With Java, is it acceptable to use an unbounded wildcard type as a method's argument, and then check and cast it to a parameterized type?

Question

If I use unbounded wildcard types for two collections (each collection will have a different type) as the arguments for a method:

private void doAssertion(List<?> testList, List<?> generatedList)

Inside this method, can I first check the type of objects in these collections, and then cast the collection to a parameterized type? This just smells bad, and I get an unchecked cast warning.

if (testList.get(0) instanceof X) {
  List<X> xList = (List<X>) testList;
  // call methods specific to X for each object
}

else if (testList.get(0) instanceof Y){
  List<Y> yList = (List<Y>) testList;
  // call methods specific to Y for each object
}

Part of my problem is that I don't have the ability to touch the code that defines classes X or Y. Otherwise, I know I can have them implement a common interface, and use a bounded type parameter. I can't overload assertEqual because both methods have the same erasure.

In my case, X and Y are always going to be children of other classes, and I'm not modifying the objects in anyway, just calling the get() methods of the objects.

Answer 1

No, you shouldn't do this. Take for example:

List<Integer> myIntList = ...;
if (myIntList.get(0) instanceof Number) {
    List<Number> myNumberList = (List<Number>)myIntList;
    myNumberList.put(Double.valueOf(100)); // BAD!
}

Generic arguments are not necessarily substitutable in sub/superclass hierarchies.