How To Instantiate a java.util.ArrayList with Generic Class Using Reflection

Question

How To Instantiate a java.util.ArrayList with Generic Class Using Reflection? I am writing a method that sets java.util.List on target object. A target object and a generic type of list is knowing in runtime:

public static void initializeList(Object targetObject, PropertyDescriptor prop, String gtype) {
    try {
        Class clazz = Class.forName("java.util.ArrayList<"+gtype+">");
        Object newInstance = clazz.newInstance();
        prop.getWriteMethod().invoke(targetObject, newInstance);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

Answer 1

An object doesn't know about its generic type at execution time. Just create a new instance of the raw type (java.util.ArrayList). The target object won't know the difference (because there isn't any difference).

Basically Java generics is a compile-time trick, with metadata in the compiled classes but just casting at execution time. See the Java generics FAQ for more information.

Answer 2

There's no need to do any reflection for creating your List. Just pass in some additional type information (usually done by passing a class of the correct type).

public static <T> List<T> createListOfType(Class<T> type) {
    return new ArrayList<T>();
}

Now you have a list of the required type you can presumably/hopefully set it directly on your targetObject without any reflection.