Java distinguishes list <object> to its interface

I have:

class Hammer Implements Part {
    public String getName() { return name; }
    public int getId() { return id; }
    ...
}

class Wrench implements Part {
    public String getName() { return name; }
    public int getId() { return id; }
    ...
}

interface Part {
    String getName();
    int getId();
}

I use a pre-created database manager written for Android SQLite, which returns a list of objects based on what I retrieve:

dataManager().getWrenchDao().getAll(); // returns List<Wrench>

I cannot change how this function works.

Now I get both lists:

List<Wrench> wrenches = dataManager().getWrenchDao().getAll();
List<Hammer> hammers = dataManager().getHammerDao().getAll();

However, I want to fill this information with spinners (Spinners are drop-down lists in android).

loadSpinner(Spinner s, List<Part> data) {
    ...
    data.ElementAt(i).getName();
    data.ElementAt(i).getId();
    ...
}

loadSpinner(wrenchSpinner, wrenches);

But this gives me a casting error that you cannot change. Why doesn't Java allow me to do this? Wrenches have all the methods that Part do so, why can't I use it in something that it implements and uses?

Error:

The method loadSpinner(Spinner, List<Part>) in the type NewActivity is not applicable for the arguments (Spinner, List<Wrench>)