To convert the ArrayCollection to an Array, just call the toArray() method:
var myArray:Array = myArrColl.toArray();
Just in case, you also may want to have the String variables initialized in your ActionScript class:
public var field:String = "";
public var op:String = "";
public var value:String = "";
If this post answers your question or helps, please mark it as such.
Greg Lafrance - Flex 2 and 3 ACE certified
Flex / AIR Development, Training, and Support Services
Thanks for your information. But the problem is: Neither ArrayCollection nor Array was recongized as null object by java side code. Any further idea?