Write a string to a file

Write one text file simplified:

private void writeToFile(String content) {
    try {
        File file = new File(Environment.getExternalStorageDirectory() + "/test.txt");

        if (!file.exists()) {
            file.createNewFile();
        }
        FileWriter writer = new FileWriter(file);
        writer.append(content);
        writer.flush();
        writer.close();
    } catch (IOException e) {
    }
}

Not having specified a path, your file will be saved in your app space (/data/data/your.app.name/).

Therefore, you better save your file onto an external storage (which is not necessarily the SD card, it can be the default storage).

You might want to dig into the subject, by reading the official docs

In synthesis:

Add this permission to your Manifest:

    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

It includes the READ permission, so no need to specify it too.

Save the file in a location you specify (this is taken from my live cod, so I'm sure it works):

public void writeToFile(String data)
{
    // Get the directory for the user's public pictures directory.
    final File path =
        Environment.getExternalStoragePublicDirectory
        (
            //Environment.DIRECTORY_PICTURES
            Environment.DIRECTORY_DCIM + "/YourFolder/"
        );

    // Make sure the path directory exists.
    if(!path.exists())
    {
        // Make it, if it doesn't exit
        path.mkdirs();
    }

    final File file = new File(path, "config.txt");

    // Save your stream, don't forget to flush() it before closing it.

    try
    {
        file.createNewFile();
        FileOutputStream fOut = new FileOutputStream(file);
        OutputStreamWriter myOutWriter = new OutputStreamWriter(fOut);
        myOutWriter.append(data);

        myOutWriter.close();

        fOut.flush();
        fOut.close();
    }
    catch (IOException e)
    {
        Log.e("Exception", "File write failed: " + e.toString());
    } 
}

[EDIT] OK Try like this (different path - a folder on the external storage):

    String path =
        Environment.getExternalStorageDirectory() + File.separator  + "yourFolder";
    // Create the folder.
    File folder = new File(path);
    folder.mkdirs();

    // Create the file.
    File file = new File(folder, "config.txt");