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From: Andreas Leitgeb <avl@logic.at>
Newsgroups: comp.lang.java.programmer
Subject: Re: class parameter with generic type on a factory
Date: Sat, 21 Jan 2017 20:11:53 -0000 (UTC)
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Philipp Kraus <philipp.kraus.flashpixx@gmail.com> wrote:
> Am Samstag, 21. Januar 2017 20:35:07 UTC+1 schrieb Andreas Leitgeb:
>> Philipp Kraus <philipp.kraus.flashpixx@gmail.com> wrote:
>> > I have got an interface
>> > interface MyInterface<T> {}
>> > based on this Interface I have a class
>> > public MyClass<T> implements MyInterface<T> {}
>> > and a factory class which generates MyInterface<T> objects with a Class parameter at the constructor
>> > [...]
>> > If I try to create an object of the factory with
>> > new MyFactor<String>( MyClass.class );
>> It is important to understand that in Java generics are primarily a
>> compile-time thing. If the compiler can see that all types fit, then it
>> can guarantee that no ClassCastException will possibly surprise you at
>> runtime, (except where you do an explicit cast).
>> 
>> Now, if you pass MyClass.class to the factory, there is no way for
>> the *compiler* to know that these instances will consistently have
>> the used factory's type param as their type param.
>> 
>> *You* on the other hand *can* tell for sure, that the instances you
>> create will only ever be used with the factory's type parameter, like
>> String.
>> 
>> This is the situation, where it is ok to "help" the compiler and tell
>> it that the instance you create (which originally is of type MyClass<?>,
>> and thus only implements MyInterface<?>) can safely be assumed to be
>> always e.g. a MyClass<String> (if String was the used factory's type
>> param).
>> 
>> You do this by using the annotation @SuppressWarnings("unchecked")
>> on the method that calls newInstance on the provided constructor and
>> casts(!) the MyClass<?> to MyClass<T> (where T here is the factory's
>> type param). It's then your responsibility that no ref to such an 
>> instance will ever be cast to a different type parameter, but that's
>> not likely to happen in your factory method, anyway.
>
> Thanks for your answer, I know that generic (or in C++ templates) are
> only exist on compile-time, but not on runtime.

You know partially wrong. It is true for Java, but C++ templates are
different. Unlike Java, C++ does know the type parameters at runtime:
it even creates a copy of the template code for each parameterization
(with some gory exceptions that I don't want to go deeper into, here).

> But the questian is, is there a possibility to pass the template
> argument to the class object. I'm working with Java 8.

Maybe my previous answer was "tl;dr", but it was meant to clarify: "no".

> For the compiler I need not a MyClass.class, but I need a
> MyClass<String>.class object, but if this is not possible
> can I do it without a type-cast?

No, you cannot specify MyClass<String>.class - Yes, you need to cast,
and yes, you can squelsh the generics-cast-warning, and you need not
even feel bad about it for this particular "class-factory" usecase.