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Here comes the funny part. In the Rolodex thought experiment, I asked, Wouldn t it be great if [you could just make this Rolodex class a generic] Remember Because one answer is Maybe, but we ll never know, because you can t. Up through Flash Player 10.1 at least, Vector holds a prized position as the one and only generic in all of ActionScript 3.0. You can t take a sweet data type you created and parameterize the type it uses. Sadly, you will see no parameterized Tree.<T>s or Heap.<T>s. For now at least, it s a special case that exists for performance tuning. Although you can build your own data types that use Vector internally for performance, you won t be able to parameterize them and take advantage of the no-cast no-cost element access that you get with a generic. The thing is, you don t really need generics. Judicious use of interfaces makes a lot more sense than a type parameter in most cases. For example, when you think of it, the Rolodex example is really terrible. How would a Rolodex work that stores emotions Cupcakes don t have phone numbers . . . how would you implement placeCall() when T is Cupcake In reality, you almost always need certain guarantees about the objects you re dealing with, even if it s something as simple as the ability to compare two of them. Nowhere does Flash Player give you a sort function that works on all
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9: Vectors
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objects. In other words, your ability to do interesting things generically is limited by the things that you re guaranteed you can do with any arbitrary object. In Flash Player, any arbitrary object means subclasses of Object, and Object is not endowed with many powers. In this case, you really don t want to be able to give Rolodex any type in the world. You want it to operate on objects that have phone numbers to call and a name to sort. Hey, you have a mechanism for that:
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interface IContact { function get phoneNumber():Number; function get fullName():String; function get lastName():String; }
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The Rolodex example is far from generic. It should be implemented to only operate on objects that have certain properties. The perfect device for holding objects to a certain contract is an interface. So Rolodex should not be generic. Its interface should be written around an IContact interface that allows it to do the interesting things a Rolodex does. Internally, you re more than welcome to use a Vector.<IContact> to make Rolodex fast and ef cient! (See Example 9-2.) You should use an interface or a common superclass in lieu of generics to keep your classes abstract. The only drawback you ll see is that you may have to do some potentially dangerous upcasts. EXAMPLE 9-2
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http://actionscriptbible.com/ch9/ex2
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The Rolodex Class
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package { import com.actionscriptbible.Example; public class ch9ex2 extends Example { public function ch9ex2() { var rolodex:Rolodex = new Rolodex(); rolodex.addEntry(new Person("Roger", "Braunstein", 7185555555)); rolodex.addEntry(new Person("Simon", "Bolivar")); rolodex.addEntry(new Person("Shimon", "Peres")); trace("B entries:", rolodex.getEntriesUnderLetter("B")); var firstB:Person = Person(rolodex.getEntriesUnderLetter("B")[0]); //note that we still have to upcast because this returns an IContact. } } } class Rolodex { protected var entries:Vector.<IContact>; public function Rolodex() { entries = new Vector.<IContact>(); } public function addEntry(contact:IContact):void { entries.push(contact); entries = entries.sort(function(a:IContact, b:IContact):Number { return (a.lastName.toLowerCase() < b.lastName.toLowerCase()) -1 : 1; }); }
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continued
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Part II: Core ActionScript 3.0 Data Types
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EXAMPLE 9-2
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(continued)
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public function getEntriesUnderLetter(ltr:String):Vector.<IContact> { ltr = ltr.toLowerCase(); return entries.filter(function(entry:IContact,...rest):Boolean { return (entry.lastName.charAt(0).toLowerCase() == ltr); }); } public function getRandomEntry():IContact { return entries[Math.floor(Math.random() * entries.length)]; } public function placeCall(contact:IContact):void { trace("calling", contact.phoneNumber); } } interface IContact { function get phoneNumber():Number; function get fullName():String; function get lastName():String; } class Person implements IContact { private var _phoneNumber:Number; private var _lastName:String; private var _firstName:String; public function Person(first:String, last:String = "", number:Number = 0) { _firstName = first; _lastName = last; _phoneNumber = number; } public function get phoneNumber():Number {return _phoneNumber;} public function get fullName():String {return _lastName + ", " + _firstName;} public function get lastName():String {return _lastName;} public function toString():String {return "[Person " + fullName + "]";} }
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Even though I ve designed Rolodex to operate on IContacts, and it uses a Vector for storage internally, you don t have the leisure of returning contacts as type Person or Company; you still have to upcast. Generics are best suited for optimized storage data types, which don t depend in any way on the contents or structure of the data. While you can t create your own, Vector, just like Array, is incredibly useful and exible.
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