contentPane : Java Glossary

2008-02-06 by Roedy Green ©1996-2009 Canadian Mind Products
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contentPane

When Swing’s JFrame class got too large it was broken into pieces. Unfortunately, the designers of Swing decided not to mask that change from the users with a facade design pattern. For some functions, such as adding a JComponent, you no longer use JFrame.add, but instead you must first get the associated Container object of the JFrame with JFrame.getContentPane(), and add to that. If you screw up, and accidentally use JFrame.add(), you won’t find out until run time, and even then the code often just quietly fails without an error message. To keep you on your toes, some Components have contentPanes and some do not. in particular JPanel does not. You add to a JPanel the some was as before.

Sun saw the error of their ways and now permits you to use JFrame. add and brethren safely in Java 1.5+. However, now you have a new problem, namely running code developed on 1.5 that works fine on 1.5+ but fails on 1.4, even when you target 1.4.

Tips

Note there is no such class as ContentPane. JFrame. getContentPane returns a vanilla Container.
JApplet, JDialog, JFrame, JInternalFrame, JOptionPane and JWindow all suffer from this contentPane gotcha.
If you do a jFrame.setBackground( Color.black ), you won’t see the effect, except when resizing. Then you will see the black frame behind the contentPane only briefly. You must do a contentPane. setBackground( Color.black ) instead to get a permanent black background. Similarly use contentPane for setForeground and setFont.
revalidate only works on JComponents, not a Container like contentPane.

The following methods should normally be used on the contentPane, not the JFrame itself.

contentPane Methods
add
addNotify
getBackground
getFont
getForeground
getLayout
invalidate
remove
repaint
setBackground
setFont
setForeground
setLayout
setVisible
validate

JDialog
JFrame
JPanel
pane
Swing

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