Let's say that I have an argument like List<Object> objectList, which is provided to me externally, and that I have absolutely no control over. Let's also say that I know every single possibility for the downcasted types of each element in this List. For example, let's say I know that objectList contains elements of these types: int, String, double, boolean.
What approaches could I use to filter objectList into separate lists of each downcasted type without using instanceof. So for this example, I want to put objectList into something that looks like this:
class SpecificTypes
{
List<Integer> intList;
List<String> stringList;
List<Double> doubleList;
List<Boolean> booleanList;
}
So say I want to write a function that looks like this:
SpecificTypes filterObjectList(List<Object> objectList)
{
SpecificTypes results = new SpecificTypes();
// Populate 'results' somehow
// Without using cascading if/else or switch logic
return results;
}
My research quickly led me to the visitor pattern, but I have come to the conclusion that I cannot use this because I would need control over the types stored in objectList (or at least be able to wrap them in my own derived types), but these are supplied to me from an external source so the visitor pattern seems impossible (right?).
The subset of derived types in the object list is fairly small (although in reality it's larger than the 4 in this example), and those types will not be changing. So it's OK if I need to write multiple specializations and dispatchers. I am trying to avoid any sort of cascading if/else or switch statement.
Is there some way to specialize functions that would sort these out properly?
No, by List<Object> objectList you are telling the language you forfeit type safety of anything that is more specific than Object which is about everything in Java.
You have to perform instanceof checks on the types you want to match. You have to perform casts because this is effectively what you're doing - casting. Hacks around it won't help.