Create Class-Object from Generic

Question

In a generic method, I don't seem to be able to access the generic type of the method at runtime (error: cannot select from a type variable).

public <A> A get(Animal a) {
    Class ac = a.getClass();
    if(ac.isAssignableFrom(A.class)) {  // <- not working!
        return (A) a;
    } else {
        // error handling
    }
}

The reason why I do this is to be able to safely downcast stuff, like:

Animal a = new Dog();
Dog d = get(a);    // <- OK
Cat c = get(a)     // <- incompatible types, caught by else block

I've found a post from over 9 years ago with the exact same problem: https://forums.oracle.com/forums/thread.jspa?threadID=1184291

There, the idea to solve this was to provide a Class object in the constructor and use this variable to check if it's assignable. Which seems a pretty stupid solution, as you could just put any class in there and all the nice typechecking was useless...

So, same question: Why should one supply <any>.class in the constructor, when during runtime the type of <A> is perfectly known? How can I access the actual type of <A>?

Found a pretty ugly solution:

public <A> A get(Animal a, Class<A> clazz) {
    // same as above...
}

So now you have to provide a Class, but the only valid class you can insert here is your return type. But at least now it's typesafe.

Just totally inconvenient...

score 4 · Answer 1 · answered Nov 13 '12 at 16:44

4

Contrary to your assertion, the type of A is not known during runtime. Take a look at the docs concerning type erasure for more details.

Briefly, the runtime knows you have an object of type Whatever. However it can't distinguish between Whatever<A> and Whatever<B> without you explicitly providing the info. That's why it's suggested that you provide a class object. Note this is distinct from the compilation phase, in which the complete type info is known.

answered Nov 13 '12 at 16:44

Brian Agnew

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What happens if you do `instanceOfA.toString()`? – 11684 Nov 13 '12 at 16:44
It will still call the correct method etc. It's just the generic info that is missing – Brian Agnew Nov 13 '12 at 16:49
Still pretty useless, as I have to declare the type as parameter, but the caller is not bound to give the _actual_ return type as parameter so type safety is nonexistent... – klamann Nov 13 '12 at 16:53
Actually, I don't even understand the explanation: Sure, `Whatever` is erased at compiletime for objects, but this is a generic method, where the type that needs to be returned must be well known during runtime, or the JVM cannot cast stuff correctly. For `Dog d = get(a);`, I know the return type has to be `Dog` and nothing else, that's how it is declared in the assignment. What am I missing here? – klamann Nov 13 '12 at 17:03
1

@Klamann: The caller knows, sure. But the method being called doesn't know. They're different, and at runtime, it's the same method being called, regardless of whether the caller knows it's acting on Dog or the Cat. – Mark Peters Nov 13 '12 at 17:40

score 1 · Answer 2 · edited May 23 '17 at 12:27

1

It is not possible to select the generic type at runtime.

See here for a possible solution: Access generic type parameter at runtime?

edited May 23 '17 at 12:27

Community

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answered Nov 13 '12 at 16:45

Felix

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Create Class-Object from Generic

2 Answers2