Ans: Here is the example write to the file system from flex

import flash.filesystem.*;

private var stream:FileStream;

private function saveFile():void{
var file:File = File.desktopDirectory.resolvePath("HelloWorldinFlex4.txt");
var stream:FileStream = new FileStream()
stream.open(file, FileMode.WRITE);
var str:String = "Hello World in Flex4";
stream.writeUTFBytes(str);
stream.close();
mx.controls.Alert.show("File has been saved to \n" + file.nativePath, "Notice");
}