Apparently "indexed" implements the "indexed property design pattern for a multi-valued property from the JavaBean specification" on the containing type.
(where Id is replaced with the name of the multi-valued property and Type is data type of your single valued property). So you would have to use getId() on the containing type to obtain an array of your "collection". There should be a containing type as every XML document only has a single root element.
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com