CODEWITHSUNDEEP
javanull
user5219532
user5219532

Reputation:

java dereferencing possible null pointer

In my code I get the above warning. Here is the part of the code where I get it,

try {
        fileFile = new File(Main.class.getProtectionDomain().getCodeSource().getLocation().toURI());
    } catch (URISyntaxException | NullPointerException e) {
    }
    finally {
        if (fileFile.getPath()!= null){
            strPathName = fileFile.getPath();
        }
        if (fileFile.getName() != null){
            strFileName = fileFile.getName();
        }
    }

The line if (fileFile.getPath()!= null){ is the one with the warning. This code is not part of the Main class. It's in another class in another class file in the same package.

I'm not very experienced with programming but I believe I did nearly everything to prevent or catch a null pointer exception. Why do I still getting it and what can I do to get rid of it? Thanks for your help.

After reading all your hints I solved it. Here is the complete code:

public static ArrayList<String> getCurrentPath() {

    File fileFile;
    String strPathName, strFileName;
    ArrayList<String> arrPathFileName;

    strFileName = null;
    strPathName = null;

    try {
        fileFile = new File(Main.class.getProtectionDomain().getCodeSource().getLocation().toURI());
        if (fileFile.getPath()!= null){
            strPathName = fileFile.getPath();
        }
        if (fileFile.getName() != null){
            strFileName = fileFile.getName();
        }
    } catch (URISyntaxException use) {
    }     
    arrPathFileName = new ArrayList<>(); 
    arrPathFileName.add(strPathName);
    arrPathFileName.add(strFileName);

    return arrPathFileName;
}

As already mentioned I simply put the if statements into the try block and removed the finally block.

BTW is also tried to combine both if blocks into one that way:

if (fileFile != null){
            strPathName = fileFile.getPath();
            strFileName = fileFile.getName();
        }

But that produced a warning that fileFile will never become null. (what was my point of view from the beginning and so the warning "dereferencing possible null pointer" was really confusing me.)

Upvotes: 1

Views: 15589

Answers (2)

Captain Man
Captain Man

Reputation: 7695

A "null pointer dereference" is computer speak for trying to call a method on a null value. It's a little more complicated, but you said you were a novice, so I wanted to keep it simple.

Let's see an example:

String s = null;
s = s.toUpperCase();

This is a simple example of what a null pointer dereference is. s is a null reference (its value is null), when we derefrence is (get the value of it) we have null, when we call toUpperCase() on null, something goes horribly wrong because null doesn't have any methods, at all! Java throws a NullPointerException to be specific.

Now, back to your code, because fileFile is assigned in the try-block I assume it was set to null before it to avoid Java yelling about an uninitialized variable. (This is all fine and correct.) In this try-block, if any of the exceptions for your catch-block occur it will stop the try-block (meaning fileFile will not get a new value, meaning it will still be null).

Now you'll notice the warning is possible null pointer dereference. That means it won't necessarily be null, but could be! (In my above example, it's always a null pointer dereference for comparison.) Specifically, if the catch catches an exception it will be null.

To be clear, the issue is this: fileFile.getPath(). It's like saying it might be null.getPath(), gross. It looks like you were trying to avoid the null pointer issue, what you should have done was if (fileFile != null) { instead. Then inside of the if do what you want.

Also, because it seems like you included it to avoid this warning, I would seriously remove the NullPointerException from the catch-block. That's not helping you avoid the warning. If you want me to explain more why it's bad you can leave a comment and I will, otherwise just take my word for it, it's not helping you.

Upvotes: 2

Brian Agnew
Brian Agnew

Reputation: 272237

So if you throw an exception on your first line, your variable will not be assigned to a File, and will retain it's previous value (null if not formerly assigned). Your exception is caught, and then you continue to use that unassigned variable. Hence the warning. See the commented code below.

try {
        fileFile = // exception thrown. Variable not assigned
} catch (URISyntaxException | NullPointerException e) {
        // exception caught
    }
    finally {
       // unassigned variable used here...
        if (fileFile.getPath()!= null){
            strPathName = fileFile.getPath();
        }
        if (fileFile.getName() != null){
            strFileName = fileFile.getName();
        }
    }

I would rather scope and use the variable within the try block, if at all practical. In your finally block, you need to be as careful as you can, since you could have come to it from most anywhere in your try block.

As an aside, this:

Main.class.getProtectionDomain().getCodeSource().getLocation().toURI();

will cause you enormous problems if you do get an NPE. Which of the above resolved to null ? I would perhaps be more explicit, such that you can check for nulls from each invocation and unambiguously determine which invocation gave you a null. Tiresome ? Unfortunately so.

Upvotes: 10

Related Questions