173. Closing the electrical panel before JDK 17

By its nature, an electrical panel is a closed unit of work. But our code from the previous problem is far from being a closed hierarchy. We can extend and implement almost any class/interface from inside or outside the hierarchy.

Using anything before JDK 17, closing a hierarchy of classes and interfaces can be done using several tools.

Applying the final modifier

For instance, we have the powerful final modifier. Once we declare a class as final, it cannot be extended, so it is completely closed to extension. Obviously, we cannot apply this technique consistently across a hierarchical model because it will lead to a non-hierarchical model.

If we scan our electrical panel model, then we can use the final modifier in several places. First, we eliminate interfaces (ElectricComponent and ElectricBreaker) since interfaces cannot be declared as final. Next, we can look at the ElectricCircuit class and its subclasses (ParallelCircuit...