[java] JFrame Exit on close Java

Calling setDefaultCloseOperation(EXIT_ON_CLOSE) does exactly this. It causes the application to exit when the application receives a close window event from the operating system. Pressing the close (X) button on your window causes the operating system to generate a close window event and send it to your Java application. The close window event is processed by the AWT event loop in your Java application which will exit the application in response to the event.

If you do not call this method the AWT event loop may not exit the application in response to the close window event but leave it running in the background.