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https://issues.apache.org/jira/browse/WICKET-3468?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12998088#comment-12998088
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Richard Emberson commented on WICKET-3468:
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Maybe better, since we know the class of the converter (IConverter<Date>):
public <C> IConverter<C> getConverter(Class<C> type) {
if (Date.class.isAssignableFrom(type)) return (IConverter<C>)converter;
else return super.getConverter(type);
}
For those Components with converters whose type is known, this is a type safe
mechanism.
If one has a Component with a type parameter and whose
converter is passed in as a construction
parameter, then one can not have compile-time type safety:
public class SomeComponent<T> extends Component {
private final IConverter<T> converter;
public SomeComponent(IConverter<T> converter) {
// ....
this.converter = converter;
}
public <C> IConverter<C> getConverter(Class<C> type) {
// can not safely return local converter because of
// type erasure
}
}
And for completeness, since Scala provides a mechanism to get
around the JVM's type erasure, one can also have type safe type
in the above case:
class SomeComponent[T](val converter: IConverter<T>)(implicit m: Manifest[T])
extends Component {
def getConverter(tpe: Class[C]): IConverter[C] =
if (m.erasure.isAssignableFrom(tpe) converter
else super.getConverter(tpe)
}
> org/apache/wicket/extensions/markup/html/form/DateTextField converter never
> null
> --------------------------------------------------------------------------------
>
> Key: WICKET-3468
> URL: https://issues.apache.org/jira/browse/WICKET-3468
> Project: Wicket
> Issue Type: Improvement
> Components: wicket-extensions
> Affects Versions: 1.5-RC1
> Environment: all
> Reporter: Richard Emberson
> Priority: Trivial
>
> As far as I can see the
> org/apache/wicket/extensions/markup/html/form/DateTextField
> converter instance variable is never null, so the getConverter can be
> re-written
> public <C> IConverter<C> getConverter(Class<C> type)
> {
> if (converter == null)
> {
> return super.getConverter(type);
> }
> else
> {
> return (IConverter<C>)converter;
> }
> }
> to
> public <C> IConverter<C> getConverter(Class<C> type)
> {
> return (IConverter<C>)converter;
> }
> And as an aside, here is another case where the Wicket code is saved by the
> fact that
> the JVM does generic type erasure. The converter instance variable is of
> type IConverter<Date>. If someone came out with a JVM that did not do type
> erasure,
> then the getConverter method would throw a cast exception for all but the
> Date class
> as a parameter. Everywhere in Wicket that the getConverter method appears has
> such
> a problem. But, saved by Java's poor generic implementation (though, the cast
> exception would probably appear in later code).
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