There is a misleading typo in the method comments for the methods:

    Preferences.userNodeForPackage(Class c);
    Preferences.systemNodeForPackage(Class c);

The last line of the first paragraph in both classes says:

    For example the absolute path name of the node associated with
    the class <tt>com.acme.widget</tt> is <tt>/com/acme/widget</tt>.

Since the last name in a class name is the class itself, and since
these methods are creating Preferences for the package, not the
class, the node path name indicated there is one name too long.