Eclipse gives dead code warning for reachable code

Question

The following code causes Eclipse to display a dead code warning although the code is reachable. Am I missing something here, or is this an Eclipse/javac bug?

import java.util.ArrayList;

public class DeadCodeDemo {

    public static class SomeClosable implements AutoCloseable {
        @Override
        public void close() throws Exception {
        }
    }

    public static ArrayList<String> throwRuntime() {
        throw new RuntimeException();
    }

    public static void main(String[] args) {
        ArrayList<String> list = null;
        try {
            try (SomeClosable c = new SomeClosable()) {
                list = throwRuntime();
            }
            try (SomeClosable c = new SomeClosable()) {
                list.clear();
            }
        } catch (Exception e) {
            if (list != null) { // Warning: Redundant null check: The variable list cannot be null at this location
                System.out.println("List is not null");
            } else {
                System.out.println("List is null"); // Warning: Dead code
            }
        }
    }
}

The code prints List is null

I'm using Eclipse 4.7.3a (Oxygen.3a) and JDK 8_162

Catima

eclipse issue looks like – Supernal

Definitely a weird interaction. Commenting out the list.clear() and the message disappears. Also you can indeed replace the ArrayList<String> with any class and do the same (just call any method on it) and the warning is shown. Remove the method call and it works. – Timmie

@Timmie I think it's this one, btw bugs.eclipse.org/bugs/show_bug.cgi?id=366277 – Supernal

Definitely sounds like it. Funny to still have that around after so many years. – Timmie

In that case related: #39599215 – Timmie

@Timmie about removing list.clear() removing the warning: it's because Eclipse thinks "list has already been derefenced before, so it can't be null or it would have thrown an NPE"; I've noticed that before. – Tomasine

I asked about this back in 2016: #39599215 – Selfimmolation

Answer 1

I think it's this issue, still open.

Just remember that this is Eclipse warning, not javac - and that is pretty much all you should care until that issue is resolved (even if it 7 years old now)

Supernal

Good find. Sadly the issue is from Dec 2011 (!), a few months after Java 7 was released. So they have this issue for as long as Java has try-with-resources. – Catima

@Catima well read the last comment, it was still reproducible lately – Supernal

The issue indeed seems likely to be related to the try-with-resources context, but this case seems even simpler than the one in the issue report. The catch block can be reached with list still null by throwRuntime() throwing an unchecked exception (as in fact it will do every time). There's no need to be concerned with the implicit close() invocation. – Flaggy

@Supernal I saw. That's why I wrote "sadly". – Catima