Question

Imagine having 2 Generic ArrayLists each storing different types. My program will be using only one of these ArrayLists at a time. Is it possible to create a general ArrayList (currentArrayList) which can store both ArrayLists, and use the two ArrayLists without casting.

ArrayList<Integer> arrInt = new ArrayList<>();
arrInt.add(10);
ArrayList<String> arrString = new ArrayList<>();
arrString.add("ten");

ArrayList<XXX> currentArrayList = arrInt;
Integer i = currentArrayList.get(0);

currentArrayList = arrString;
String str = currentArrayList.get(0);

Thanks.

Answer 1

The problem is the part where you want to:

use the two ArrayLists without casting.

Definitely not going to happen. The whole point of a static type system is to stop you from being able to treat a general type as a specific type without explicitly asking to do so.

So for example, you could say:

ArrayList<?> currentArrayList = arrInt;
Object i = currentArrayList.get(0);

But notice that i is just an Object, so the compiler is happy – we don’t know anything about the things in the array except that they must be Object-based because this is a Java program. But you want to say:

ArrayList<?> currentArrayList = arrInt;
Integer i = currentArrayList.get(0);

It’s that second line that isn’t ever going to happen without a cast.

UPDATE Question from comments:

Can’t the compiler easily infer the type of the ArrayList by
looking at the generic type of arrInt ?

Suppose it did that for us. What should it then do with the type of currentArrayList when it sees the line:

currentArrayList = arrString;

Your code as it stands assumes that the compiler is not going to do that kind of inference. Suppose you comment-out that second assignment. Now the compiler could make the inference you suggest, and allow your code to compile. But then in the future if you put back the second assignment, the rest of your code would stop compiling! This would be needlessly confusing.