Specifying generic collection type param at runtime (Java Reflection)

Question

I\'d like to get the generic type of a collection, using reflection, at runtime.

Code (JAVA):

Field collectionObject = object.getClass().getDeclaredF         


        

Answer 1

Type erasure means that information about the generic type of an object simply isn't present at execution time.

(The link is to the relevant section of Angelika Langer's Java Generics FAQ which should answer virtually every question you could possibly ask about Java generics :)

However, you're not really interested in the type of an object - you're interested in the type of a field. I misread the question, and although the answer has been accepted I hope to make amends by fixing it now :)

If the field doesn't use a type parameter itself, it can be done. For example:

import java.lang.reflect.*;
import java.util.*;

public class Test
{
    public List names;

    public static void main(String [] args)
        throws Exception // Just for simplicity!
    {
        Field field = Test.class.getDeclaredField("names");

        ParameterizedType type = (ParameterizedType) field.getGenericType();

        // List
        System.out.println(type.getRawType());

        // Just String in this case
        for (Type typeArgument : type.getActualTypeArguments())
        {
            System.out.println("  " + typeArgument);
        }
    }
}

If the field were in a class T with the field being List then you'd have to know the type argument for the instance in order to know the type argument for the collection.

Translating this into your required code is somewhat tricky though - you really need to know the type argument at the point of the collection class. For instance, if someone declared:

public class StringCollection implements Collection

and then had a field of type StringCollection, that field itself wouldn't have any type arguments. You'd then need to check getGenericSuperType and getGenericInterfaces recursively until you found what you wanted.

It's really not going to be easy to do that, even though it's possible. If I were you I'd try to change your design so that you don't need this.