Imtiyaz.m.s has compiled a comparison between Java 5.0 and ActionScript 3.0 in his blog. Here is the link http://msimtiyaz.wordpress.com/flex/java-and-actionscript/
And don’t forget to view the comments here http://msimtiyaz.wordpress.com/2007/08/05/mfug-2/
Detailed comparison can be found in the book Rich Internet Applications with Adobe Flex and Java .
This book is written by Yakov Fain, Dr. Victor Rasputnis, Anatole Tartakovsky.