FileNotFound while using InputSource

I get an Error when using InputSource for XPath.

InputSource inputSource = null;
try {
    inputSource = new InputSource(new FileInputStream(filename));
} catch (FileNotFoundException e) {
    e.printStackTrace();
}
String s = readFromFile(filename);
String uuid = taskItems.get(position).get("uuid");
XPath xPath = XPathFactory.newInstance().newXPath();
try {
    Node taskNode = (Node) xPath.evaluate("//task[@uuid='" + uuid + "']", inputSource, XPathConstants.NODE);
    Document document = taskNode.getOwnerDocument();
    //Füge neue Zeile ein
    Node noteNode = document.createElement("task_note");
    noteNode.setTextContent(taskItems.get(position).get("task_note"));
    taskNode.appendChild(noteNode);
    //Speichere Datei
    Source input = new DOMSource(document);
    Result output = new StreamResult(new File(filename));
    TransformerFactory.newInstance().newTransformer().transform(input, output);
} catch (XPathExpressionException e) {
    e.printStackTrace();
} catch (TransformerException e) {
    e.printStackTrace();
}

I don't know why but when using String s = readFromFile(filename);', I get the File insideString s`.

readFromFile:

private String readFromFile(String fileName) {
    String ret = "";
    String UTF8 = "UTF-8";
    int BUFFER_SIZE = 8192;

    try {
        InputStream inputStream = openFileInput(fileName);

        if (inputStream != null) {

            BufferedReader bufferedReader1 = new BufferedReader(new InputStreamReader(inputStream, UTF8), BUFFER_SIZE);
            String receiveString = "";
            StringBuilder stringBuilder = new StringBuilder();

            while ((receiveString = bufferedReader1.readLine()) != null) {
                stringBuilder.append(receiveString);
            }

            inputStream.close();
            ret = stringBuilder.toString();
        }
    } catch (FileNotFoundException e) {
        Log.e("readFromFile: ", "Datei nicht gefunden: " + e.toString());
        e.printStackTrace();
    } catch (IOException e) {
        Log.e("readFromFile: ", "Kann Datei nicht lesen: " + e.toString());
        e.printStackTrace();
    }
    return ret;
}

writeToFile:

private void writeToFile(String data, String fileName) {
    try {
        String UTF8 = "UTF-8";
        int BUFFER_SIZE = 8192;
        FileOutputStream fileOutputStream = openFileOutput(fileName, Context.MODE_PRIVATE);
        BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(fileOutputStream, UTF8), BUFFER_SIZE);
        bufferedWriter.write(data);
        bufferedWriter.close();
    } catch (IOException e) {
        Log.e("writeToFile: ", "Datei-Erstellung fehlgeschlagen: " + e.toString());
    }
}

So what do I have to change, to make InputSource find the File?