> > 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?
