createFileAndroid.java

try {
            File nuevaCarpeta = new File(Environment.getExternalStorageDirectory(), "NombreNuevaCarpeta");
            if (!nuevaCarpeta.exists()) {
                nuevaCarpeta.mkdir();
            }
            try {
                File file = new File(nuevaCarpeta, "ArchivoDePrueba" + ".txt");
                file.createNewFile();

                FileOutputStream stream = new FileOutputStream(file);
                try {
                    stream.write(new Gson().toJson("objeto o string sin Gson")).getBytes());
                } finally {
                    stream.close();
                }

            } catch (Exception ex) {
                Log.e("Error", "ex: " + ex);
            }
        } catch (Exception e) {
            Log.e("Error", "e: " + e);
        }