 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.

Use an unscaled value both initially and in every callback: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. The NonProfit update uses factor one.

The word “always” requires a qualification in the supplied design. In the intended single-threaded notification protocol, $v=\text{service.value}$ 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.