Overriding redo method in drawing arrows

Question

I have to undo and redo of path and arrows to them.at a time i need only either left arrow undo or right arrow undo depending on the direction in which path is drawn ie if its frm left to right then arrowhead left
and if its from right to left tthen arrowheadtright.
Undo is ok But problem comes with redo.Since if the last drawn path is from left to right then arrowright becomes null.pls suggest me a method overriding redo method for left and right arrows
private void redoButtonActionPerformed(java.awt.event.ActionEvent evt) {                                          
  try { m_edit.redo(); }
        catch (CannotRedoException cre)
        { cre.printStackTrace(); }
drawpanel.repaint();
undoButton.setEnabled(m_edit.canUndo());
redoButton.setEnabled(m_edit.canRedo());  
private void undoButtonActionPerformed(java.awt.event.ActionEvent evt) {                                          
        try { m_edit.undo(); }
         catch (CannotRedoException cre)
         { cre.printStackTrace(); }
         drawpanel.repaint();
         undoButton.setEnabled(m_edit.canUndo());
         redoButton.setEnabled(m_edit.canRedo());
class Edit extends AbstractUndoableEdit {
       protected Vector drawarc;
      protected Curve path;
      protected Vector vectlabel;
      protected drawlabel labelarc;
     protected Vector arrowheadleft;
     protected drawArrowHeads arrow1;
     protected Vector arrowheadright;
     protected  drawArrowHeadsecond arrow2;
       public Edit( Vector drawarc, Curve path ,Vector vectlabel,drawlabel labelarc,Vector arrowheadleft,drawArrowHeads arrow1,Vector arrowheadright,drawArrowHeadsecond arrow2)
          this.drawarc = drawarc;
          this.path = path;
                this.vectlabel=vectlabel;
                this.labelarc=labelarc;
                this.arrowheadleft=arrowheadleft;
                this.arrow1=arrow1;
                this.arrowheadright=arrowheadright;
                this.arrow2=arrow2;
     public void undo() throws CannotUndoException {
          drawarc.remove(path);
                vectlabel.remove(labelarc);
                arrowheadleft.remove(arrow1);
                arrowheadright.remove(arrow2);
     public void redo() throws CannotRedoException {
          drawarc.add( path);
                vectlabel.add(labelarc);
               // arrowheadleft.add(arrow1);
               // System.out.println("The arrowheadleft vector inside Edit method contains "+arrowheadleft.toString());
               arrowheadright.add(arrow2);
               System.out.println("The arrowheadright vector inside Edit method contains "+arrowheadright.toString());
         public boolean canUndo() {
          return true;
     public boolean canRedo() {
          return true;
     public String getPresentationName() {
          return "Add Arc";
m_edit=new Edit(drawarc, path , vectlabel, labelarc, arrowheadleft, arrow1, arrowheadright, arrow2);
undoButton.setText(m_edit.getUndoPresentationName());
redoButton.setText(m_edit.getRedoPresentationName());
undoButton.setEnabled(m_edit.canUndo());
redoButton.setEnabled(m_edit.canRedo());

Answer 1

..arrowright becomes null..Check for null value. If null, then do nothing with it.