Both javax.xml.transform.Source and org.xml.sax.InputSource can be empty.

javax.xml.transform.Source has four subtypes: DOMSource, SAXSource, StAXSource and StreamSource. Three of them, DOMSource, SAXSource, and StreamSource, include the no-argument constructor that when used, creates empty Source.

org.xml.sax.InputSource also include a no-argument constructor.

Since a Source object contains multiple inputs (publicId, systemId, byte or character stream), a convenient method isEmpty() would make it easier to evaluate whether a Source object is empty.