populating from mongodb to jtable

i dont know how many keys the documents have...so i make a program like..

 private void showresultsActionPerformed(java.awt.event.ActionEvent evt) {                                            

       

        

        Object dbs=db.getSelectedItem();

        Object colls=coll.getSelectedItem();

      String  distances=distance.getText();

      String skillss=skills.getText();

      

       

        MongoClient m = null;

        try {

            m=new MongoClient();

                        

        } catch (UnknownHostException ex) {

            Logger.getLogger(NewJFrame.class.getName()).log(Level.SEVERE, null, ex);

        }

      DB dbv = m.getDB((String) dbs);

      DBCollection collv = dbv.getCollection((String) colls);

      

     DBObject doc = collv.findOne();

         Set<String> keys= doc.keySet();

         

        DBCursor cursor=collv.find();

        

     String[] columnnames=keys.toArray(new String[keys.size()]);

     DefaultTableModel tb=new DefaultTableModel(columnnames,0);

     while(cursor.hasNext()) {

              DBObject obj = cursor.next();

            int i;

              for(i=0;i<columnnames.length;i++)

              {

                  Object a = obj.get(columnnames[i]);

              tb.addColumn(columnnames[i], (Object[]) a);

              } 

                             

     }

      main.setModel(tb);

     

    }                                           

to fill jtable with mongo data..but upon clicking  button this error happens:----

Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: org.bson.types.ObjectId cannot be cast to [Ljava.lang.Object;

at NewJFrame.showresultsActionPerformed(NewJFrame.java:217)

at NewJFrame.access$100(NewJFrame.java:12)

at NewJFrame$2.actionPerformed(NewJFrame.java:74)

at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)

at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)

at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)

at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)

at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)

at java.awt.Component.processMouseEvent(Component.java:6516)

at javax.swing.JComponent.processMouseEvent(JComponent.java:3320)

at java.awt.Component.processEvent(Component.java:6281)

at java.awt.Container.processEvent(Container.java:2229)

at java.awt.Component.dispatchEventImpl(Component.java:4872)

at java.awt.Container.dispatchEventImpl(Container.java:2287)

at java.awt.Component.dispatchEvent(Component.java:4698)

at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)

at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)

at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)

at java.awt.Container.dispatchEventImpl(Container.java:2273)

at java.awt.Window.dispatchEventImpl(Window.java:2719)

at java.awt.Component.dispatchEvent(Component.java:4698)

at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)

at java.awt.EventQueue.access$200(EventQueue.java:103)

at java.awt.EventQueue$3.run(EventQueue.java:694)

at java.awt.EventQueue$3.run(EventQueue.java:692)

at java.security.AccessController.doPrivileged(Native Method)

at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)

at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)

at java.awt.EventQueue$4.run(EventQueue.java:708)

at java.awt.EventQueue$4.run(EventQueue.java:706)

at java.security.AccessController.doPrivileged(Native Method)

at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)

at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)

at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)

at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)

at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)

at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)

at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)

at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: org.bson.types.ObjectId cannot be cast to [Ljava.lang.Object;

at NewJFrame.showresultsActionPerformed(NewJFrame.java:217)

at NewJFrame.access$100(NewJFrame.java:12)

at NewJFrame$2.actionPerformed(NewJFrame.java:74)

at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)

at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)

at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)

at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)

at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)

at java.awt.Component.processMouseEvent(Component.java:6516)

at javax.swing.JComponent.processMouseEvent(JComponent.java:3320)

at java.awt.Component.processEvent(Component.java:6281)

at java.awt.Container.processEvent(Container.java:2229)

at java.awt.Component.dispatchEventImpl(Component.java:4872)

at java.awt.Container.dispatchEventImpl(Container.java:2287)

at java.awt.Component.dispatchEvent(Component.java:4698)

at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)

at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)

at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)

at java.awt.Container.dispatchEventImpl(Container.java:2273)

at java.awt.Window.dispatchEventImpl(Window.java:2719)

at java.awt.Component.dispatchEvent(Component.java:4698)

at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)

at java.awt.EventQueue.access$200(EventQueue.java:103)

at java.awt.EventQueue$3.run(EventQueue.java:694)

at java.awt.EventQueue$3.run(EventQueue.java:692)

at java.security.AccessController.doPrivileged(Native Method)

at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)

at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)

at java.awt.EventQueue$4.run(EventQueue.java:708)

at java.awt.EventQueue$4.run(EventQueue.java:706)

at java.security.AccessController.doPrivileged(Native Method)

at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)

at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)

at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)

at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)

at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)

at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)

at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)

at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

BUILD SUCCESSFUL (total time: 7 seconds)

can someone tell me why..i have imported import org.bson.types.ObjectId;