ExtensionElement is now a marker interface that requires all
implementation non-abstract classes to carry a static final QNAME
field (of type QName). This is verified by a new unit test.
Also FullyQualifiedElement is renamed to simply XmlElement. XmlElement
is used over ExtensionElement when implementing classes do not
statically know the qualified name of the XML elements they
represent. In general, XmlElement should be used sparingly, and every
XML element should be modeled by its own Java class (implementing
ExtensionElement).
This means that users get now exceptions with helpful error messages
instead of the dreaded ClassCastException, like
java.lang.ClassCastException: org.jivesoftware.smack.packet.StandardExtensionElement cannot be cast to org.jivesoftware.smackx.mam.element.MamElements$MamResultExtension
at
when StanzaView.getExtension(Class) is used to retrieve the extension.
Ensure that he returned extension element is actually of the correct
type. It should not be common, but in theory, Smack's provider
architecture would allow for different types to be returned than the
ones one may expect.
Returning a generic would allow for
List<ExtensionElement> list = stanza.getExtension("foo", "bar");
to compile (Note the we are calling getExtension(), not
getExtension*s*()).
Users are encouraged to use the type safe getExtension(Class<? extends
ExtensionElement) variant instead.
Fixes SMACK-825.