Is this a memory leak or a false positive?

If you split the code like this, will the warning go away?

  Scanner scanner = new Scanner(a);  scanner.useDelimiter(",");  scanner.close();

Yes, your code has a potential (but not real) memory leak. You assign the return value of useDelimiter(a) to the local variable scanner, but the constructor result is thrown away. That is why you get the warning.

In practice, the return value of useDelimiter(a) is exactly the same object as the one returned from the constructor call, so your code closes the resource just fine. But this is something the compiler/code analysis tool cannot detect as it would have to know the useDelimiters implementation for that.

And a really good code analysis tool should have shown you an additional warning, because you are closing a resource which has not been opened in this method (the return value of useDelimiter). If you had those 2 messages together, the symptoms might have been more clear to you.

Have you try :

Scanner scanner = new Scanner(new BufferedReader(new FileReader("a"))).useDelimiter(",");

If it does not work you have to add a.close();