How could I get the number of visible windows of my app?

Question

Or more exact:

I need to know on a window close event, if any other window is still visible.

If not, System.exit(0) would be called.

Answer 1

1) I need to know on a window close event

there are WindowConstants and WindowEvent

2) if any other window is still visible.

you can get number of Top-Level Containers by using Window[] wins = Window.getWindows(); for testing their visibility or by adding WindowStateListener

some important notice here

Answer 2

Try

if(Frame.getFrames().length == 0) {
    // work here.
}

(Frame is java.awt.Frame, that is the parent of JFrame, so you will capture them, too).