RE: DisplayRealType ClassCastException

• To: <visad-list@xxxxxxxxxxxxx>
• Subject: RE: DisplayRealType ClassCastException
• From: "John Caron" <caron@xxxxxxxx>
• Date: Tue, 31 Aug 1999 13:44:44 -0600

> > Why is it that these work:
> >
> > ScalarType type = (ScalarType)
> ScalarType.getScalarTypeByName("XAxis");
> > RealType type   = (RealType)
> ScalarType.getScalarTypeByName("XAxis");
> >
> > but
> >
> > DisplayRealType type = (DisplayRealType)
> > ScalarType.getScalarTypeByName("XAxis");
> >
> > gives:
> >
> > java.lang.ClassCastException: visad.RealType
> >
> > Is this a bug or a feature?
>
> A feature.  DisplayRealType extends RealType but the DisplayRealType
> constructor prepends "Display" to the name String before invoking
> the RealType super constructor.  To avoid confusion, of course ;)
>
> So:
>
>     DisplayRealType type = (DisplayRealType)
>       ScalarType.getScalarTypeByName("DisplayXAxis");
>
> works.

I would answer this question in a more general way:

ScalarType.getScalarTypeByName() returns a ScalarType.

In general you cannot cast a type to its subtype, because subtypes are
subsets of types (remember your Venn diagrams!) so you are just "lucky"
in the case of

        RealType type   = (RealType) ScalarType.getScalarTypeByName("XAxis")


generally you have to do

        ScalarType st   = ScalarType.getScalarTypeByName("XAxis");
        if (st instanceof RealType)
           RealType type   = (RealType) st;

Relying on knowledge that ScalarType.getScalarTypeByName("XAxis")
returns a RealType may make your code more brittle; the method signature
is a contract only to return a ScalarType.

I'm not sure how Bill's answer fits into this "standard type theory"
answer. Is it equivalent using different language?

• Follow-Ups:
  • Re: DisplayRealType ClassCastException
    • From: From: Bill Hibbard <hibbard@xxxxxxxxxxxxxxxxx>

• References:
  • Re: DisplayRealType ClassCastException
    • From: From: Bill Hibbard <hibbard@xxxxxxxxxxxxxxxxx>