First you need to get the file as InputStream and then get XmlSchemaCollection of that InputStream. Using this XmlSchemaCollection object you can read XmlSchema. You can find Sample code below, Which get sample.xml XSD file to XmlSchema Object.
Now you can use this XmlSchema object to iterate the XSD file. If you need to know how to do it then have a look my previous post "How to process Apache commons XmlSchema"