Flippant reply:

I would have expected that you would get a compiler error with the above
code.  You need an object reference or class name before the ".".

Serious reply:

If you want some help with a problem you are having, you ought to provide
a description of what you are trying to accomplish, what you have tried,
including *real* code samples, a description of what the result was and
how it differs from what you expected.  If you want someone to take the
time to help you, you should be willing to take the time to write out a
description with complete, legible sentences.

That said, I am guessing that you are adding checkboxes to a combo box and
expecting them to render and function as checkboxes.  The items in a combo
box are not components, so they do not work that way.  Check out the Swing
Tutorial trail on combo boxes:

 http://java.sun.com/docs/books/tutorial/uiswing/components/combobox.html

JComboBox can contain items of any class, but by default they still
display as text.  If you want them to display differently, then the
correct place to look is the *renderer*, not the actual object that's
been added.  So the actual objects in the JComboBox can be of any type,
and you'd implement a renderer that shows them as check boxes.

Renderers are a strange beast, and you should understand how they work
before using them, or you will run into all manner of weird things.  
Although you can return a new component each time you're asked for a
renderer, the normal approach is to use one component, and just keep
modifying its properties to act as a "rubber stamp" for each item.  So
you could have:

public class MyRenderer extends JCheckBox implements ListCellRenderer
{
   ...
}

and getListCellRendererComponent would set up the state of the object
(by calling this.setSelected, this.setText, etc.) and then just
"return this".

Note that a renderer only affects how the combo box items look.  You'll
need your own logic for how they change state.  Clicking on an item in
the drop-down list for a combo box generally only selects that item,
rather than changing its state.  However, you could add an ItemListener
to change the state, or you could do something more elaborate and
intuitive with MouseListener instead.

Hope that helps,