Why can I not check instance type after class cast exception?

Question

Can anybody explain to me why the following block of code is highlighted as a mistake in Android Studio?

The IDE is saying elem instanceof SubtypeB is always false - is this just a mistake by the inspector or is this a real language detail I need to learn about?. Can I safely suppress the warning and keep the current code?

Example code:

for (Parcelable elem : list) {
    try {
        listOfSubtypeA.add( (SubtypeA) elem );
    } catch (ClassCastException cce) {
        //Line below is highlighted as always false
        if (elem instanceof SubtypeB) {
            ... //just logging
        }
    }
}

What relationship is there between `SubtypeB` and `Parcelable` ? — k5_, Sep 29 '19 at 19:59
I can't reproduce the problem. Do you get the error with your paraphrased version? Can you provide a minimal example with all relevant code? — k5_, Sep 29 '19 at 20:08
@k5 it's trivial to reproduce, I've already given the minimal example but I added a picture just for you — Nick Cardoso, Sep 29 '19 at 20:13
If list actually contained a `SubtypeB` the if is reached an true. So the check is thrown off by something. As your picture seems to contain compile errors without compile error markings. It isn't that trustworthy. — k5_, Sep 29 '19 at 20:19
There's no compile errors, it's just marking the items as unused since it's just a demo block — Nick Cardoso, Sep 29 '19 at 20:20
@Willem that's even more confusing then, but may be the answer to my question - the IDE is probably making a mistake (I've already tried invalidating caches) — Nick Cardoso, Sep 29 '19 at 20:25
@Temporary I did not downvote. I prefer making suggestions for improving the question in comments. If you post a demo program as text, I can copy-paste it into an IDE window and compile exactly what you are compiling, with no risk of typos. If you post a partial program and I try to expand it to a compilable program there is a risk I'll make choices that prevent the problem. If you post a complete demo program, that risk is also eliminated. — Patricia Shanahan, Sep 29 '19 at 20:26

score 1 · Accepted Answer · answered Sep 29 '19 at 20:25

It is incorrect from your IDE. Just run it:

 interface Test123  { }
static class SubtypeA implements Test123{ }
static class SubtypeB implements Test123 { }

public static void main(String[] args) {
    List<Test123> list = new ArrayList<>();
    list.add(new SubtypeB());
    List<SubtypeA> subList = new ArrayList<>();

    for (Test123 elem : list) {
        try {
            subList.add( (SubtypeA) elem );
        } catch (ClassCastException cce) {
            //Line below is highlighted as always false
            if (elem instanceof SubtypeB) {
                System.out.println("cce = " + cce);
        //just logging
            }
        }
    }
}

You will see that the statement if (elem instanceof SubtypeB) turns out to be true so your IDE doesn't detect it correct

This is what I'd expect, my own code also compiles, I just don't want to risk that it crashes in production. I tested your code out of curiosity and it is also highlighted in my IDE. I think I'm just going to suppress it, thanks — Nick Cardoso, Sep 29 '19 at 20:31

Why can I not check instance type after class cast exception?

1 Answers1