Polymorphic solution

In object-oriented programming, polymorphism provides a powerful way to abstract an interface of many forms. Polymorphism literally means many forms in Greek.

The common interface of the households contains a function called performService to signal the start of the service, as well as a function called wasServicePerformed to return true if the service was performed as per the agreement:

interface Service {
    fun performService(time: Instant)
    fun wasServicePerformed(): Boolean
}

This solution can be illustrated in the following UML class diagram:

Figure 3.2 – Polymorphic solution

The Plumbing class is relatively simple. It provides a function for the household to start the service, a function to report the service completed, and a function for the other household to confirm that the service has been performed. Meanwhile, timestamps are recorded that indicate when the service...