1. Data persistence How to save data using SharedPreferences, Files, and SQLite database 1Data persistence

2. SharedPreferences Saving simple application data like – Text strings – Preferred font size, background color, etc. – Example: PreferenceMenuExample Reading preferences, example SharedPreferences prefs = getSharedPreferences(PREF_NAME, MODE_PRIVATE); float fontSize = prefs.getFloat(FONT_SIZE_KEY, 12); // default value: 12 editText.setText(prefs.getString(TEXT_VALUE_KEY, "")); // default value: empty string Writing preferences, example SharedPreferences prefs = getSharedPreferences(PREF_NAME, MODE_PRIVATE); // MODE_PRIVATE: Only accessible to this application SharedPreferences.Editor editor = prefs.edit(); editor.putFloat(FONT_SIZE_KEY, editText.getTextSize()); editor.putString(TEXT_VALUE_KEY, editText.getText().toString()); editor.commit(); OR BETTER editor.apply(); 2Data persistence

3. SharedPreferences: Obtaining the object The way to get access to a SharedPreferences object it through the method – SharedPreferences getSharedPreferences(String name, int mode) String name: The name of the preferences object Int mode: Decides which application can access the object – Factory method design pattern Modes – MODE_PRIVATE Only accessible to the creating application Any Activity in the creating application can read and write to the object – MODE_WORLD_READABLE Any application can read the object. Only the creating application can write to the object – MODE_WORLD_WRITEABLE Any application can read and write to the object Data persistence3

4. SharedPreferences: Some methods Int getInt(String key, int defaultValue) General – Type getType(String key, Type defaultValue) – Type: float, long, int, String, etc. boolean contains(String key) Map getAll() SharedPreferences.Editor edit() Data persistence4

5. SharedPreferences.Editor: some methods SharedPreferences.Editor putType(Type element) – Type: float, long, int, String, etc. SharedPreferences.Editor remove(String key) – Removes one entry SharedPreferences.Editor clear() – Removes all entries Void apply() + boolean commit() Data persistence5

6. Using SharedPreferences In most cases a SharedPreferences object should only be accessible in a single application – MODE_PRIVATE Can be used to hold information about the users preferences or personal data – Background color, birth day, name etc. – Username + password for easy login. Security?? Can be used to transport data between Activities – An alternative to putting the data into an Intent object. – However, data is going to last longer with SharedPreferences You can find the preferences file – From Android Studio open Device Monitor → File Explorer → data → data → appName → shared_prefs – To see the contents of the file: Pull a file from device (small red/black icon top-left) Data persistence6

7. Files The basic file API classes from the ordinary Java package java.io – FileInputStream, InputStreamReader, BufferedReader, etc. However, opening a file is different – FileInputStream fis = openFileInput(someName, MODE_PRIVATE); – openFileInput is yet another method from the Activity class You can create files (text or binary) and read them Files can be stored – On the device’s internal store – On an SD memory card For transportation to another device Usage – High score lists, pictures, etc. Example: FilesExample + FilesExampleAsyncTask Data persistence7

8. Databases: SQLite Android comes with a small scale relational DBMS called SQLite – http://sqlite.org/ http://sqlite.org/ Uses ordinary SQL The database file is stored in the folder – /data/data/ /databases – Can be seen in the debugger tab File Explorer File Explorer can be opening from the menu Window -> Show View A database is only accessible to the application which created the database – Secure – The database must be created by the application using it Usually you will create the database the first time you use the application – No DBMS tools/user interfaces for your phone! Example: DatabaseExample Data persistence8

9. Database programming practice It’s good practice with SQLite (and any other DBMS) to put the database related code in a separate class – This calls is usually called DBAdapter This class is NOT an Activity, but a plain Java class Data persistence9

10. SQLite database versions When you create a SQLite database you should must give it a version number – Start using version 1. You cannot alter the tables, etc. In the database What you can do is that you can create a new database with a new version number – And all your old data is lost Data persistence10

11. Foreign keys SQLite claims to have support for foreign keys starting from version 3.6.19 – http://www.sqlite.org/foreignkeys.html http://www.sqlite.org/foreignkeys.html – http://www.sqlite.org/faq.html#q22 http://www.sqlite.org/faq.html#q22 Foreign key checking must explicitly be switched on – PRAGMA foreign_keys = on http://www.sqlite.org/foreignkeys.html#fk_enable Data persistence11

12. SQLite, a few more words SQLite can run on ordinary computers as well as on mobile devices. SQLite comes with a lot of options – Some of which can make SQLite run faster Data persistence12