opaque : Java Glossary

by Roedy Green ©1996-2008 Canadian Mind Products
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opaque

In Swing, you can do a JComponent.setOpaque( true ) to notify everyone that when your component paints, it covers the entire region with no background showing through. This replaces the AWT update/ paint mechanism.

setOpaque Usage
setOpaque(?)	call super.paintComponent in paintComponent?	Meaning
false		There is no point in doing this.
false		You will paint only some of the pixels or use some transparent pixels to deliberately let what is painted underneath show through.
true		You paint every pixel of the region, with no transparent pixels, and you want Swing to clear the region to the background colour for you automatically.
true		You paint every pixel of the region, with no transparent pixels, and you will clear the background as needed with clearRect yourself in your paintComponent method.

The symptom of failing to call setOpaque( true ) is flicker, where the image gets repeatedly repainted to the background colour. The symptom of calling setOpaque( true ), when you should not are artifacts — little bits of left over painting from other work in where your component should be. These are areas you did not paint, which should have been background.

If in your TableCellRenderer you create your own JLabels to return, rather than using the one from DefaultTableCellRenderer, make sure you call JLabel.setOpaque( true ) or else your JLabel.setBackground will be ignored.

alpha channel
paint
paintComponent
TableCellRenderer gotchas
transparent
transparent background
update

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