Design patterns exercise (command)

Calculator.scala

import java.awt.Dimension

import scala.swing.BorderPanel.Position
import scala.swing.event.ButtonClicked
import scala.swing.{BorderPanel, Button, GridPanel, MainFrame, TextField}


object Calculator {
  def main(args: Array[String]): Unit = {
    val frame = new MainFrame() {
      title = "Calculator"
      preferredSize = new Dimension(320, 240)
      contents = new Calculator
    }
    frame.visible = true
  }
}

class Calculator() extends BorderPanel {
  private val display = new TextField("0")
  private var result: Double = 0
  private var operator = "="
  private var calculating = true

  display.editable = false
  layout(display) = Position.North

  val panel = new GridPanel(4, 4) {
    val buttonLabels = "789/456*123-0.=+"
    var i = 0
    while (i < buttonLabels.length) {
      val b = new Button(buttonLabels.substring(i, i + 1))

      contents += b
      listenTo(b)
      reactions += {
        case ButtonClicked(`b`) =>
          buttonClicked(b.text)
      }
      i += 1
    }
  }
  layout(panel) = Position.Center

  private def buttonClicked(cmd: String): Unit = {
    if ('0' <= cmd.charAt(0) && cmd.charAt(0) <= '9' || cmd == ".") {
      if (calculating)
        display.text = cmd
      else
        display.text = display.text + cmd
      calculating = false
    } else {
      if (calculating) {
        if (cmd == "-") {
          display.text = cmd
          calculating = false
        } else
          operator = cmd
      } else {
        val x = display.text.toDouble
        calculate(x)
        operator = cmd
        calculating = true
      }
    }
  }

  private def calculate(n: Double): Unit = {
    if (operator == "+") result += n
    else if (operator == "-") result -= n
    else if (operator == "*") result *= n
    else if (operator == "/") result /= n
    else if (operator == "=") result = n
    display.text = "" + result
  }
}