What you want to do is call JFrame.setDefaultCloseOperation(javax.swing.WindowConstants.DO_NOTHING_ON_CLOSE). This will prevent your window from being cleaned-up and disposed by Java. After creation, you can store this JFrame in your hashtable.

Next, you'll want to create an implementation of a java.awt.event.WindowListener which watches for windowClosing events. When that event is caught, call JFrame.setVisible(false).

The next time you want to show the window, search your hashtable for the appropriate one. If it's there, call JFrame.setVisible(true) and your window should be back.

Don't forget to re-initialize the fields on the window before you make it visible, or your user will be confused.

-chris