File.list() returns files in a different order for 4.0 than 2.2

Question

If i am using Android 2.2 and call File.list() method in BookGenerator.java, then pages and chapters come in exact sequence, but whenever I execute on Android 4.0 it gives me reverse pages list or reverse pages order.

Is there a compatibility issue between 2.2 and 4.0?

Answer 1

You shouldn't rely on listFiles() for an ordered list of the pages:

http://docs.oracle.com/javase/1.4.2/docs/api/java/io/File.html#listFiles()

"There is no guarantee that the name strings in the resulting array will appear in any specific order;"

You have to create your own ordering system, based on file name or lastModified or file's size. You can either use a Comparator < File > or Comparator < String > for sorting the files in a SortedSet, or as mentionned earlier, create an own class for the items to sort that implements Comparable. I would suggest the first solution as it's a bit stupid to wrap the File or String class into an other one just for this functionality.

An example with a lot of memory overhead:

TreeSet<File> pages = new TreeSet<File>(new Comparator<File>(){
   public int compare(File first, File second) {
      return first.getName().compareTo(second.getName());
   }
});

for (File file : allFiles) {
   pages.add(file());
}

allFiles = pages.toArray();

If you want a more efficient one, you have to implement your own method for sorting an array in its place.

Answer 2

The list() method does not guarantee any specific order for the items. The Android documentation lacks this point but the official Java SE API javadoc warns about it:

There is no guarantee that the name strings in the resulting array will appear in any specific order; they are not, in particular, guaranteed to appear in alphabetical order.

You should sort the array with Collections.sort() before using it.

File fChapters = new File(internalStorage + bookName + "/Chapters");
// Obtain the chapters file names list (the files in the directory)
chapters = fChapters.list();
// Sort the file names according to default alphabetic ordering
Collections.sort(chapters)
// The chapters list is now sorted from A to Z

With the sort(List list, Comparator c) overload of this method you can define whatever order you need. For example, ignore the case of the letters in the titles:

Collections.sort(chapters, new Comparator<String>() {
    @Override
    public int compare(String chapter1, String chapter2) {
        return chapter1.compareToIgnoreCase(chapter2);
    }
});