* Scenarios:
- Install latest jre 8-b83 promotion
- Install latest jre 7u21-b10 promotion
- Select to disable jre 8 from the JCP-->Java-->View-->User
- Uninstall jre7u21

There is no way to enable jre 8 again from the Java control panel..
Go back to JCP-->Java-->View-->User and the jre8 enable checkbox is NOT selectable.
It is a dead end as the installed jre 8 is not usable from user perspective,

If trying to invoke an applet in this case, there is exception:

Exception in thread "Thread-2" java.lang.IndexOutOfBoundsException: Index: 0
ze: 0
        at java.util.ArrayList.rangeCheck(ArrayList.java:603)
        at java.util.ArrayList.get(ArrayList.java:381)
        at sun.plugin2.main.server.JVMManager.getBestJREInfo(Unknown Source)
        at sun.plugin2.main.server.JVMManager.getBestJREInfo(Unknown Source)
        at sun.plugin2.main.server.JVMManager.getOrCreateBestJVMInstance(Unk
 Source)
        at sun.plugin2.main.server.JVMManager.startAppletImpl(Unknown Source
        at sun.plugin2.main.server.JVMManager.startApplet(Unknown Source)
        at sun.plugin2.main.server.JVMManager.startApplet(Unknown Source)
        at sun.plugin2.main.server.MozillaPlugin.maybeStartApplet(Unknown So
)
        at sun.plugin2.main.server.MozillaPlugin.setWindow(Unknown Source)