Java: null comparison always yields false: incorrect warning?

Question

Interesting situation. I have a section of code that creates several ZipOutputStreams. As a safety check before I even think about writing anything I check thta my output streams have been correctly initialised:

ZipOutputStream countStream = null;
File countFile = null;
// other files

try {
    countFile =
    new File(savePath.getCanonicalPath() + savePath.separator + outputTag
        + "_COUNT_OUTPUTS.zip");
    // other files
} catch (Exception e) {
    outputLog.add("UNABLE TO FIND SAVE PATH");
    return util.FILE_INVALID;
}

try {
    // prepare outputs
    if (countFile.exists() == true) {
    countFile.delete();
    } else {
    }
    countStream = new ZipOutputStream(new FileOutputStream(countFile));
    // other files
} catch (Exception e) {
    e.printStackTrace();
    outputLog.add("UNABLE TO CREATE OUTPUT FILES");
    return util.FILE_SAVE_FAIL;
}

if (countStream == null) {
    outputLog.add("UNABLE TO CREATE OUTPUT FILES");
    return util.FILE_SAVE_FAIL;
} else {
}

I dont especially see where the problem is in this code, but it throws up a warning on the null test: "Null comparison always yields false: The variable countStream cannot be null at this location". As far as I can see it, I have a variable initialised to null, then the creation of an outputstream is attempted, but isn't guaranteed to happen. ignoring the warning is easy enough, but I'd rather know how the compiler comes to the conclusion that countStream is guaranteed to be successfully created

K.Barad

Answer 1

The compiler is able to see that you cannot be null there since:

1. in your try you've initialized it (countStream = new ....) and
2. in your catch you've "return"ed out of the method.

Hence, the code "if (countStream == null)" can only be attained through normal flow after countStream has been initialized to a non-null ZipOutputStream.

Answer 2

When there is no Exception, the variable countStream will not be null. When there is an Exception, you return util.FILE_SAVE_FAIL and the null check will not be performed. Hence, whenever the null check is performed, the result will always be false.

Slightly more formal: There are no code paths by which execution can reach your null check that result in countStream being null when the check is performed.