Internal Storage

Internal storage is by far the easiest to understand. Internal storage is a private directory located on the app's internal memory unit that can only be accessed by a single app. Each application installed on a given device has their own private, internal storage directory that can be used to store settings, configuration, or any other private data. The data stored in this directory cannot be accessed by any other application nor can it be accessed via a file browser. That limitation goes away, however, if a device is rooted. As such, be sure to not store any highly sensitive data (e.g. username/password values) in this directory unless it's encoded or encrypted in some way. This is a general rule regardless of where you are storing data, but worthy of calling to attention under the lure of the "private" data label.

The types of data you should be storing in this directory should be relatively small. This is not the recommended location to store large files such as images, audio clips, or videos. This location is meant more so to store object or JSON file output that could be used to load private application data. For instance, if you were making a journal app, the list of entries would be stored out to a file in internal storage using either a JSONArray text file or maybe a serialized ArrayList of Entry objects (more on Serializable in a later section). This way other apps can't read the private journal entries created by your application. It's important to note that any files stored in this directory are deleted when the app is uninstalled or if the user clears data from the App Info page in the system settings.

To access files in internal storage, you would use the openFileInput() and openFileOutput() methods of the Context (or Activity) class. These methods return streams which can be used to read from and write to files in internal storage as covered in the following section of this book.

Example of Internal Storage

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle _savedInstanceState) {
        super.onCreate(_savedInstanceState);
        setContentView(R.layout.activity_main);

        // Read in a private file
        FileInputStream fis = this.openFileInput("some_file.txt");
        // Create new or open existing private file
        FileOutputStream fos = this.openFileOutput("some_other_file.txt", Context.MODE_PRIVATE);
    }
}

References

http://developer.android.com/guide/topics/data/data-storage.html