setDefaultCloseOperation()

The setDefaultCloseOperation() method is used to specify one of several options for the close button. Use one of the following constants to specify your choice:

• JFrame.EXIT_ON_CLOSE — Exit the application.
• JFrame.HIDE_ON_CLOSE — Hide the frame, but keep the application running.
• JFrame.DISPOSE_ON_CLOSE — Dispose of the frame object, but keep the application running.
• JFrame.DO_NOTHING_ON_CLOSE — Ignore the click.

If you forget to call setDefaultCloseOperation() you will get JFrame.HIDE_ON_CLOSE by default. This can be frustrating, because it looks like you have "killed" the program, but it keeps on running, and you see no frame.