FULL PRODUCT VERSION :
Java(TM) SE Runtime Environment (build 1.6.0_02-b05)
Java HotSpot(TM) Client VM (build 1.6.0_02-b05, mixed mode, sharing)


ADDITIONAL OS VERSION INFORMATION :
Linux anchor 2.6.20-1.2962.fc6 #1 SMP Tue Jun 19 18:24:12 EDT 2007 i686 i686 i386 GNU/Linux

A DESCRIPTION OF THE PROBLEM :
I  executed Swing program in GNOME environment on Linux.

UIManager.getSystemLookAndFeelClassName() returns following in GNOME.

com.sun.java.swing.plaf.gtk.GTKLookAndFeel

This is valid value.
I set this value to UIManager.setLookAndFeel().
So exception was trowm.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1.Compile and execute following code.

import javax.swing.UIManager;

public class Test {
        public static void main(String[] args) throws Exception {
                UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        }
}


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
ACTUAL -
Finish normally.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at com.sun.java.swing.plaf.gtk.GTKLookAndFeel.initSystemColorDefaults(GTKLookAndFeel.java:1267)
        at com.sun.java.swing.plaf.gtk.GTKLookAndFeel.loadStyles(GTKLookAndFeel.java:1509)
        at com.sun.java.swing.plaf.gtk.GTKLookAndFeel.access$000(GTKLookAndFeel.java:37)
        at com.sun.java.swing.plaf.gtk.GTKLookAndFeel$WeakPCL$1.run(GTKLookAndFeel.java:1449)
        at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)
Exception in thread "main" java.lang.NullPointerException
        at javax.swing.plaf.synth.SynthLookAndFeel$AATextListener.propertyChange(SynthLookAndFeel.java:793)
        at java.beans.PropertyChangeSupport.firePropertyChange(PropertyChangeSupport.java:339)
        at java.beans.PropertyChangeSupport.firePropertyChange(PropertyChangeSupport.java:347)
        at java.beans.PropertyChangeSupport.firePropertyChange(PropertyChangeSupport.java:276)
        at java.awt.Toolkit.setDesktopProperty(Toolkit.java:1784)
        at sun.awt.SunToolkit.fireDesktopFontPropertyChanges(SunToolkit.java:1698)
        at sun.awt.SunToolkit.setAAFontSettingsCondition(SunToolkit.java:1743)
        at sun.swing.SwingUtilities2$AATextInfo.getAATextInfo(SwingUtilities2.java:119)
        at com.sun.java.swing.plaf.gtk.GTKLookAndFeel.initComponentDefaults(GTKLookAndFeel.java:1258)
        at com.sun.java.swing.plaf.gtk.GTKLookAndFeel.getDefaults(GTKLookAndFeel.java:294)
        at javax.swing.UIManager.setLookAndFeel(UIManager.java:537)
        at javax.swing.UIManager.setLookAndFeel(UIManager.java:581)
        at W.main(W.java:17)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at com.sun.java.swing.plaf.gtk.GTKLookAndFeel.initSystemColorDefaults(GTKLookAndFeel.java:1267)
        at com.sun.java.swing.plaf.gtk.GTKLookAndFeel.loadStyles(GTKLookAndFeel.java:1509)
        at com.sun.java.swing.plaf.gtk.GTKLookAndFeel.access$000(GTKLookAndFeel.java:37)
        at com.sun.java.swing.plaf.gtk.GTKLookAndFeel$WeakPCL$1.run(GTKLookAndFeel.java:1449)
        at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import javax.swing.UIManager;

public class Test {
        public static void main(String[] args) throws Exception {
                UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Call UIManager.getInstalledLookAndFeels() before getSystemLookAndFeelClassName().

See following.

import javax.swing.UIManager;

public class Test {
        public static void main(String[] args) throws Exception {
                UIManager.getInstalledLookAndFeels();
                UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        }
}