This is a Flex HashMap. It uses the flash.utils.Dictionary class. The original version is written by Eric J. Feminella. This version is slightly modified after I've written a unit test for the HashMap class and found some unexpected results. More details can be found in http://zuro.blogspot.com/2012/01/getting-started-with-tdd-test-driven.html . flash.utils.Dictionary -- http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/utils/Dictionary.html Eric J. Feminella HashMap -- http://code.ericfeminella.com/classes/as3/HashMap.as.html