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 was.

Now, if the ArrayList is not empty, I could take one of the objects in it and use the instanceof operator to learn what the actual type is. But what of the case where the ArrayList is empty? How do I determine what type Object actually is then? Is it possible?

Edit: In retrospect, I suppose it doesn't strictly matter what type an empty ArrayList holds. I can discard the old one and construct a new empty ArrayList of the proper type I was looking for in the first place.

It's a bit messy, so if anyone has alternate suggestions for how to allow a large variety of potential types which may not share a relevant common superclass (Ex. Integer and ArrayList), I'm open to suggestions. The thing is, I have a class that will contain other classes specifically designed to be interchangably used within it. The contained classes can make assumptions as to the type, as they're the ones defining it. The surrounding class cannot make such assumptions, but must confirm that it is specifying the same type as the contained classes that have been slotted into it.

Thus my question, as the containing class is generic (the generic specifying what sort of type it is handling), but it must ensure the contained classes it was passed are returning and operating on the type that was specified to it (they are designed to be created separately and as such the fact that they match must be confirmed when they are slotted in, not at the time of their creation).

I think a better question would be to ask if how you're doing what you're trying to do is the best way, because I've seen a line or two of code in my day and I've seen nothing that would require the solution you've roughly described. — Paul
– Paul, Commented Nov 16, 2011 at 2:25
To clarify further, it's a modular design. The problem is that the user could easily pass the wrong kind of modules. — Christopher Griffith
– Christopher Griffith, Commented Nov 16, 2011 at 2:36
ugh, hit enter too early. Had an example: EX. Hill Climbing search algorithm module, it contains two sub-modules: a state module and a heuristic module. The state is designed with an exact structure in mind, but the Hill-Climbing algorithm could operate on many kinds of states. The hill climbing algorithm has to handle whatever form the state module may pass up to it. Although in retrospect the design could probably just use Object since hill-climbing need not strictly do anything but pass it around... — Christopher Griffith
– Christopher Griffith, Commented Nov 16, 2011 at 2:42
What are you going to do when you find out that type? Do a case analysis? What is the meaningful thing to do with an empty list in your application? — Saintali
– Saintali, Commented Nov 16, 2011 at 15:33

Jeffrey · Accepted Answer · 2011-11-16 01:48:25Z

3

Nope, type erasure makes sure of that.

answered Nov 16, 2011 at 1:48

Jeffrey

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Paul Over a year ago

You, sir, are correct. Bonus points for taking only one sentence to fully answer his question.

Saintali · Accepted Answer · 2011-11-16 15:48:12Z

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...

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