如何自定义JComboBox，使弹出式是JTree(而不是列表)?

I am trying to create a combo box so that I can put whatever control I prefer within the pop-up, in my specific case a JTree. Having a look at how the JComboBox is implement, the pop-up is really created by the UI delegate. The problem in changing that is that it would need to be re-implemented for each look and feel, which is something I do not want to do...I am trying to create a combo box so that I can