing/scala/swing/test/ButtonApp.scala
http://booksites.artima.com/programming_in_scala_2ed/examples/html/ch34.html