= Solution
The <UML class diagram> shows the subject's collection of observers and the added concrete observer. Each concrete company also retains the `SomeService` reference shown in its attributes; notifications call `meth()` through `Company`.
\Image[/past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2016/imperative-resit-observer-uml.png]
{title=<Observer pattern> for service values, extended with a NonProfit company}
Use an unscaled value both initially and in every callback:
``
class NonProfit(service: SomeService) extends Company {
private var v: Double = service.value
service.addCompany(this)
override def meth(): Unit = {
v = service.value
printf("New service value: %f\n", v)
}
}
``
Initializing `v` before registration prevents a callback from observing an uninitialized field if registration is later extended to notify immediately. For a service initially valued at `100.0`, this initializes `v` to `100.0`; after `updateValue(125.0)`, the callback sets `v` to `125.0` and prints `New service value: 125.000000` under a decimal-point locale. \b[The NonProfit update uses factor one.]
The word “always” requires a qualification in the supplied design. In the intended single-threaded notification protocol, $v=\texttt{service.value}$ holds after construction and after a completed `SomeService.updateValue`. However, the printed public `value_=` setter changes the subject without calling `sendMessage()`. Calling `service.value = ...` can therefore leave every cached observer value stale. No implementation that merely caches `v` in `meth()` can guarantee literal equality after that unannounced change. If equality on every query is required, replace the field by `private def v: Double = service.value`, retaining registration and the same printing callback; alternatively every mutation route must notify observers. The source's distinct private/public company multipliers are not changed.
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