Class.forName usage

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Moiristo - 25 Sep 2006 17:31 GMT

A short question about forName:

I have an interface called 'GeometryPainter' and a subclass that
implements it, which is called 'DefaultGeometryPainter'. I'm
instantiating the DefaultGeometryPainter as following (Java5):

Class painter = Class.forName(lpainterClass);
GeometryPainter gm = GeometryPainter.class.cast(painter);

It finds the class, because I don't get a ClassNotFoundException, but I
cannot cast it to GeometryPainter (it throws a ClassCastException). Can
someone tell me why? What should I do?

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jeffcoat - 25 Sep 2006 18:35 GMT

> A short question about forName:
>
[quoted text clipped - 8 lines]
> I cannot cast it to GeometryPainter (it throws a
> ClassCastException). Can someone tell me why? What should I do?

You don't want a cast, you want to call Class.newInstance().

Something like ...

try {
Class painter = Class.forName("DefaultGeometryPainter");
GeometryPainter gm = (GeometryPainter) painter.newInstance();
} catch ...

If you want something other than the default constructor,
look into Class.getConstructor().

Signature

Mark Jeffcoat

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Moiristo - 26 Sep 2006 00:12 GMT

> You don't want a cast, you want to call Class.newInstance().
>
[quoted text clipped - 7 lines]
> If you want something other than the default constructor,
> look into Class.getConstructor().

Thank you both, it's working now.

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jagonzal@gmail.com - 25 Sep 2006 22:35 GMT

> A short question about forName:
>
[quoted text clipped - 8 lines]
> cannot cast it to GeometryPainter (it throws a ClassCastException). Can
> someone tell me why? What should I do?

This code: Class painter = Class.forName(lpainterClass);

Gives you the object that represents the _class_ of that object, not an
instance of the class.

For that you need to call newInstance() on the Class object (if you
have a no-argument constructor), or get the constructors, and call one
of them according to the parameters it takes.

Check the Class javadocs.

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Class.forName usage

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