FULL PRODUCT VERSION :
java version "1.8.0_66"
Java(TM) SE Runtime Environment (build 1.8.0_66-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.66-b17, mixed mode)


ADDITIONAL OS VERSION INFORMATION :
Darwin Chrystas-Air.home 15.0.0 Darwin Kernel Version 15.0.0: Sat Sep 19 15:53:46 PDT 2015; root:xnu-3247.10.11~1/RELEASE_X86_64 x86_64

A DESCRIPTION OF THE PROBLEM :
The number of items enumerated over by UIDefaults.keys() is different than the number of items included in UIDefaults.keySet(). I expect these to be the same.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
This is demonstrated by compiling and running the following code:

import java.util.Enumeration;
import javax.swing.UIManager;

public class UIManagerTest {

    public static void main(String[] args) {
        int ie = 0;
        int is = 0;
        Enumeration e = UIManager.getDefaults().keys();
        while (e.hasMoreElements()) {
            ie = ie + 1;
            System.out.println("Enumerated: " + e.nextElement());
        }
        for (Object k : UIManager.getDefaults().keySet()) {
            is = is + 1;
            System.out.println("Set: " + k);
        }
        System.out.println("UIDefaults.keys() contains " + ie + " elements.");
        System.out.println("UIDefaults.keySet() contains " + is + " elements.");
    }

}

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
I was expecting the last two lines to read:

UIDefaults.keys() contains 719 elements.
UIDefaults.keySet() contains 719 elements.
ACTUAL -
The last two lines read:

UIDefaults.keys() contains 719 elements.
UIDefaults.keySet() contains 0 elements.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.util.Enumeration;
import javax.swing.UIManager;

public class UIManagerTest {

    public static void main(String[] args) {
        int ike = 0;
        int iks = 0;
        Enumeration e = UIManager.getDefaults().keys();
        while (e.hasMoreElements()) {
            ike = ike + 1;
            System.out.println("Enumerated: " + e.nextElement());
        }
        for (Object k : UIManager.getDefaults().keySet()) {
            iks = iks + 1;
            System.out.println("Set: " + k);
        }
        System.out.println("UIDefaults.keys() contains " + ike + " elements.");
        System.out.println("UIDefaults.keySet() contains " + iks + " elements.");
    }

}

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

CUSTOMER SUBMITTED WORKAROUND :
The workaround is to not use enhanced for loops or streams to iterate over the keys in UIDefaults.