I missed something

Classes can extend other classes and they implement interfaces. Interfaces can extend other interfaces. They can't implement anything. With Classes, you can only inherit one other class. With both classes and interfaces, however, you can inherit (extent or implement) as many interfaces as possible. The reason why you are limited to one and only one class is to avoid ambiguity with parent classes. Here is a quote that helped me understand: "Suppose I have one class, which is a Toaster, and another class, which is NuclearBomb. They both have an on() method. (One has an off(), the other doesn't.) If I want to create a class that's a subclass of both of these, then call the on() method.... which one will it call? As you can see, this is a problem that could really blow up in my face here." With interfaces the ambiguity won't matter since interfaces do not contain instruction. If you add ten interfaces that all have an on() method defined, only the class that implements those interfaces determines what will happen if you call the on() method. (Just because each of those 10 interfaces have a duplicate method, like 10 on() methods, the class that implements them would only have 1 method to satisfy all the interfaces)