FULL PRODUCT VERSION :
java version "1.6.0"
Java(TM) SE Runtime Environment (build 1.6.0-b105)
Java HotSpot(TM) Client VM (build 1.6.0-b105, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

A DESCRIPTION OF THE PROBLEM :
The UIDefaults class contains one or more StringBuffer objects as keys (!), where it should be String objects instead. This used to work in JSE 1.5.0_10 and earlier.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run and compile the following program below.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Up to JSE 1.5.0_10, this app printed the UI defaults in sorted order. It relied on the keys of the UIDefaults to be comparable. As these are actually String objects, this used to work fine.
ACTUAL -
With JSE 1.6.0-b105 (final), this prints an exception because the UIDefaults table contains a StringBuffer as a key.

The resolution would be to fix the run time type of the key (shouldn't be a StringBuffer anyway because its mutable) and probably declare the UIDefaults class to extend from HashTable<String, Object> rather than HashTable<Object, Object>.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
Exception in thread "main" java.lang.ClassCastException: java.lang.StringBuffer cannot be cast to java.lang.Comparable
        at java.util.TreeMap.put(TreeMap.java:542)
        at java.util.TreeSet.add(TreeSet.java:238)
        at java.util.AbstractCollection.addAll(AbstractCollection.java:305)
        at java.util.TreeSet.addAll(TreeSet.java:295)
        at java.util.TreeSet.<init>(TreeSet.java:143)
        at PrintSortedSwingDefaults.main(PrintSortedSwingDefaults.java:7)


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.util.*;
import javax.swing.*;

public class PrintSortedSwingDefaults {
    public static void main(String[] args) {
        final UIDefaults defaults = UIManager.getDefaults();
        final Set keys = new TreeSet(Collections.list(defaults.keys()));
        for (Iterator it = keys.iterator(); it.hasNext();) {
            final Object key = it.next();
            System.out.println(key + "=" + defaults.get(key));
        }
    }
}

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

CUSTOMER SUBMITTED WORKAROUND :
import java.util.*;
import javax.swing.*;

public class PrintSortedSwingDefaults {
    public static void main(String[] args) {
        final UIDefaults defaults = UIManager.getDefaults();
        final Set keys = new TreeSet(Collections.list(new Enumeration() {
            final Enumeration keys = defaults.keys();

            public boolean hasMoreElements() {
                return keys.hasMoreElements();
            }

            public Object nextElement() {
                return keys.nextElement().toString();
            }
        }));
        for (Iterator it = keys.iterator(); it.hasNext();) {
            final Object key = it.next();
            System.out.println(key + "=" + defaults.get(key));
        }
    }
}

Release Regression From : 5.0u9
The above release value was the last known release where this
bug was not reproducible. Since then there has been a regression.