Re: Generics and Polymorphism

From:

Daniel Pitts <newsgroup.spamfilter@virtualinfinity.net>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 28 Apr 2008 17:14:50 -0700

Message-ID:

<48166860$0$30749$7836cce5@newsrazor.net>

Jason Cavett wrote:

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;
  }
}

The problem is that DERIVED_PREFERENCE.getPreference() returns
Preference<? extends Object>, who's setValue() method accepts only E,
which can't be statically determined from the context...

Another issue is that enums can't have type parameters, so that makes
what you're trying to do specifically impossible using "enum"......

What you *can* do is instead of "enum", use a plain old class.

class PreferencesEnum<E> {
    private final Preference<E> pref;

    public static final DERIVED_PREFERENCE = new
PreferencesEnum<String>(new DerivedPreference());

    private PreferenceEnum(Preference<E> pref) {
      this.pref = pref;
    }
}

etc...

Hope this helps.