Reflection - inherited fields all null or empty

I have a class (with superclass) for which I want to check that all fields are either NULL, or if it's a Collection, that it's empty. With the following code I am able to check for NULL and whether it's a Collection, but I can't seem to cast the Collection to check it's size:

public static boolean objectIsEmpty(Object object) {
    for (Class<?> c = object.getClass(); c != null; c = c.getSuperclass()) {
        Field[] fields = c.getDeclaredFields();
        for (Field field : fields) {
            field.setAccessible(true);
            try {
                if (field.get(object) != null) {
                    if (field.getType().equals(List.class)) {
                        // System.err.println("len " +
                        // Array.getLength(field.get(object)));
                    }
                    if (Collection.class.isAssignableFrom(field.getType())) {
                        System.err.println(Collection.class.cast(field).size());
                        // ClassCastException thrown here
                    }
                }
            } catch (IllegalAccessException e) {
                // Should not occur with setAccessible(true), return false just
                // in case
                e.printStackTrace();
                return false;
            }
        }
    }
    return true;

But that results in java.lang.ClassCastException: Cannot cast java.lang.reflect.Field to java.util.Collection

How do I get the size of the Lists?