0.5 Interface polymorphism - the big picture  (Page 2/9)

Supplemental material

I recommend that you also study the other lessons in my extensive collection of online programming tutorials. You will find a consolidated index at www.DickBaldwin.com .

General background information

What is an ActionScript interface?

In an earlier lesson titled "Inheritance - The Big Picture" , I told you that unlike C++,

"ActionScript 3 does not support multiple inheritance. Instead it supports a different mechanism called an interface that provides most ofthe benefits of multiple inheritance without most of the problems."

I promised to explain the ActionScript interface in a future lesson, and that time has come.

What does the documentation have to say?

According to About interfaces ,

"Interfaces are a type of class that you design to act as an outline for your components. When you write an interface, you provide only the namesof public methods rather than any implementation. For example, if you define two methods in an interface and then implement that interface, theimplementing class must provide implementations of those two methods.

Interfaces in ActionScript can declare methods and properties only by using setter and getter methods; they cannot specify constants. The benefitof interfaces is that you can define a contract that all classes that implement that interface must follow. Also, if your class implements aninterface, instances of that class can also be cast to that interface."

According to Interfaces ,

"An interface is a collection of method declarations that allows unrelated objects to communicate with one another...

Interfaces are based on the distinction between a method's interface and its implementation. A method's interface includes all the information necessary to invoke that method, including the name of the method, all ofits parameters, and its return type. A method's implementation includes not only the interface information, but also the executable statements thatcarry out the method's behavior. An interface definition contains only method interfaces, and any class that implements the interface isresponsible for defining the method implementations...

Another way to describe an interface is to say that it defines a data type just as a class does. Accordingly, an interface can be used as a typeannotation, just as a class can. As a data type, an interface can also be used with operators, such as the is and as operators, that require a datatype. Unlike a class, however, an interface cannot be instantiated. This distinction has led many programmers to think of interfaces as abstract datatypes and classes as concrete data types."

According to this author...

An interface is like a class in which all of the methods are abstract, meaning that only their user interface is declared. Their behavior orimplementation is not defined. (Recall from an earlier lesson that it is not possible to declare an abstract method in an ActionScript class.All methods must be defined as concrete methods in ActionScript classes.)