For those looking for a general strategy for reading and writing a string to file:

credit to : https://stackoverflow.com/users/559634/sharkalley

First, get a file object

You’ll need the storage path. For the internal storage, use:

File path = context.getFilesDir();

For the external storage (SD card), use:

File path = context.getExternalFilesDir(null);

Then create your file object:

File file = new File(path, "my-file-name.txt");

Write a string to the file

FileOutputStream stream = new FileOutputStream(file);
try {
    stream.write("text-to-write".getBytes());
} finally {
    stream.close();
}

Or with Google Guava

String contents = Files.toString(file, StandardCharsets.UTF_8);

Read the file to a string

int length = (int) file.length();

byte[] bytes = new byte[length];

FileInputStream in = new FileInputStream(file);
try {
    in.read(bytes);
} finally {
    in.close();
}

String contents = new String(bytes);   

Or if you are using Google Guava

String contents = Files.toString(file,"UTF-8");

For completeness I’ll mention

String contents = new Scanner(file).useDelimiter("\\A").next();

which requires no libraries, but benchmarks 50% – 400% slower than the other options (in various tests on my Nexus 5).

Notes

For each of these strategies, you’ll be asked to catch an IOException.

The default character encoding on Android is UTF-8.

If you are using external storage, you’ll need to add to your manifest either:

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

or

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

Write permission implies read permission, so you don’t need both.