Scanner class in Java5 throw java.lang.NullPointerException

Question

I am using scanner class in java5, and the following code will throw an exception:

Scanner scanner = new Scanner
        (new File(args[0]));
int dealId;
while (scanner.hasNextLine()) {
    dealId = scanner.nextInt();
    System.out.println(dealId);
}
scanner.close();

The stacktrace is:

Exception in thread "main" java.lang.NullPointerException
   at java.util.regex.Matcher.toMatchResult(libgcj.so.10)
   at java.util.Scanner.myCoreNext(libgcj.so.10)
   at java.util.Scanner.myPrepareForNext(libgcj.so.10)
   at java.util.Scanner.myNextLine(libgcj.so.10)
   at java.util.Scanner.hasNextLine(libgcj.so.10)

Does anybody knows what caused this exception?

It would have been useful to mention that you're using GCJ rather than the Sun/Oracle JDK... (You were aware that you were, right?) — Jon Skeet, Mar 08 '12 at 10:06
@JonSkeet Yes, I am using GCJ...I just relized that because it was modified by someone else. Is there any solution to this problem except go back to Sun/Oracle JDK? — cheng, Mar 08 '12 at 10:21

score 2 · Accepted Answer · answered Mar 08 '12 at 10:20

2

The GCJ Home Page suggest it "supports most of the 1.4 libraries plus some 1.5 additions. "

Scanner was added in version 1.5 and I suspect you have hit a piece of functionality GCJ doesn't support. You need to try something different to see what you can get to work.

Is there any reason you are not using OpenJDK/Oracle Java 6 or 7? (Please don't say its for performance reasons ;)

answered Mar 08 '12 at 10:20

Peter Lawrey

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The java on the machine is configured by someone else, it is out of my control... Is there anyother solution except go back to Sun JDK? – cheng Mar 08 '12 at 10:23
You can install Java yourself (in your home directory) Either you need to do this and the person who controls the system should set up the machine correctly, or you don't need to do it. You can't succeed in a project unless you are given the right support. You might be able to get this to work, but this is likely to be a recurring problem, even after you think you have finished, you could come across strange problems which I suspect will be your problem. ;) – Peter Lawrey Mar 08 '12 at 10:29
Ok, I know what is the problem now. Thanks for your kind help. – cheng Mar 08 '12 at 10:31
If you want to install Java into your home directory, down load the self extracting `.bin` and unpack it into your home. You can add this to your path and it will use that instead. I have even unpacked into `/tmp` where I couldn't use my home directory. ;) – Peter Lawrey Mar 08 '12 at 10:37

score 0 · Answer 2 · answered Jul 21 '17 at 03:17

I reproduced the error and found a work around

Here is the code, compiled on x86_64 GNU/Linux, Fedora with Java 1.5.0:

Scanner r = new Scanner(f, "ISO-8859-1");
while(r.hasNext()){
    String line = r.nextLine();   //The guts of nextLine(), specifically: 
                                  //Matcher.toMatchResult bubbles up a 
                                  //nullPointerException
}

The file just contains two ascii words separated by a newline. The runtime Exception only occurs when nextLine processes the last line of the file, whether it has characters or not:

java.lang.NullPointerException
   at java.util.regex.Matcher.toMatchResult(libgcj.so.10)
   at java.util.Scanner.myCoreNext(libgcj.so.10)
   at java.util.Scanner.myPrepareForNext(libgcj.so.10)
   at java.util.Scanner.myNextLine(libgcj.so.10)
   at java.util.Scanner.nextLine(libgcj.so.10)
   at Main.parseFile(Main.java:1449)
   at Main.construct(Main.java:1420)
   at Main.populateBlogPosts(Main.java:1399)
   at Main.main(Main.java:263)

Here is a bug report on this issue: https://bugs.openjdk.java.net/browse/JDK-6178785

Diagnosis

It's a bug in libgcj.so.10, perfectly legitimate ascii input as well as blankstring causes it to puke an NPE on the last line of a file.

Workaround

Since this bug only occurs on the very last line of the file, the hacky workaround is to initially make sure there is at least one newline at the end of the file, then catch and ignore the nullPointerException bubbled up from toMatchResult and exit the loop when that happens.

Scanner class in Java5 throw java.lang.NullPointerException

2 Answers2