Using isAssignableFrom() with generic interface

Question

I try to find a way to do it but i didn't find out, please let me know if my question has already been asked.

Basically i have a generic interface Evaluator<E, T> with a single method E evaluate(T a, T b) which will be implemented by lot of classes (for each Evaluator implementation correspond a specific algorithm). The class which use those evaluators use an XML configuration file which indicate which Evaluator class to use. To get an Evaluator from a given class name i decided to set a EvaluatorProvider class which build a evaluator from a given classe name.

This method is also generic :

public static <E, T> Evaluator<E, T> newInstance(String className)

I want to ensure that the class found for this name is a valid subclass of Evaluator using isAssignableFrom() :

    Evaluator<E, T> evaluatorInstance = null;
    try {
        Class<?> evaluatorClass = Class.forName(className);
        if (!Evaluator.class.isAssignableFrom(evaluatorClass)) {

        }
        evaluatorInstance = (Evaluator<E, T>) evaluatorClass.newInstance();
    }
    // Catch clause, etc ...

But it gives me a warning about unchecked cast, i was wondering then how to take in account the generic type provided for the class checking (using isAssignableFrom() or any other valid java mecanism).

Hope it was clear :).

PS : If you have other suggestion for the solution design please let me know, i am curious about the best way to modelize such system in Java, without run in ServiceProvider pattern which is too big for the project need.

score 2 · Accepted Answer · answered Sep 20 '13 at 20:45

2

Generics are a compile time check and are not meaningful at runtime. It doesn't make sense to add a runtime check for something only the compiler honours.

answered Sep 20 '13 at 20:45

Peter Lawrey

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So it would be better to implement a method for each generic combinaison ? – Faylixe Sep 20 '13 at 20:48
@Faylixe I don't see how that would help. With some complexity you can examine the generic types for the class, but it is unlikely to be worth the effort. Either way you will get a runtime error for the mistake. – Peter Lawrey Sep 20 '13 at 20:50
@PeterLawrey What about writing a dispatcher method that explicitly knows about all the implementations? Like, ` Thing getThing(Class cls) { if(cls.isAssignableFrom(Integer.class)) { return (???)new [class implementing Thing](); } }` – Random832 Jun 08 '16 at 17:08
@Random832 you are explicitly passing the type as an argument, not checking the type via a generic. In C# for example, you could write a method like ` T getDefaultValue() { ... }` – Peter Lawrey Jun 10 '16 at 12:02
Yes but the signature of my method is a generic. How do I do this cast without triggering the "unchecked" warning? – Random832 Jun 10 '16 at 12:33

score 1 · Answer 2 · answered Sep 20 '13 at 20:50

1

This information is erased at compile time. See the JLS:

http://docs.oracle.com/javase/specs/jls/se7/html/jls-4.html#jls-4.6

answered Sep 20 '13 at 20:50

Rag

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Using isAssignableFrom() with generic interface

2 Answers2