I have a method createFoo()
that creates instances of Foo<T>
using the Class<T>
instance for that T
. Now I want to extend that method to forward calls that are made using an enum type to the method createEnumFoo()
. Calling the second method from the first one seems to be non-trivial. Below is an example of how I managed to do it using two unchecked casts and an extra method, all of which I would like to get rid of.
The method castEnumType()
is required because I couldn't find a way to cast a Class<?>
to a Class<E extends Enum<E>>
without having the E
bound somewhere. This involves an unchecked cast because I have not found a way to do it using Class.asSubclass()
. After creating the instance of Foo
, I need to cast it from Foo<E>
to Foo<T>
event though E
and T
will always be the same types.
I can't weaken the signature of createEnumFoo()
because it is calling Enum.valueOf(enumType, ...)
and requires the result of this to be of type E
.
final class Example {
<E extends Enum<E>> Foo<E> createEnumFoo(Class<E> enumType) {
// This makes use of e.g. Enum.valueOf(enumType, ...).
return null;
}
<E extends Enum<E>> Class<E> castEnumType(Class<?> enumType) {
return (Class<E>) enumType;
}
<T> Foo<T> createFoo(Class<T> type) {
if (Enum.class.isAssignableFrom(type))
return (Foo<T>) createEnumFoo(castEnumType(type));
else
// Here we would do something else or maybe throw an exception.
return null;
}
interface Foo<T> {
}
}
Is there a simpler way to do this?
To clarify the problem I'm facing, I'll explain how this problem actually arose in a project I'm working on:
In the code where I came across this problem, Foo<T>
is actually Converter<T>
, which is an interface which allows an instance of T
to be serialized and de-serialized from and to a JSON value:
public interface Converter<T> {
JsonObject encode(T value);
T decode(JsonObject data);
}
And createFoo()
is actually a method converterForType()
which takes a Class<T>
instance and dynamically dispatches to a bunch of static methods and fields that create/contain converters for common Java types and types specific to the project. Normally when a converter is needed, the appropriate method/field is accessed directly but there are some places where the type is only known at runtime, which is where converterForType()
is used.
Now I wanted to extend that method to automatically handle enum types by converting those to JSON strings containing the name of the enum constant. This is why I need to call the method enumConverter()
from converterForType()
. This is the implementation of enumConverter()
:
public static <E extends Enum<E>> Converter<E> enumConverter(final Class<E> enumClass) {
return new Converter<E>() {
public JsonObject encode(E value) {
return Json.convert(value.name());
}
public E decode(JsonObject data) {
return Enum.valueOf(enumClass, data.asString());
}
};
}
What about this, use raw types for createEnumFoo method
Edit: fixed compile error reported by @Feuermurmel in comments
@SuppressWarnings({ "unchecked", "rawtypes" })
final class Example
{
<E extends Enum<E>> Foo<E> createEnumFoo(Class enumType)
{
// This makes use of e.g. Enum.valueOf(enumType, ...).
Enum x = Enum.valueOf(enumType, "x");
return (Foo<E>) x;
}
<T extends Enum> Foo<T> createFoo(Class<T> type)
{
if (Enum.class.isAssignableFrom(type))
return (Foo<T>) createEnumFoo(type);
else
// Here we would do something else or maybe throw an exception.
return null;
}
interface Foo<T>
{
}
}
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