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CHAPTER 9 File Storage Shared Preferences SQLite Chapter objectives: • • • • • Data storage methods Understand what Shared Preferences are used for Key-value pairs File-based Storage The differences between internal storage and external storage • How to build applications using SQLite databases 9.1 Storing Data • Applications often store information about a user’s preferences in order to provide a more sophisicated level of personalization and responsiveness • A user can log into and out of an application and have it remember them when the application is relaunched • Users expect to choose settings according to their personal needs, such as specific background sounds and images, and have them automatically persist across user sessions • Advanced applications are frequently datadriven and require the management of a larger volume of data • Databases and files can be used by an application to store structured data or information that will be made available to other applications • Choosing the manner in which data is stored depends upon the specific needs of the application, such as the amount of data that needs to be stored and whether the data will be kept private • The Android platform allows data files to be saved on the device’s internal memory and on an external storage media • Files that are saved within the device’s internal storage memory tend to be small, as opposed to external storage, which typically holds larger volume files 9.2 Shared Preferences • Shared Preferences refers to the storage of a limited set of primitive data used to make persistent changes in an Android application • A simple way to read and write key-value pairs of data • Used to store a user’s personal settings and a small amount of application session data • SharedPreference information is backed up in XML files in internal storage • Proprietary and inaccessible to outside applications • Allows preference data values to automatically persist throughout sessions, regardless of how they end • This includes exit of an application, shutdown of the device, or system crash • A key-value pair is a data representation model that uses a set of key identifiers along with associated data values • Model is frequently used in hash tables and configuration files • Provides access to a data value or object by specifying its associated key • SharedPreferences API provides a set of methods for reading and writing key-value pairs to files • Shared preferences supports the String data type and Java primitive data types, such as int, float, long, and boolean 9.3 File Storage – Internal and External Storage • In Android, a file-based storage option allows data to be written to an actual file structure • This storage method requires more control regarding read and write permissions • Internal storage allows data to be stored directly onto the device’s memory • This storage is always available, assuming there is space • External storage may not always be obtainable on a device. • There are significant differences in how external and internal storage is utilized in an application • Internal storage files can be configured to be readable and writeable by the application • Typically, internal storage is utilized when processing an image, video and audio elements, and large data files • By default, files saved to internal storage are private to the application • External storage is publicly shared storage, which means that it can be made available to external applications • Unlike internal storage, once an application is uninstalled, external storage files will continue to exist 9.4 Android Database with SQLite • SQLite provides the foundation for managing private and embedded databases in an Android application • The Android SDK includes the SQLite software library that implements the SQL (Structured Query Language) database engine • This library is used for defining the database structure and adding and managing the database content as required by the application • The Android SDK also includes an SQLite database tool for explicit database debugging purposes • As a condensed version of SQL (Structured Query Language), SQLite supports the standard SQL syntax and database transactions • Though SQLite is not a full-featured database, it supports a large set of the SQL standard and is sufficient for Android developers needing a simple database engine to plug into their applications • An SQLite database file is a collection of data organized in a table • The SQLite database table is structured by a set of rows and columns • A row represents a single record, the implicitly structured data entry in the table • A column represents a data field, an attribute of a data record • A field is a single item that exists at the intersection between one row and one column • The possible data types for an individual field of data consist of NULL, INTEGER, REAL, TEXT, and BLOB • BLOB is a data type used to store a large array of binary data (bytes) • SQLite does not provide specific data storage classes for values that need to be represented as Boolean, date, or time. Instead, these values can be stored as an INTEGER or TEXT An outline for a students database table schema 9.5 SQLiteOpenHelper • The Android SDK provides a set of classes for working with SQLite databases • SQLiteOpenHelper is essential for the creation and management of database content, as well as database versioning • In an Android SQLite application, SQLiteOpenHelper must be subclassed • It must contain the implementation of onCreate() and onUpgrade() 9.6 Adapters and AdapterViews • An Adapter object is a mechanism that binds a data source, such as a database, to an AdapterView • An AdapterView is a container widget that is populated by a data source, determined by an Adapter • Common AdapterViews are ListViews, GridViews, and Spinners • A ListView is an AdapterView that provides a simple approach for an application to display a scrolling list of records • These records can be displayed with a default format, using a built-in style, or customized extensively • As an AdapterView, a ListView object requires an Adapter to provide it with data • A ListView consists of a collection of sequential items.