Java Generics: casting to ? (or a way to use an arbitrary Foo<?>)

Question

So, I have some code that looks approximately like (truncated for brevity - ignore things like the public member variables):

  public class GenericThingy<T> {
    private T mValue;
    public final T[] mCandidates;

    public GenericThingy(T[] pCandidates, T pInitValue) {
      mCandidates = pCandidates;
      mValue = pInitValue;
    }

    public void setValue(T pNewValue) {
      mValue = pNewValue;
    }
  }

  public class GenericThingyWidget {

    private final GenericThingy<?> mThingy;
    private final JComboBox mBox;

    public GenericThingyWidget (GenericThingy<?> pThingy) {
      mThingy = pThingy;
      mBox = new JComboBox(pThingy.mCandidates);
      //do stuff here that makes the box show up
    }

    //this gets called by an external event
    public void applySelectedValue () {
      mThingy.setValue(mBox.getSelectedItem());
    }
  }
}

My problem is that the mThingy.setValue(mBox.getSelectedItem()); call generates the following error:

The method setValue(capture#4-of ?) in the type Generics.GenericThingy<capture#4-of ?> is not applicable for the arguments (Object)

I can get around this by removing the <?> from the declaration of mThingy and pThingy in GenericThingyWidget - which gives me a "GenericThingy is a raw type. References to GenericThingy should be parameterized" warning.

I also tried replacing the setValue call with

mThingy.setValue(mThingy.mCandidates[mBox.getSelectedIndex()]);

which I genuinely expected to work, but that produced a very similar error:

The method setValue(capture#4-of ?) in the type Generics.GenericThingy<capture#4-of ?> is not applicable for the arguments (capture#5-of ?)

Is there any way to do this without generating "raw type" warnings ("unchecked cast" warnings I'm OK with) and without making GenericThingyWidget into a generic type? I'd think I could cast the return of mBox.getSelectedItem() to something, but I can't figure out what that would be.

As a bonus question, why does the replacement call to mThingy.setValue not work?

Answer 1

Update

OK, try this:

  public class GenericThingy<T> {
    private Class<T> mClazz;
    private T mValue;
    public final T[] mCandidates;

    public GenericThingy(Class<T> clazz, T[] pCandidates, T pInitValue) {
      mClazz = clazz;
      mCandidates = pCandidates;
      mValue = pInitValue;
    }

    public void setValue(Object newValue) throws ClassCastException {
      mValue = mClazz.cast(newValue);
    }
  }

Answer 2

You lack information in GenericThingyWidget. The ? you put means : any class extending object. Which means any, not some particular one but I don't know which one. Java can't relate one ? to another, they can not be related one to the other in a class hierarchy tree. So

mThingy.setValue(mThingy.mCandidates[mBox.getSelectedIndex()]);

this tries to put an object of any class in the setValue, which is waiting for any other class, but the ? can not tell Java these two any should be the same class.

Without parameterizing GenericThingyWidget, I don't see any way to work around it.

What I would do : parameterize GenericThingyWidget, and create a Factory static parameterized method :

public static <T> GenericThingyWidget<T> make(T someObject){
    ...
}

Answer 3

What you need to to is parameterize GenericThingyWidget like so:

public class GenericThingyWidget<T> {

private final GenericThingy<? super T> mThingy;
private final JComboBox mBox;

public GenericThingyWidget (GenericThingy<? super T> pThingy) {
  mThingy = pThingy;
  mBox = new JComboBox(pThingy.mCandidates);
  //do stuff here that makes the box show up
}

//this gets called by an external event
public void applySelectedValue () {
  mThingy.setValue((T) mBox.getSelectedItem());
 }
}
}

Technically, you don't need the ? super T for your example, and would be fine with just a T, and perhaps it would be better in real code if you ever want to get from the GenericThingy instead of just inserting into it.

Answer 4

As KLE said, You can just de-parameterize GenericThingy (replace all the T's with objects). In fact, I think you have to unless you plan to pass the class of T to the constructor of GenericThingyWidget, and then dynamically cast from your mbox.getSelectedItem(), since as far as I can tell, getSelectedItem() only returns Object.