What is the primary function of an Interface in Actionscript? - actionscript-3

Question

I know that packages are collection of Classes and Interfaces, designed to both organize and classify its contents.
I also know that Classes are the definition of Objects, and the instructions for both them, their attributes/variables, and their functions/methods.
However, I have yet to understand what an Interface is, or what it is really for...
I have read this definition on Adobe's website..:
interface
Usage
interface InterfaceName [extends InterfaceName ] {}
Defines an interface. Interfaces are data types that define a set of methods; the methods must be defined by any class that implements the interface.
An interface is similar to a class, with the following important differences:
• Interfaces contain only declarations of methods, not their implementation. That is, every class that implements an interface must provide an implementation for each method declared in the interface.
• Interface method definitions cannot have any attribute such as public or private, but implemented methods must be marked as public in the definition of the class that implements the interface.
• Multiple interfaces can be inherited by an interface by means of the extends statement, or by a class through the implements statement.
Unlike ActionScript 2.0, ActionScript 3.0 allows the use of getter and setter methods in interface definitions.
...However, this is too vague to be helpful to me.
Does anyone know the purpose and proper implementation and/or design of an Interface in ActionScript?

Answer 1

Interfaces basically let you announce "This class can do these things."
As a real world example, you might want to make a tutorial for a game which highlights each of the different controls on screen one by one. Each control might flash or bounce to highlight itself, so you can say they implement an "IHighlightable" interface, and let them take care of the rest:
public interface IHighlightable {
function highlight():void;
}
Then in your controls:
public class Control implements IHighlightable {
public function highlight():void {
// Bounce and flash!
}
}
This means you can do things like:
private function highlightControl(tutorialItem:IHighlightable):void {
tutorialItem.highlight();
}
Importantly, you can have a class implement multiple interfaces, which is useful when classes share abilities, but it doesn't make sense to have them all extend a common base class.

Answer 2

Interfaces are a contract. It is compile time mechanism to force your to implement methods. In large OOP code bases, it is the best practice to have other classes depend on interfaces rather than other classes, so you can swap the implementation without changing the code that consumes the interface (this advantage is discutable in practice, since very often the interface will change too).
I believe interfaces are borrowed from Java which introduced them to achieve polymorphism (ClassA can be of type IFoo and IBar at the same time) without inheriting from multiple abstract classes (you can only extend one class, but you can implement any number of interfaces).

Answer 3

Although I'm technically wrong, I consider interfaces to be similar to C++ abstract classes.
Wikipedia article on Abstract Types
Oracle Java documentation on Abstracts
MSDN article on C# Interfaces
Interfaces are meant to define properties and methods without actually implementing them. Also, they cannot be instantiated
var test:FooInterface = new FooInterface() // would error