Java type-casting -- Q3

On 26 Sep, 00:45, markspace <nos...@nowhere.com> wrote:


OK, Mark, since I dont want to
risk anyones physical wellbeing :)
I'll re-phrase the question
(starting from scratch with a new thread).

The problem I started out with was from a real programming task.

I wanted to make a static function
that returns a Pair of Lists,
where the first component
(lets say, for simplicity)
is known to be: List<Integer>
and the other component is: List<String>

So the return-type really should be:
  Pair<List<Integer>,List<String>>

But since I dont think Java has any type for Pair<A,B> (?) and I dont
like to create yet another custom-class only for this trivial purpose,
I decided instead to try a return-type like either

  List<List<Object>> or
  List<List<?>>

where the "returnvalue.get(0)" is of type List<Integer>
and "returnvalue.get(1)" is of type List<String>
(is there a better candidate for a return-type, without creating a new
class?)

Now, if I try:

    public static List<List<Object>> returnListList() {

        List<List<Object>> result = new ArrayList<List<Object>>();

        List<Integer> iList = new ArrayList<Integer>();
        // ... add some objects to iList...
        List<String> sList = new ArrayList<String>();
        // ... add some objects to sList...

        result.add(iList);
        result.add(sList);

        return result;
    }

the .add() method-calls give a type-error.
And it also does *not* work to try this instead:

        result.add((List<Object>)iList);
        result.add((List<Object>)sList);

Of course, I could declare iList and sList as List<Object> instead,
but I would really like to keep the type-info there, for clarity.

After some experimentation, I found this one *does* works:

    public static List<List<?>> returnListList01() {

        List<List<?>> result = new ArrayList<List<?>>();

        List<Integer> iList = new ArrayList<Integer>();
        // ... add some objects to iList...
        List<String> sList = new ArrayList<String>();
        // ... add some objects to sList...

        result.add(iList);
        result.add(sList);

        return result;
    }

But unfortunately, the last function gives me a problem when I try to
*consume* it, and extract the two lists from the returnvalue.

If I do

    public static void consume(){
        List<List<?>> myList = returnListList();
        List<Integer> iList = (List<Integer>)myList.get(0);
        List<String> sList = (List<String>)myList.get(0);
    }

I again get the warning:

  Type safety: Unchecked cast from List<capture#1-of ?> to
List<Integer>

And I dont seem to be able to rewrite it, without getting some other
error or warning.

Any hints, what is the best way to do what I want to achieve? I.e
return a pair of lists of known, but different types (without
creating a new class) and then split the returnvalue into the two
separate lists?

/ grz01