AS3: Dictionary Object

// Arrays use numeric indexes:
var arr:Array = new Array();
arr[0] = "value";
// Generic objects use string indexes:
var obj:Object = new Object();
obj["key"] = "value";
// Dictionary uses object keys:
var dict:Dictionary = new Dictionary();
dict[myObj] = "value";

It’s important to understand that Dictionary uses strict equality to match the object, not the reference, as the key. This means that different references to the same object will act as the same key:

import flash.utils.Dictionary;
var a:Object = new Object();
var b:Object = a; // reference to the same object
var dict:Dictionary = new Dictionary();
dict[a] = "fun!";
trace(dict[b]); // traces 'fun!' because a===b
You can use any type of object as a key, not just generic objects (you could use a Sprite, or an Array, for example). This includes primitives (string, boolean, number, int, uint) which are matched based on value (again, strict equality):

var dict:Dictionary = new Dictionary();
dict["string"] = "joy!";
trace(dict["string"]); // traces "joy" because "string"==="string"
Dictionary objects are enumerable with “for in”, and “for each” loops:

for (var key:Object in dict) {
// iterates through each object key
}
for each (var value:Object in dict) {
// iterates through each value
}
You can also set a Dictionary to use weak references as keys. This is pretty cool, because it means that if you clear all references to an object except those in a weakly referenced dictionary, that object will be available for garbage collection, which in turn will release the reference to its value.

var a:Sprite = new Sprite();
// create a weakly referenced dictionary by passing true as first param:
var dict:Dictionary = new Dictionary(true);
dict[a] = new Object();
a = null; // clear original reference to the Sprite.