Why getDoctype() returns null?
===

I have the following bookstore.xml file:

    <?xml version="1.0" ?>
    <!DOCTYPE Bookstore SYSTEM "assert/Bookstore.dtd">
    <Bookstore>
        <Book>
            ..... Some nodes here .....
        </Book>
    </Bookstore>

With the following DTD:

    <!ELEMENT Bookstore (Book | Magazine)*>
       <!ELEMENT Book (Title, Authors, Remark?)>
       .... etc ....

So after i've loaded and parsed this XML in Java `xmlDocument.getDoctype()` method return [Bookstore: null]. Where is my mistake and how to fix this? 