How to write/save a XML document to a file in GWT

Hi,

I am new to GWT..Trying small applications..
Task:
To have collection of widgets(like textbox,checkbox...)
Get the values.Convert those values to an XML document.
Then write\save the XML document in a file with .xml extn..

Currently i have parsed it into a XML document through DOM and XMLparser...
I am not able write it to a file...
I tried with Transformer but I am getting error......