Generics and Polymorphism

I'm having some issues with generics and polymorphism. I thought this
was possible in Java - maybe someone can clear up what I'm doing
wrong. Basically, when I actually try to use the preference, the code
will not compile and I get the following error. How can I do what I'm
trying to do?

Here is the code that has the error:

PreferencesEnum.DERIVED_PREFERENCE.getPreference().setValue(new
String());

The error is:
The method setValue(capture#2-of ? extends Object) in the type
Preference<capture#2-of ? extends Object> is not applicable for the
arguments (String)

Thanks,
Jason

--- CLASS LISTINGS ---

I have an enum:

PreferencesEnum {
  DERIVED_PREFERENCE(new DerivedPreference());

  private final Preference<? extends Object> pref;

  private PreferencesEnum(Preference<? extends Object> pref) {
   this.pref = pref;
  }

  public Preference<? extends Object> getPreference() {
   return pref;
  }
}

And I have the generic Preference:
public abstract class Preference<E extends Object> {

    // provides access to the preferences per application, per user
    protected static Preferences prefs =
Preferences.userNodeForPackage(Main.class);

    /**
     * Default constructor.
     */
    public Preference() {
    }

    /**
     * Perform a refresh when the preferences change.
     */
    public abstract void refresh();

    /**
     * Set the value of the preference.
     *
     * @param value
     * the value to set
     */
    public abstract void setValue(E value);

    /**
     * Get the value of the preference.
     *
     * @return the associated preference value
     */
    public abstract E getValue();
}

And here's a derived preference:

public class DerivedPreference extends Preference<String> {

    private static final String KEY = "derived";

    private static final String DEFAULT = "DEFAULT VALUE";

    /**
     * Default constructor
     */
    public DerivedPreference() {
        super();
    }

    @Override
    public String getValue() {
        return prefs.get(DerivedPreference.KEY,
                DerivedPreference.DEFAULT);
    }

    @Override
    public void refresh() {
    }

    @Override
    public void setValue(String value) {
        prefs.put(DerivedPreference.KEY, value);
    }
}