How to create a new List<T> knowing the Type through reflection?

Question

I have a Field f, after I check it and it turns out that it is of type List, I can get its generic type.

But how do I create a new List with this generic type and put new instances inside?

if (List.class.isAssignableFrom(f.getType())) {

    Type genericType = f.getGenericType();
    List<genericType> a = new ArrayList(); // ?????? Error

}

---

For example I have a field with type List<MyObject>

Is it possible to create an new instance of ArrayList<MyObject> through reflections?

Answer 1

You can't do that at runtime. The best you could do is to create a List<?> and do runtime type checking yourself.

If you're curious why this is, check out "type erasure". http://docs.oracle.com/javase/tutorial/java/generics/erasure.html

Alternatively, you could attempt to clone the old list, but only the implementations of list have a clone() method, not the List interface, and it will only perform a shallow copy.

Good luck!

Answer 2

What I normally do is:

public void populateListAndSetToField(Field field,Class<?> type, Object instance, Object... params) throws IllegalArgumentException, IllegalAccessException {
    List<?> list = getGenericList(type, params);
    field.set(instance, list);
}

private <Type> List<Type> getGenericList(Class<Type> type, Object... params) {
    List<Type> l = new ArrayList<Type>();
    for (int i = 0; i < params.length; i++) {
        l.add((Type) params[i]);
    }
    return l;
}

There are runtime constrains and checks you need to apply but I've used this too many times and it works, thing is when I get to these implementations, the entire surrounding architecture relay on unknown object types, it is tricky.