Null Pointer Exception while file searching

Question

I am programming code that searches my computer's storage for a specific file (or multiple of the same name) indicated by a starting path, all of which is user console inputted. My code seems to work fine for some testing but then I when I tried using a more general directory (C:\Users\andy) to search for a file, i get this null pointer exception below.

Exception in thread "main" java.lang.NullPointerException
    at FindFile.search(FindFile.java:44)
    at FindFile.search(FindFile.java:48)
    at FindFile.search(FindFile.java:48)
    at FindFile.search(FindFile.java:48)
    at FindFile.directorySearch(FindFile.java:32)
    at Driver.main(Driver.java:33)

The program runs for a quite a bit of time while it searches through my C drive but then after a few seconds I get thrown the exception above. I am totally lost and any help would be appreciated! (sorry i am new at this)

44: for (File temp : filename.listFiles()

48: search(temp);

32: search(dir);

33: fileSearch.directorySearch(targetFile, pathToSearch);

The class that contains the problematic code:

public class Driver {

    public static void main(String[] args) {

        String targetFile = "";
        String pathToSearch = "";
        int maxNumber = 0;

        Scanner input = new Scanner(System.in);


            System.out.println("max number of files to look for");


                maxNumber = input.nextInt();

                System.out.println("directory to start in");
                pathToSearch = input.next();

                System.out.println("What is the file name");
                targetFile = input.next();

                FindFile fileSearch = new FindFile(maxNumber);


                try {

                    fileSearch.directorySearch(targetFile, pathToSearch);

                } catch (IllegalArgumentException e) {
                    System.out.println("you have reach max number of files found");

                }

                int count = fileSearch.getFileResult().size();
                if (count == 0) {
                    System.out.println("no results of that file was found");
                } else {
                    System.out.println("found " + count + "of file " + targetFile);
                    for (String match : fileSearch.getFileResult()) {
                        System.out.println("location: " + match);
                    }
                }



    }
}


public class FindFile {

    private int maxFiles;
    private List<String> fileResult = new ArrayList<String>();
    private String nameOfFile;
    private int currentNumOfFiles = 0;

    /*
     * accepts the max number of files to find
     */
    public FindFile(int maxNum) {
        maxFiles = maxNum;

    }

    public FindFile() {

    }

    /*
     * @param target file name to look for and directory to start search
     */
    public void directorySearch(String target, String directory) {

        File dir = new File(directory);
        setNameOfFile(target);
        if (dir.isDirectory()) {
            search(dir);
        } else {
            System.out.println("not a directory ");
        }
    }

    public void search(File filename) throws IllegalArgumentException {
        if (filename.isDirectory()) {
            System.out.println("searching directory..." + filename.getAbsoluteFile());
            System.out.println();

            if (filename.canRead()) {
                for (File temp : filename.listFiles()) {
                    if (currentNumOfFiles == maxFiles) {
                        throw new IllegalArgumentException();
                    } else if (temp.isDirectory()) {
                        search(temp);

                    } else {

                        if (getNameOfFile().equals(temp.getName().toLowerCase())) {
                            fileResult.add(temp.getAbsolutePath().toString());
                            currentNumOfFiles = getFileResult().size();
                        }
                    }

                }
            } else {
                System.out.println("cannot read into this directory");
            }
        }

    }

    public int getCount() {
        return currentNumOfFiles;
    }

    public int getMaxFiles() {
        return maxFiles;
    }

    public void setMaxFiles(int maxFiles) {
        this.maxFiles = maxFiles;
    }

    public List<String> getFileResult() {
        return fileResult;
    }

    public String getNameOfFile() {
        return nameOfFile;
    }

    public void setNameOfFile(String nameOfFile) {
        this.nameOfFile = nameOfFile;
    }

    public int getCurrentNumOfFiles() {
        return currentNumOfFiles;
    }

    public void setCurrentNumOfFiles(int currentNumOfFiles) {
        this.currentNumOfFiles = currentNumOfFiles;
    }

}

Answer 1

In the line

for (File temp : filename.listFiles()) {

the method filename.listFiles() can sometimes return null, which leads to a NullPointerException in the for Loop.

This can e.g. arise, if that directory existed shortly before, but was removed inbetween. If you scan your C: drive this can occur, since typically some programs run in the Background, which can do this.

Answer 2

As @Thomas notes, the listFiles() can return null. According to the javadocs this can occur:

... if [the File] does not denote a directory, or if an I/O error occurs.

Here are some possibilities:

1. There is a race condition whereby the directory is deleted, replaced with a file or made read-only while the parent directory is being iterated.

2. The directory cannot be listed despite canRead() returning true. There are scenarios¹ where this call may return true but the file or directory cannot be read anyway.

3. There is some other other cause for the IOException; e.g. a network error or hard disk IO error.

You should not assume that (canRead() && isDirectory()) == true means that you won't get a null. Test for the null.

---

^{1 - For example: http://bugs.java.com/view_bug.do?bug_id=6203387. I also recall seeing some rather strange behavior happen when a syscall is blocked by SELinux in "enforcing" mode. That could have a bearing here.}