code: private void addGroups(MpxjTreeNode parentNode, ProjectFile file)
     {
        for (Group group : file.getGroups())
        {
           final Group g = group;
           MpxjTreeNode childNode = new MpxjTreeNode(group)
           {
              @Override public String toString()
              {
                 return g.getName();
              }
           };
           parentNode.add(childNode);
        }
     }
docs: Add groups to the tree.
  
  @param parentNode parent tree node
  @param file group container