Before turning our attention to Java 9, let's take a look at one more significant language feature: default methods. Since the beginning of Java, an interface was used to define how a class looks, implying a certain type of behavior, but was unable to implement that behavior. This made polymorphism much simpler in a lot of cases, as any number of classes could implement a given interface, and the consuming code treats them as that interface, rather than whatever concrete class they actually are.
One of the problems that have confronted API developers over the years, though, was how to evolve an API and its interfaces without breaking existing code. For example, take the ActionSource interface from the JavaServer Faces 1.1 ...
