Android™ Application Development - Bahar Ali Khan

Chapter 6: Data Storage, Retrieval, and Sharing
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Content Providers offer a generic interface to any data source. They effectively decouple the underlying
data storage technique from the application layer.
By default, access to all fi les, databases, and preferences is restricted to the application that created
them. Content Providers offer a managed way for your applications to share private data with other
applications. As a result, your applications can use the Content Providers offered by others, including
native providers.
Android Techniques for Saving Data
The data persistence techniques in Android provide options for balancing speed, effi ciency, and
robustness:
❑ Shared Preferences When storing the UI state, user preferences, or application settings, you
want a lightweight mechanism to store a known set of values. Shared Preferences let you save
groups of key/value pairs of primitive data as named preferences.
❑ Files It’s not pretty, but sometimes writing to, and reading from, fi les directly is the only way
to go. Android lets you create and load fi les on the device’s internal or external media.
❑ SQLite Databases When managed, structured data is the best approach, Android offers the
SQLite relational database library. Every application can create its own databases over which it
has total control.
❑ Content Providers Rather than a storage mechanism in their own right, Content Providers let
you expose a well-defi ned interface for using and sharing private data. You can control access
to Content Providers using the standard permission system.
Saving Simple Application Data
There are two lightweight techniques for saving simple application data for Android applications —
Shared Preferences and a pair of event handlers used for saving Activity instance details. Both mechanisms
use a name/value pair (NVP) mechanism to store simple primitive values.
Using SharedPreferences, you can create named maps of key/value pairs within your application
that can be shared between application components running in the same Context.
Shared Preferences support the primitive types Boolean, string, fl oat, long, and integer, making them
an ideal way to quickly store default values, class instance variables, the current UI state, and user
preferences. They are most commonly used to persist data across user sessions and to share settings
between application components.
Alternatively, Activities offer the onSaveInstanceState handler. It’s designed specifi cally to persist
the UI state when the Activity becomes eligible for termination by a resource-hungry run time.
The handler works like the Shared Preference mechanism. It offers a Bundle parameter that represents
a key/value map of primitive types that can be used to save the Activity’s instance values. This Bundle