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      1 page.title=Application Fundamentals
      2 @jd:body
      3 
      4 <div id="qv-wrapper">
      5 <div id="qv">
      6 
      7 <h2>Quickview</h2>
      8 <ul>
      9   <li>Android applications are composed of one or more application components (activities,
     10 services, content providers, and broadcast receivers)</li>
     11   <li>Each component performs a different role in the overall application behavior, and each
     12 one can be activated individually (even by other applications)</li>
     13   <li>The manifest file must declare all components in the application and should also declare
     14 all application requirements, such as the minimum version of Android required and any hardware
     15 configurations required</li>
     16   <li>Non-code application resources (images, strings, layout files, etc.) should include
     17 alternatives for different device configurations (such as different strings for different
     18 languages and different layouts for different screen sizes)</li>
     19 </ul>
     20 
     21 
     22 <h2>In this document</h2>
     23 <ol>
     24 <li><a href="#Components">Application Components</a>
     25   <ol>
     26     <li><a href="#ActivatingComponents">Activating components</a></li>
     27   </ol>
     28 </li>
     29 <li><a href="#Manifest">The Manifest File</a>
     30   <ol>
     31     <li><a href="#DeclaringComponents">Declaring components</a></li>
     32     <li><a href="#DeclaringRequirements">Declaring application requirements</a></li>
     33   </ol>
     34 </li>
     35 <li><a href="#Resources">Application Resources</a></li>
     36 </ol>
     37 </div>
     38 </div>
     39 
     40 <p>Android applications are written in the Java programming language. The Android SDK tools compile
     41 the code&mdash;along with any data and resource files&mdash;into an <i>Android package</i>, an
     42 archive file with an {@code .apk} suffix. All the code in a single {@code .apk} file is considered
     43 to be one application and is the file that Android-powered devices use to install the
     44 application.</p>
     45 
     46 <p>Once installed on a device, each Android application lives in its own security sandbox: </p>
     47 
     48 <ul>
     49  <li>The Android operating system is a multi-user Linux system in which each application is a
     50 different user.</li>
     51 
     52 <li>By default, the system assigns each application a unique Linux user ID (the ID is used only by
     53 the system and is unknown to the application). The system sets permissions for all the files in an
     54 application so that only the user ID assigned to that application can access them. </li>
     55 
     56 <li>Each process has its own virtual machine (VM), so an application's code runs in isolation from
     57 other applications.</li>
     58 
     59 <li>By default, every application runs in its own Linux process. Android starts the process when any
     60 of the application's components need to be executed, then shuts down the process when it's no longer
     61 needed or when the system must recover memory for other applications.</li>
     62 </ul>
     63 
     64 <p>In this way, the Android system implements the <em>principle of least privilege</em>. That is,
     65 each application, by default, has access only to the components that it requires to do its work and
     66 no more. This creates a very secure environment in which an application cannot access parts of
     67 the system for which it is not given permission.</p>
     68 
     69 <p>However, there are ways for an application to share data with other applications and for an
     70 application to access system services:</p>
     71 
     72 <ul>
     73   <li>It's possible to arrange for two applications to share the same Linux user ID, in which case
     74 they are able to access each other's files.  To conserve system resources, applications with the
     75 same user ID can also arrange to run in the same Linux process and share the same VM (the
     76 applications must also be signed with the same certificate).</li>
     77   <li>An application can request permission to access device data such as the user's
     78 contacts, SMS messages, the mountable storage (SD card), camera, Bluetooth, and more. All
     79 application permissions must be granted by the user at install time.</li>
     80 </ul>
     81 
     82 <p>That covers the basics regarding how an Android application exists within the system. The rest of
     83 this document introduces you to:</p>
     84 <ul>
     85   <li>The core framework components that define your application.</li>
     86   <li>The manifest file in which you declare components and required device features for your
     87 application.</li>
     88   <li>Resources that are separate from the application code and allow your application to
     89 gracefully optimize its behavior for a variety of device configurations.</li>
     90 </ul>
     91 
     92 <!--
     93 <p class="note"><strong>Tip:</strong> If you're new to Android development, we suggest that you
     94 follow the Beginner's Path link at the bottom of this page. For each document in the Application
     95 Fundamentals, the Beginner's Path points you to the document we suggest you read next, in order
     96 to get up to speed on the core Android concepts.</p>
     97 -->
     98 
     99 
    100 <h2 id="Components">Application Components</h2>
    101 
    102 <p>Application components are the essential building blocks of an Android application. Each
    103 component is a different point through which the system can enter your application. Not all
    104 components are actual entry points for the user and some depend on each other, but each one exists
    105 as its own entity and plays a specific role&mdash;each one is a unique building block that
    106 helps define your application's overall behavior.</p>
    107 
    108 <p>There are four different types of application components. Each type serves a distinct purpose
    109 and has a distinct lifecycle that defines how the component is created and destroyed.</p>
    110 
    111 <p>Here are the four types of application components:</p>
    112 
    113 <dl>
    114 
    115 <dt><b>Activities</b></dt>
    116 
    117 <dd>An <i>activity</i> represents a single screen with a user interface. For example,
    118 an email application might have one activity that shows a list of new
    119 emails, another activity to compose an email, and another activity for reading emails. Although
    120 the activities work together to form a cohesive user experience in the email application, each one
    121 is independent of the others. As such, a different application can start any one of these
    122 activities (if the email application allows it). For example, a camera application can start the
    123 activity in the email application that composes new mail, in order for the user to share a picture.
    124 
    125 <p>An activity is implemented as a subclass of {@link android.app.Activity} and you can learn more
    126 about it in the <a href="{@docRoot}guide/components/activities.html">Activities</a>
    127 developer guide.</p>
    128 </dd>
    129 
    130 
    131 <dt><b>Services</b></dt>
    132 
    133 <dd>A <i>service</i> is a component that runs in the background to perform long-running
    134 operations or to perform work for remote processes. A service
    135 does not provide a user interface. For example, a service might play music in the background while
    136 the user is in a different application, or it might fetch data over the network without
    137 blocking user interaction with an activity. Another component, such as an activity, can start the
    138 service and let it run or bind to it in order to interact with it.
    139 
    140 <p>A service is implemented as a subclass of {@link android.app.Service} and you can learn more
    141 about it in the <a href="{@docRoot}guide/components/services.html">Services</a> developer
    142 guide.</p>
    143 </dd>
    144 
    145 
    146 <dt><b>Content providers</b></dt>
    147 
    148 <dd>A <i>content provider</i> manages a shared set of application data. You can store the data in
    149 the file system, an SQLite database, on the web, or any other persistent storage location your
    150 application can access. Through the content provider, other applications can query or even modify
    151 the data (if the content provider allows it). For example, the Android system provides a content
    152 provider that manages the user's contact information. As such, any application with the proper
    153 permissions can query part of the content provider (such as {@link
    154 android.provider.ContactsContract.Data}) to read and write information about a particular person.
    155 
    156 <p>Content providers are also useful for reading and writing data that is private to your
    157 application and not shared. For example, the <a
    158 href="{@docRoot}resources/samples/NotePad/index.html">Note Pad</a> sample application uses a
    159 content provider to save notes.</p>
    160 
    161 <p>A content provider is implemented as a subclass of {@link android.content.ContentProvider}
    162 and must implement a standard set of APIs that enable other applications to perform
    163 transactions. For more information, see the <a
    164 href="{@docRoot}guide/topics/providers/content-providers.html">Content Providers</a> developer
    165 guide.</p>
    166 </dd>
    167 
    168 
    169 <dt><b>Broadcast receivers</b></dt>
    170 
    171 <dd>A <i>broadcast receiver</i> is a component that responds to system-wide broadcast
    172 announcements.  Many broadcasts originate from the system&mdash;for example, a broadcast announcing
    173 that the screen has turned off, the battery is low, or a picture was captured.
    174 Applications can also initiate broadcasts&mdash;for example, to let other applications know that
    175 some data has been downloaded to the device and is available for them to use. Although broadcast
    176 receivers don't display a user interface, they may <a
    177 href="{@docRoot}guide/topics/ui/notifiers/notifications.html">create a status bar notification</a>
    178 to alert the user when a broadcast event occurs. More commonly, though, a broadcast receiver is
    179 just a "gateway" to other components and is intended to do a very minimal amount of work. For
    180 instance, it might initiate a service to perform some work based on the event.
    181 
    182 <p>A broadcast receiver is implemented as a subclass of {@link android.content.BroadcastReceiver}
    183 and each broadcast is delivered as an {@link android.content.Intent} object. For more information,
    184 see the {@link android.content.BroadcastReceiver} class.</p>
    185 </dd>
    186 
    187 </dl>
    188 
    189 
    190 
    191 <p>A unique aspect of the Android system design is that any application can start another
    192 applications component. For example, if you want the user to capture a
    193 photo with the device camera, there's probably another application that does that and your
    194 application can use it, instead of developing an activity to capture a photo yourself. You don't
    195 need to incorporate or even link to the code from the camera application.
    196 Instead, you can simply start the activity in the camera application that captures a
    197 photo. When complete, the photo is even returned to your application so you can use it. To the user,
    198 it seems as if the camera is actually a part of your application.</p>
    199 
    200 <p>When the system starts a component, it starts the process for that application (if it's not
    201 already running) and instantiates the classes needed for the component. For example, if your
    202 application starts the activity in the camera application that captures a photo, that activity
    203 runs in the process that belongs to the camera application, not in your application's process.
    204 Therefore, unlike applications on most other systems, Android applications don't have a single entry
    205 point (there's no {@code main()} function, for example).</p>
    206 
    207 <p>Because the system runs each application in a separate process with file permissions that
    208 restrict access to other applications, your application cannot directly activate a component from
    209 another application. The Android system, however, can. So, to activate a component in
    210 another application, you must deliver a message to the system that specifies your <em>intent</em> to
    211 start a particular component. The system then activates the component for you.</p>
    212 
    213 
    214 <h3 id="ActivatingComponents">Activating Components</h3>
    215 
    216 <p>Three of the four component types&mdash;activities, services, and
    217 broadcast receivers&mdash;are activated by an asynchronous message called an <em>intent</em>.
    218 Intents bind individual components to each other at runtime (you can think of them
    219 as the messengers that request an action from other components), whether the component belongs
    220 to your application or another.</p>
    221 
    222 <p>An intent is created with an {@link android.content.Intent} object, which defines a message to
    223 activate either a specific component or a specific <em>type</em> of component&mdash;an intent
    224 can be either explicit or implicit, respectively.</p>
    225 
    226 <p>For activities and services, an intent defines the action to perform (for example, to "view" or
    227 "send" something) and may specify the URI of the data to act on (among other things that the
    228 component being started might need to know). For example, an intent might convey a request for an
    229 activity to show an image or to open a web page. In some cases, you can start an
    230 activity to receive a result, in which case, the activity also returns
    231 the result in an {@link android.content.Intent} (for example, you can issue an intent to let
    232 the user pick a personal contact and have it returned to you&mdash;the return intent includes a
    233 URI pointing to the chosen contact).</p>
    234 
    235 <p>For broadcast receivers, the intent simply defines the
    236 announcement being broadcast (for example, a broadcast to indicate the device battery is low
    237 includes only a known action string that indicates "battery is low").</p>
    238 
    239 <p>The other component type, content provider, is not activated by intents. Rather, it is
    240 activated when targeted by a request from a {@link android.content.ContentResolver}. The content
    241 resolver handles all direct transactions with the content provider so that the component that's
    242 performing transactions with the provider doesn't need to and instead calls methods on the {@link
    243 android.content.ContentResolver} object. This leaves a layer of abstraction between the content
    244 provider and the component requesting information (for security).</p>
    245 
    246 <p>There are separate methods for activating each type of component:</p>
    247 <ul>
    248   <li>You can start an activity (or give it something new to do) by
    249 passing an {@link android.content.Intent} to {@link android.content.Context#startActivity
    250 startActivity()} or {@link android.app.Activity#startActivityForResult startActivityForResult()}
    251 (when you want the activity to return a result).</li>
    252   <li>You can start a service (or give new instructions to an ongoing service) by
    253 passing an {@link android.content.Intent} to {@link android.content.Context#startService
    254 startService()}. Or you can bind to the service by passing an {@link android.content.Intent} to
    255 {@link android.content.Context#bindService bindService()}.</li>
    256   <li>You can initiate a broadcast by passing an {@link android.content.Intent} to methods like
    257 {@link android.content.Context#sendBroadcast(Intent) sendBroadcast()}, {@link
    258 android.content.Context#sendOrderedBroadcast(Intent, String) sendOrderedBroadcast()}, or {@link
    259 android.content.Context#sendStickyBroadcast sendStickyBroadcast()}.</li>
    260   <li>You can perform a query to a content provider by calling {@link
    261 android.content.ContentProvider#query query()} on a {@link android.content.ContentResolver}.</li>
    262 </ul>
    263 
    264 <p>For more information about using intents, see the <a
    265 href="{@docRoot}guide/components/intents-filters.html">Intents and
    266 Intent Filters</a> document. More information about activating specific components is also provided
    267 in the following documents: <a
    268 href="{@docRoot}guide/components/activities.html">Activities</a>, <a
    269 href="{@docRoot}guide/components/services.html">Services</a>, {@link
    270 android.content.BroadcastReceiver} and <a
    271 href="{@docRoot}guide/topics/providers/content-providers.html">Content Providers</a>.</p>
    272 
    273 
    274 <h2 id="Manifest">The Manifest File</h2>
    275 
    276 <p>Before the Android system can start an application component, the system must know that the
    277 component exists by reading the application's {@code AndroidManifest.xml} file (the "manifest"
    278 file). Your application must declare all its components in this file, which must be at the root of
    279 the application project directory.</p>
    280 
    281 <p>The manifest does a number of things in addition to declaring the application's components,
    282 such as:</p>
    283 <ul>
    284   <li>Identify any user permissions the application requires, such as Internet access or
    285 read-access to the user's contacts.</li>
    286   <li>Declare the minimum <a href="{@docRoot}guide/topics/manifest/uses-sdk-element.html#ApiLevels">API Level</a>
    287 required by the application, based on which APIs the application uses.</li>
    288   <li>Declare hardware and software features used or required by the application, such as a camera,
    289 bluetooth services, or a multitouch screen.</li>
    290   <li>API libraries the application needs to be linked against (other than the Android framework
    291 APIs), such as the <a
    292 href="http://code.google.com/android/add-ons/google-apis/maps-overview.html">Google Maps
    293 library</a>.</li>
    294   <li>And more</li>
    295 </ul>
    296 
    297 
    298 <h3 id="DeclaringComponents">Declaring components</h3>
    299 
    300 <p>The primary task of the manifest is to inform the system about the application's components. For
    301 example, a manifest file can declare an activity as follows: </p>
    302 
    303 <pre>
    304 &lt;?xml version="1.0" encoding="utf-8"?&gt;
    305 &lt;manifest ... &gt;
    306     &lt;application android:icon="@drawable/app_icon.png" ... &gt;
    307         &lt;activity android:name="com.example.project.ExampleActivity"
    308                   android:label="@string/example_label" ... &gt;
    309         &lt;/activity&gt;
    310         ...
    311     &lt;/application&gt;
    312 &lt;/manifest&gt;</pre>
    313 
    314 <p>In the <code><a
    315 href="{@docRoot}guide/topics/manifest/application-element.html">&lt;application&gt;</a></code>
    316 element, the {@code android:icon} attribute points to resources for an icon that identifies the
    317 application.</p>
    318 
    319 <p>In the <code><a
    320 href="{@docRoot}guide/topics/manifest/activity-element.html">&lt;activity&gt;</a></code> element,
    321 the {@code android:name} attribute specifies the fully qualified class name of the {@link
    322 android.app.Activity} subclass and the {@code android:label} attributes specifies a string
    323 to use as the user-visible label for the activity.</p>
    324 
    325 <p>You must declare all application components this way:</p>
    326 <ul>
    327   <li><code><a
    328 href="{@docRoot}guide/topics/manifest/activity-element.html">&lt;activity&gt;</a></code> elements
    329 for activities</li>
    330   <li><code><a
    331 href="{@docRoot}guide/topics/manifest/service-element.html">&lt;service&gt;</a></code> elements for
    332 services</li>
    333   <li><code><a
    334 href="{@docRoot}guide/topics/manifest/receiver-element.html">&lt;receiver&gt;</a></code> elements
    335 for broadcast receivers</li>
    336   <li><code><a
    337 href="{@docRoot}guide/topics/manifest/provider-element.html">&lt;provider&gt;</a></code> elements
    338 for content providers</li>
    339 </ul>
    340 
    341 <p>Activities, services, and content providers that you include in your source but do not declare
    342 in the manifest are not visible to the system and, consequently, can never run.  However,
    343 broadcast
    344 receivers can be either declared in the manifest or created dynamically in code (as
    345 {@link android.content.BroadcastReceiver} objects) and registered with the system by calling
    346 {@link android.content.Context#registerReceiver registerReceiver()}.</p>
    347 
    348 <p>For more about how to structure the manifest file for your application, see <a
    349 href="{@docRoot}guide/topics/manifest/manifest-intro.html">The AndroidManifest.xml File</a>
    350 documentation. </p>
    351 
    352 
    353 
    354 <h3 id="DeclaringComponentCapabilities">Declaring component capabilities</h3>
    355 
    356 <p>As discussed above, in <a href="#ActivatingComponents">Activating Components</a>, you can use an
    357 {@link android.content.Intent} to start activities, services, and broadcast receivers. You can do so
    358 by explicitly naming the target component (using the component class name) in the intent. However,
    359 the real power of intents lies in the concept of intent actions. With intent actions, you simply
    360 describe the type of action you want to perform (and optionally, the data upon which youd like to
    361 perform the action) and allow the system to find a component on the device that can perform the
    362 action and start it. If there are multiple components that can perform the action described by the
    363 intent, then the user selects which one to use.</p>
    364 
    365 <p>The way the system identifies the components that can respond to an intent is by comparing the
    366 intent received to the <i>intent filters</i> provided in the manifest file of other applications on
    367 the device.</p>
    368 
    369 <p>When you declare a component in your application's manifest, you can optionally include
    370 intent filters that declare the capabilities of the component so it can respond to intents
    371 from other applications. You can declare an intent filter for your component by
    372 adding an <a href="{@docRoot}guide/topics/manifest/intent-filter-element.html">{@code
    373 &lt;intent-filter&gt;}</a> element as a child of the component's declaration element.</p>
    374 
    375 <p>For example, an email application with an activity for composing a new email might declare an
    376 intent filter in its manifest entry to respond to "send" intents (in order to send email). An
    377 activity in your application can then create an intent with the send action ({@link
    378 android.content.Intent#ACTION_SEND}), which the system matches to the email applications send
    379 activity and launches it when you invoke the intent with {@link android.app.Activity#startActivity
    380 startActivity()}.</p>
    381 
    382 <p>For more about creating intent filters, see the <a
    383 href="{@docRoot}guide/components/intents-filters.html">Intents and Intent Filters</a> document.
    384 </p>
    385 
    386 
    387 
    388 <h3 id="DeclaringRequirements">Declaring application requirements</h3>
    389 
    390 <p>There are a variety of devices powered by Android and not all of them provide the
    391 same features and capabilities. In order to prevent your application from being installed on devices
    392 that lack features needed by your application, it's important that you clearly define a profile for
    393 the types of devices your application supports by declaring device and software requirements in your
    394 manifest file. Most of these declarations are informational only and the system does not read
    395 them, but external services such as Google Play do read them in order to provide filtering
    396 for users when they search for applications from their device.</p>
    397 
    398 <p>For example, if your application requires a camera and uses APIs introduced in Android 2.1 (<a
    399 href="{@docRoot}guide/topics/manifest/uses-sdk-element.html#ApiLevels">API Level</a> 7), you should declare these as
    400 requirements in your manifest file. That way, devices that do <em>not</em> have a camera and have an
    401 Android version <em>lower</em> than 2.1 cannot install your application from Google Play.</p>
    402 
    403 <p>However, you can also declare that your application uses the camera, but does not
    404 <em>require</em> it. In that case, your application must perform a check at runtime to determine
    405 if the device has a camera and disable any features that use the camera if one is not available.</p>
    406 
    407 <p>Here are some of the important device characteristics that you should consider as you design and
    408 develop your application:</p>
    409 
    410 <dl>
    411   <dt>Screen size and density</dt>
    412   <dd>In order to categorize devices by their screen type, Android defines two characteristics for
    413 each device: screen size (the physical dimensions of the screen) and screen density (the physical
    414 density of the pixels on the screen, or dpi&mdash;dots per inch). To simplify all the different
    415 types of screen configurations, the Android system generalizes them into select groups that make
    416 them easier to target.
    417 <p>The screen sizes are: small, normal, large, and extra large.<br/>
    418 The screen densities are: low density, medium density, high density, and extra high density.</p>
    419 
    420 <p>By default, your application is compatible with all screen sizes and densities,
    421 because the Android system makes the appropriate adjustments to your UI layout and image
    422 resources. However, you should create specialized layouts for certain screen sizes and provide
    423 specialized images for certain densities, using alternative layout resources, and by declaring in
    424 your manifest exactly which screen sizes your application supports with the <a
    425 href="{@docRoot}guide/topics/manifest/supports-screens-element.html">{@code
    426 &lt;supports-screens&gt;}</a> element.</p>
    427 <p>For more information, see the <a
    428 href="{@docRoot}guide/practices/screens_support.html">Supporting Multiple Screens</a>
    429 document.</p></dd>
    430 
    431   <dt>Input configurations</dt>
    432   <dd>Many devices provide a different type of user input mechanism, such as a hardware keyboard, a
    433 trackball, or a five-way navigation pad. If your application requires a particular kind of input
    434 hardware, then you should declare it in your manifest with the <a
    435 href="{@docRoot}guide/topics/manifest/uses-configuration-element.html">{@code
    436 &lt;uses-configuration&gt;}</a> element. However, it is rare that an application should require
    437 a certain input configuration.</dd>
    438 
    439   <dt>Device features</dt>
    440   <dd>There are many hardware and software features that may or may not exist on a given
    441 Android-powered device, such as a camera, a light sensor, bluetooth, a certain
    442 version of OpenGL, or the fidelity of the touchscreen. You should never assume that a certain
    443 feature is available on all Android-powered devices (other than the availability of the standard
    444 Android library), so you should declare any features used by your application with the <a
    445 href="{@docRoot}guide/topics/manifest/uses-feature-element.html">{@code &lt;uses-feature&gt;}</a>
    446 element.</dd>
    447 
    448   <dt>Platform Version</dt>
    449   <dd>Different Android-powered devices often run different versions of the Android platform,
    450 such as Android 1.6 or Android 2.3. Each successive version often includes additional APIs not
    451 available in the previous version. In order to indicate which set of APIs are available, each
    452 platform version specifies an <a
    453 href="{@docRoot}guide/topics/manifest/uses-sdk-element.html#ApiLevels">API Level</a> (for example, Android 1.0 is API Level
    454 1 and Android 2.3 is API Level 9). If you use any APIs that were added to the platform after
    455 version 1.0, you should declare the minimum API Level in which those APIs were introduced using the
    456 <a href="{@docRoot}guide/topics/manifest/uses-sdk-element.html">{@code &lt;uses-sdk&gt;}</a>
    457 element.</dd>
    458 </dl>
    459 
    460 <p>It's important that you declare all such requirements for your application, because, when you
    461 distribute your application on Google Play, the store uses these declarations to filter which
    462 applications are available on each device. As such, your application should be available only to
    463 devices that meet all your application requirements.</p>
    464 
    465 <p>For more information about how Google Play filters applications based on these (and other)
    466 requirements, see the <a href="{@docRoot}google/play/filters.html">Filters on Google Play</a>
    467 document.</p>
    468 
    469 
    470 
    471 <h2 id="Resources">Application Resources</h2>
    472 
    473 <p>An Android application is composed of more than just code&mdash;it requires resources that are
    474 separate from the source code, such as images, audio files, and anything relating to the visual
    475 presentation of the application. For example, you should define animations, menus, styles, colors,
    476 and the layout of activity user interfaces with XML files. Using application resources makes it easy
    477 to update various characteristics of your application without modifying code and&mdash;by providing
    478 sets of alternative resources&mdash;enables you to optimize your application for a  variety of
    479 device configurations (such as different languages and screen sizes).</p>
    480 
    481 <p>For every resource that you include in your Android project, the SDK build tools define a unique
    482 integer ID, which you can use to reference the resource from your application code or from
    483 other resources defined in XML. For example, if your application contains an image file named {@code
    484 logo.png} (saved in the {@code res/drawable/} directory), the SDK tools generate a resource ID
    485 named {@code R.drawable.logo}, which you can use to reference the image and insert it in your
    486 user interface.</p>
    487 
    488 <p>One of the most important aspects of providing resources separate from your source code
    489 is the ability for you to provide alternative resources for different device
    490 configurations. For example, by defining UI strings in XML, you can translate the strings into other
    491 languages and save those strings in separate files. Then, based on a language <em>qualifier</em>
    492 that you append to the resource directory's name (such as {@code res/values-fr/} for French string
    493 values) and the user's language setting, the Android system applies the appropriate language strings
    494 to your UI.</p>
    495 
    496 <p>Android supports many different <em>qualifiers</em> for your alternative resources. The
    497 qualifier is a short string that you include in the name of your resource directories in order to
    498 define the device configuration for which those resources should be used. As another
    499 example, you should often create different layouts for your activities, depending on the
    500 device's screen orientation and size. For example, when the device screen is in portrait
    501 orientation (tall), you might want a layout with buttons to be vertical, but when the screen is in
    502 landscape orientation (wide), the buttons should be aligned horizontally. To change the layout
    503 depending on the orientation, you can define two different layouts and apply the appropriate
    504 qualifier to each layout's directory name. Then, the system automatically applies the appropriate
    505 layout depending on the current device orientation.</p>
    506 
    507 <p>For more about the different kinds of resources you can include in your application and how
    508 to create alternative resources for various device configurations, see the <a
    509 href="{@docRoot}guide/topics/resources/index.html">Application Resources</a> developer guide.</p>
    510 
    511 
    512 <!--
    513 <h2>Beginner's Path</h2>
    514 
    515 <p>For a close look at implementing activities&mdash;the components your users use to
    516 interact with your application&mdash;continue with the <b><a
    517 href="{@docRoot}guide/components/activities.html">Activities</a></b> document.</p>
    518 -->
    519