You may have noticed there is no Frame.close() or JFrame.close() method. How then do you close a JFrame? Closing a JFrame is difficult to get your head around mainly because there are so many variants on how you can do it. You can:

• Get rid of the JFrame object entirely.
• Just make the JFrame invisible, but ready to be made visible again an instant later.
• Make the JFrame invisible, and put in into cold storage, ready to be made visible again with a little thawing.
• System.exit( 0 ) the program immediately.

• when the user clicks the closing X.
• when the user clicks a close menu item.
• programmatically when you decide to close the JFrame.
• programmatically when you decide to shut down the entire program.

When the user clicks the close X, that does not make your JFrame object mysteriously disappear or force it to close, or go invisible. All it does is send your JFrame a WindowClosing event. Your JFrame is free to ignore the event, close the JFrame in any of the four usual ways, or do something else entirely.

The event in no way closes your JFrame or changes its state. It is totally up to you what happens. Receiving a WindowClosingEvent is not an eviction notice; it is just notification the user idly clicked an X. It does not mean the JFrame is closing or that it has to close, just that the user would like it to.

Final Close

• call dispose() to release associated GUI resources, and automatically call setVisible( false ). If you fail to do this, you will create a memory leak in the GUI since the native GUI may not be smart enough to do its own garbage collection.
• Remove the JFrame from any Collections or Containers.
• Delete all references to the JFrame.

Temporary Close

Cold Storage Close

WindowClosingEvent

DO_NOTHING_ON_CLOSE is a bit of a misnomer. It should have been called USE_EVENT_HANDLER_ON_CLOSE. It means Swing does nothing to handle the events automatically, but hand the problem over to you at your windowClosing method.

If you put no code at all in your windowClosing method, the event would be ignored. The user could click the X, but nothing would happen. You are free to put any code your heart desires in there, whatever you want to happen when the user clicks X.

Shorthand WindowClosingEvent Handlers

• If all you want to do is call dispose() you can use this, instead of setting up an event handler.

  this.setDefaultCloseOperation ( JFrame.DISPOSE_ON_CLOSE );

• If all you want to do is call setVisible( false ) you can use this, instead of setting up an event handler.

  this.setDefaultCloseOperation ( JFrame.HIDE_ON_CLOSE );

  Believe it or not, this is the default. Beware!
• If all you want to do is call System.exit( 0 ) you can use this, instead of setting up an event handler.

  this.setDefaultCloseOperation ( JFrame.EXIT_ON_CLOSE );

• If all you want to write your own custom event handler, you need to use this, as well as setting up an event handler.

  this.setDefaultCloseOperation ( JFrame.DO_NOTHING_ON_CLOSE );

AWT and Frame

Caveats

this.setDefaultCloseOperation ( JFrame.DISPOSE_ON_CLOSE );

Be careful not to cast adrift an undisposed JFrame with no references to it. It can never be garbage collected.

To iconify (minimise), maximize etc. programmatically see Frame.setExtendedState( Frame.ICONIFIED ); Other constants you can use include Frame.MAXIMIZED_HORIZ, Frame.MAXIMIZED_VERT, Frame.MAXIMIZED_BOTH, and Frame.NORMAL. Prior to Java 1.14 you had to use Frame.setState( Frame.ICONIFIED );

Oddly, for JInternalFrames, you have to use a different technique entirely using the DeskTopManager class.