FULL PRODUCT VERSION :
java version "1.6.0-rc"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.6.0-rc-b65)
Java HotSpot(TM) Client VM (build 1.6.0-rc-b65, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Linux 2.6.14

A DESCRIPTION OF THE PROBLEM :
1) we must call UIManager.getInstalledLookAndFeels(); before UIManager.setLookAndFeel("com.sun.java.swing.plaf.gtk.GTKLookAndFeel");

Try the source code below, Executing it gives a java.lang.NullPointerException Exception.

2) if we uncomment UIManager.getInstalledLookAndFeels(); it works.
but with no antialiasing text in the button.

ERROR MESSAGES/STACK TRACES THAT OCCUR :

Exception in thread "main" java.lang.NullPointerException
at javax.swing.plaf.synth.SynthLookAndFeel$AATextListener.propertyChange(Unknown Source)
at java.beans.PropertyChangeSupport.firePropertyChange(Unknown Source)
at java.beans.PropertyChangeSupport.firePropertyChange(Unknown Source)
at java.beans.PropertyChangeSupport.firePropertyChange(Unknown Source)
at java.awt.Toolkit.setDesktopProperty(Unknown Source)
at sun.awt.SunToolkit.fireDesktopFontPropertyChanges(Unknown Source)
at sun.awt.SunToolkit.setAAFontSettingsCondition(Unknown Source)
at sun.swing.SwingUtilities2$AATextInfo.getAATextInfo(Unknown Source)
at com.sun.java.swing.plaf.gtk.GTKLookAndFeel.initComponentDefaults(Unknown Source)
at com.sun.java.swing.plaf.gtk.GTKLookAndFeel.getDefaults(Unknown Source)
at javax.swing.UIManager.setLookAndFeel(Unknown Source)
at javax.swing.UIManager.setLookAndFeel(Unknown Source)
at Appmain.main(Appmain.java:18)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at com.sun.java.swing.plaf.gtk.GTKLookAndFeel.initSystemColorDefaults(Unknown Source)
at com.sun.java.swing.plaf.gtk.GTKLookAndFeel.loadStyles(Unknown Source)
at com.sun.java.swing.plaf.gtk.GTKLookAndFeel.access$000(Unknown Source)
at com.sun.java.swing.plaf.gtk.GTKLookAndFeel$WeakPCL$1.run(Unknown Source)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
public class Appmain {
public static void main(String[] args) throws Exception {
// UIManager.getInstalledLookAndFeels();
UIManager.setLookAndFeel("com.sun.java.swing.plaf.gtk.GTKLookAndFeel");


JFrame frame = new JFrame("Testing");
JButton bn = new JButton("my label");
frame.add(bn);
frame.setSize(300, 100);
frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
frame.setVisible(true);
}
}
---------- END SOURCE ----------