Why are the DOM APIs “interfaces” rather than “classes”?
Interfaces are widely used in many object-oriented languages, for example Java, and have several advantages when designing an API. They are similar to abstract classes, but all the methods are abstract. Variables in an interface must be constants. The key point with interfaces is that they do not constrain implementations. The methods defined in an interface must give the correct results, but the implementation is free to do anything it needs to. Thus, for example, even if one interface inherits from another, this does not mean that the implementation must share any code. Interfaces are implemented by classes. Any given class is free to implement more than one interface (e.g., an interface specified by the DOM and some extensions). When a class implements more than one interface, it must provide an implementation for all the abstract methods in each interface, but again, need not share code or any other implementation details.
