Is it possible to retrieve the generic type used for a type token (i.e. Class) at runtime?

Question

Neal Gafter introduced type tokens (for example Class<String>). Assuming one has access to an instance of Class<String> at runtime, is it possible to retrieve the generic type (String) at runtime?

I am looking for something similar to Method.getGenericReturnType().

not sure what you mean. if you have the Class instance already, what more do you want? — jtahlborn, Jul 30 '12 at 18:19
of _what_? a Class instance or some type name? maybe you could add some pseudo code indicating what you are looking for... — jtahlborn, Jul 30 '12 at 18:41
in your question you are using "Class". do you mean the Java class `Class`, or do you mean, "some unspecified class type"? — jtahlborn, Jul 30 '12 at 18:42
Of the class. In my example, that would be String as indicated in the question. — Jérôme Verstrynge, Jul 30 '12 at 18:42
In what form do you want it? What do you expect to be returned? A `String` holding the class name? If you want "the type" to be returned, the only way for that desire to make sense is...to get a `Class`, which you already have. — Louis Wasserman, Jul 30 '12 at 20:42
A Type tt where System.out.println(tt) would print something like this: java.lang.String. — Jérôme Verstrynge, Jul 30 '12 at 20:48
The only thing similar to `Method.getGenericReturnType` for a class is `Class.getGenericSuperclass()` and `Class.getGenericSuperinterfaces()`, which means you can only retrieve type information for an object that is passed into a superclass as part of that object's class declaration. Fully resolving this type information can be a pain since it can consist of TypeParameters that are scattered around your type hierarchy. See my response below for more info... — Jonathan, Jul 30 '12 at 21:21

score 2 · Answer 1 · edited May 23 '17 at 11:55

2

I think it is only possible for Fields/Methods. We can't get class specific generic type at runtime due to type erasure. It seems there is hack you can do if you have access to class. Read this discussion.

edited May 23 '17 at 11:55

Community

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answered Jul 30 '12 at 18:17

kosa

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Actually you can retrieve type information at runtime so long as it is part of a type declaration. That is what the hack refers to... – Jonathan Jul 30 '12 at 21:31
Well, I found a solution... So it is also possible for classes! – Jérôme Verstrynge Jul 30 '12 at 21:39

score 1 · Answer 2 · answered Jul 30 '12 at 18:20

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Unlike C#, Generics don't exist at run-time in Java. So you cannot try to create an instance of generic type or try to find type of a generic type at run-time.

answered Jul 30 '12 at 18:20

Hari Gudigundla

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Wrong, the generic information is available as metadata, otherwise, how could Method.getGenericReturnType() work? – Jérôme Verstrynge Jul 30 '12 at 18:27
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I'm talking about some generic type T and you are talking about methods with parameter or return type with a generic type T. – Hari Gudigundla Jul 30 '12 at 18:38
Assuming a class has a method returning the generic type, it is retrievable. – Jérôme Verstrynge Jul 30 '12 at 18:43

score 1 · Answer 3 · answered Jul 30 '12 at 21:02

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It sounds like what you want is ParameterizedType.

You get these by reflecting on a Class and objects that come from it (Method, Field). However, you can't get a ParameterizedType from any old Class object; you can obtain one from a Class instance that represents a type that extends a generic class or interface.

answered Jul 30 '12 at 21:02

erickson

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Ok, but then, how to proceed specifically? – Jérôme Verstrynge Jul 30 '12 at 21:37

score 0 · Accepted Answer · answered Jul 30 '12 at 21:35

It is possible using a variation of Bob Lee's elaboration of Gafter's Gadget pattern:

public class GenericTypeReference<T> {

    private final Type type;

    protected GenericTypeReference() {
        Type superclass = getClass().getGenericSuperclass();
        if (superclass instanceof Class) {
            throw new RuntimeException("Missing type parameter.");
        }
        this.type = ((ParameterizedType) superclass).getActualTypeArguments()[0];
    }

    public Type getType() {
        return this.type;
    }   

    public static void main(String[] args) {

        // This is necessary to create a Class<String> instance
        GenericTypeReference<Class<String>> tr =
            new GenericTypeReference<Class<String>>() {};

        // Retrieving the Class<String> instance
        Type c = tr.getType();

        System.out.println(c);
        System.out.println(getGenericType(c));

    }

    public static Type getGenericType(Type c) {
        return ((ParameterizedType) c).getActualTypeArguments()[0];
    }

}

The above code prints:

java.lang.Class<java.lang.String>
class java.lang.String

Is it possible to retrieve the generic type used for a type token (i.e. Class) at runtime?

4 Answers4