In Java 6, there is unfortunately no way to determine why a file cannot be deleted. With Java 7, you can use java.nio.file.Files#delete() instead, which will give you a detailed cause of the failure, if the file or directory cannot be deleted.

Note that file.list() may return entries for directories, which can be deleted. The API documentation for delete says that only empty directories can be deleted, but a directory is considered empty, if the contained files are e.g. OS specific metadata files.

Hmm, best I could do:

public String getReasonForFileDeletionFailureInPlainEnglish(File file) {
    try {
        if (!file.exists())
            return "It doesn t exist in the first place.";
        else if (file.isDirectory() && file.list().length > 0)
            return "It s a directory and it s not empty.";
        else
            return "Somebody else has it open, we don t have write permissions, or somebody stole my disk.";
    } catch (SecurityException e) {
        return "We re sandboxed and don t have filesystem access.";
    }
}

Be aware that it can be your own application that prevents a file from being deleted!

If you wrote to the file previously and didn t close the writer, you are locking the file yourself.

Java 7 java.nio.file.Files class can be also used:

http://docs.oracle.com/javase/tutorial/essential/io/delete.html

try {
    Files.delete(path);
} catch (NoSuchFileException x) {
    System.err.format("%s: no such" + " file or directory%n", path);
} catch (DirectoryNotEmptyException x) {
    System.err.format("%s not empty%n", path);
} catch (IOException x) {
    // File permission problems are caught here.
    System.err.println(x);
}

A deletion may fail due to one or more reasons:

1. File does not exist (use File#exists() to test).
2. File is locked (because it is opened by another app (or your own code!).
3. You are not authorized (but that would have thrown a SecurityException, not returned false!).

So whenever deletion fails, do a File#exists() to check if it is caused by 1) or 2).

Summarized:

if (!file.delete()) {
    String message = file.exists() ? "is in use by another app" : "does not exist";
    throw new IOException("Cannot delete file, because file " + message + ".");
}

As pointed out in File.delete()

you may use a SecurityManager that throws the exeception for you.