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Why getContentPane()  ??

Posted on 2002-04-01
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Why getContentPane() for the Swing components to be added to the frame? Why can't we simply add the components directly like in the case of AWT components? The method returns Container object instead of JComponent which is the super class of all Swing Components. WHY?


regards,
killi
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Question by:killisrinivas
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Jim Cakalic earned 200 total points
ID: 6910485
The Swing JFrame is much more sophisticated than the AWT Frame. The content pane wasn't merely inserted to cause programming inconvenience but to permit more precise programmatic specification of the UI construction and enable addition of capabilities not present in the AWT. Instead of repeating this material, I've provided links to articles which attempt to explain the content pane and its role in the JFrame design. I hope they are of assistance to you.

http://www.javaworld.com/javaworld/javaqa/2000-05/01-qa-0505-jfc.html
http://java.sun.com/products/jfc/tsc/articles/containers/index.html
http://home.att.net/~baldwin.rg/Intermediate/Java087.htm

Best regards,
Jim Cakalic
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Expert Comment

by:Igor Bazarny
ID: 6912206
Hi,

My speculation about Container use:
While JComponent is subclass of Container, most swing components would not let you to add something to them. So, use of Container instead of JComponent is a hint that you can add components there.

As for add(), I don't understand why can't it be shortcut to getContentPane().add(). But the only thing we can do is use JFrame in a way it's supposed to be used.

Regards,
Igor Bazarny,
Brainbench MVP for Java 1
www.brainbench.com

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Expert Comment

by:heyhey_
ID: 6912451
> most swing components would not let you to add something

any examples ?
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by:Igor Bazarny
ID: 6912483
Hi,

heyhey_, thank you a lot! It was funny to know about...

I thought it's a crazy idea to call JButton.add(). I was wrong. JButton appears to have FlowLayout inside and adds components. JTextArea appears to have null layout, but still adds components. Don't know what JComboBox does to component I trying to add. Let's say that in general result of JComponent.add() is undefined.

I still feel that most JComonents are not supposed to contain other components, that's why Container looks like a better hint... It was really fun to find that JComponent subclasses Container when I was learning swing.

Igor.
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by:heyhey_
ID: 6912497
I agree :)

but " ... not supposed to contain ... " is quite different than " ... would not let you to add something ... "

JLabel is pretty good container if you want to add background Image btw.

re ContentPane - you can set your own ContentPane which is Container, but not JComponent
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by:heyhey_
ID: 6912503
>  JButton appears to have FlowLayout inside

you can always change the layout.
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Expert Comment

by:0xDEADBEEF
ID: 6912735
The method returns Container object instead of JComponent which is the super class of all Swing Components. WHY?

Because in case you want to set ur own ContentPane, you might want to use, say, CellRendererPane, which is not derived from JComponent. JComponent implements some functionalities a content pane doesn't need, that's why you rather use its subclass.
If I asked what it takes to make a forest, you'd say 'trees', not 'oaks'. Same thing here.
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by:Moondancer
ID: 6961455
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by:Venci75
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No comment has been added lately, so it's time to clean up this TA.
I will leave a recommendation in the Cleanup topic area that this question is:
Answered by: jim_cakalic
Please leave any comments here within the next seven days.
 
PLEASE DO NOT ACCEPT THIS COMMENT AS AN ANSWER!
 
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by:Jim Cakalic
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OK by me if everyone else agrees (or declines to comment).
Jim
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by:SpideyMod
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per recommendation

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by:Jim Cakalic
ID: 7989597
Thanks :-)
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