Recently I wanted to create a form on top of a JWindow since I thought that it would look good for a popup window to have no title bar and title bar buttons. JWindow is usually good for splash screens, those sort of thing. After much researching, a JDialog actually can be set to look and act like a JWindow.
Just use the setUndecorated() method and set it to either true or false. If true, then it will look like a JWindow. The bonus here is that you can also set the container to modal with ease.