java - Extend a JDOM Document -

for project @ university, need parse gml file. gml files xml based use jdom2 parse it. fit purposes, extended org.jdom2.document so:

package datenbank;  import java.io.file; // more imports  public class gmldatei extends org.jdom2.document {      public void saveasfile() {         // ...     }      public gmlknoten getrootelement(){         return (gmlknoten) this.getdocument().getrootelement();     }      public void setrootelement(gmlknoten root){         this.getdocument().setrootelement(root);     } } 

i extended org.jdom2.element , named subclass gmlknoten not matter question.

when testing, try load gml file. when using native document , element classes, loads fine, when using subclasses, following scenario:

i load file using:

saxbuilder saxbuilder = new saxbuilder(); file inputfile = new file("gml/roads_munich_route_lines.gml"); gmldatei document = null;  arraylist<string> types = new arraylist<string>();  try {     document = (gmldatei) saxbuilder.build(inputfile); } catch (jdomexception e) {     // todo auto-generated catch block     e.printstacktrace(); } catch (ioexception e) {     // todo auto-generated catch block     e.printstacktrace(); } 

in line

document = (gmldatei) saxbuilder.build(inputfile); 

i cast-exception:

exception in thread "main" java.lang.classcastexception: org.jdom2.document cannot cast datenbank.gmldatei @ datenbank.gmltest.main(gmltest.java:27) 

i thought casting schould no problem subclassing org.jdom2.document. missing?

vat

in general want "challenge" requirement extend document - value custom classes not part of native implementation? ask 2 reasons:

• as maintainer of jdom, should adding new feature?
• i curious.....

jdom has system in place allowing extend it's core classes , have different implementation of them when parsing document. done extending jdomfactory.

consider code here: jdomfactory interface. when saxparser parses document uses methods build document.

there default, overridable implementation in defaultjdomfactory can extend, and, example, in implementation, must override non-final "element" methods like:

@override public element element(final int line, final int col, final string name,       string prefix, string uri) {   return new element(name, prefix, uri); } 

and instead have:

@override public element element(final int line, final int col, final string name,         string prefix, string uri) {     return new gmlknoten (name, prefix, uri); } 

note have override methods non-final , return content customised (for example, have override 4 element methods count.

with own gmljdomfactory can use saxbuilder either using full constructor new saxbuilder(null, null, new gmpjdomfactory()) or setting jdomfactory after have constructred setjdomfactory(...)