getFile function

[Stormuzi]

Hi，I feel that there may be a problem with this code.
If the line is short, for example ,"a", when the file is exhausted, starting recursion, "" will be passed in.
There will be a stringIndexOutOfBoundsException

protected File getFile(String prefix, String line) {
    char start = line.charAt(0);
    String filename = (prefix + start).toLowerCase();

    File file = fileSplitter.getTempFiles().get(filename);
    if (file == null) {
        file = Paths.get(tempFilesDir, filename + ".txt").toFile();
        file.deleteOnExit();
        fileSplitter.addTempFile(filename, file);
    } else {
        Long maxTempFileSize = Long.valueOf(ExternalSortingProperties.MAX_TEMP_FILE_SIZE.value());
        if (fileSplitter.isFileExhausted(filename) || file.length() >= maxTempFileSize) {
            fileSplitter.addExhaustedFile(filename);
            file = getFile(filename, line.substring(1));
        }
    }
    fileSplitter.checkSameFirstCharFilename(file);
    return file;
}