= Solution
Choose the following explicit policy: an edit of pounds recomputes euros; an edit of euros recomputes pounds; an edit of the rate keeps pounds fixed and recomputes euros. Amounts may be any finite signed values, while a rate must be finite and strictly positive. Invalid text, non-finite numbers and overflowing conversions leave the last accepted state intact and restore its display.
``
import scala.swing._
import scala.swing.event.EditDone
import scala.util.Try
object CurrencyConverter extends SimpleSwingApplication {
def top: Frame = new MainFrame {
title = "Currency converter"
val pounds = new TextField(8)
val euros = new TextField(8)
val rate = new TextField(8)
private var poundsValue = 10.0
private var rateValue = 1.15
private var updating = false
private def finite(x: Double): Boolean =
java.lang.Double.isFinite(x)
private def money(x: Double): String =
java.lang.String.format(java.util.Locale.ROOT, "%.2f",
java.lang.Double.valueOf(x))
private def showValues(): Unit = {
updating = true
try {
pounds.text = money(poundsValue)
euros.text = money(poundsValue * rateValue)
rate.text = java.lang.Double.toString(rateValue)
} finally updating = false
}
private def commit(field: TextField): Unit = {
if (!updating) {
val proposed = Try(field.text.trim.toDouble).toOption
.filter(finite).map { value =>
if (field eq pounds) (value, rateValue)
else if (field eq euros) (value / rateValue, rateValue)
else (poundsValue, value)
}
proposed.foreach { case (newPounds, newRate) =>
if (finite(newPounds) && finite(newRate) && newRate > 0.0 &&
finite(newPounds * newRate)) {
poundsValue = newPounds
rateValue = newRate
}
}
showValues()
}
}
contents = new FlowPanel {
contents += new Label("Pounds")
contents += pounds
contents += new Label("Euros")
contents += euros
contents += new Label("Euros per pound")
contents += rate
}
listenTo(pounds, euros, rate)
reactions += {
case EditDone(source) if source eq pounds => commit(pounds)
case EditDone(source) if source eq euros => commit(euros)
case EditDone(source) if source eq rate => commit(rate)
}
showValues()
}
}
``
The numeric state has the <invariant> $eurosValue=poundsValue\cdot rateValue$ and $rateValue>0$; euros are derived instead of being stored independently. A proposal is validated before either stored value changes. The update flag prevents a synchronous programmatic display update from recursively committing another edit. Currency amounts display two decimal places, while the rate retains its parsed precision; display rounding is not repeatedly fed back into the numeric model. `Locale.ROOT` ensures that formatted decimal points match the input parser.
The code handles the `EditDone` events used by the paper rather than trying to parse every intermediate keystroke. It uses <Scala> 2 style and <Java> 8-or-newer numeric helpers. No <Scala> compiler or Swing runtime execution was available in the checking environment; the state transitions were checked independently, and the API usage was compared with the supplied contract and official <Scala>/Swing documentation.<Java 8 Double API documentation>[isFinite(double), parseDouble(String) and toString(double)]{ref}
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