Finding the specific type held in an ArrayList<Object> (ie. Object = String, etc.)

Question

Say I have an ArrayList that I have cast to an ArrayList of objects. I know that all the objects that were in the ArrayList I cast were of the same type, but not what the type w

Answer 1

Nope, type erasure makes sure of that.

Answer 2

As Jeffrey mentioned, this is impossible due to magics of type erasure. Your best choice I guess is to add an additional case for the empty list:

    if (list.isEmpty()) {
        // process empty ...
    } else {
        Object o = list.get(0);
        if (o instanceof Integer) {
            List<Integer> integers = (List<Integer>) list;
            // process Integer ...
        } else if (o instanceof String) {
            List<String> strings = (List<String>) list;
            // process String ...
        } // etc...  
    }

But beware! This instanceof chain is not generally considered good OO practice. Rather than passing around bare lists and then trying to guess their constituent types consider creating a wrapper class for the lists which may also hold a reference to a Class object. That way you may also be able to refactor your different processing algorithms as overriding a single process() method...