xref: /frameworks/base/docs/html/guide/components/activities.jd

page.title=Activities
page.tags=activity,intent
@jd:body

<div id="qv-wrapper">
<div id="qv">
<h2>In this document</h2>
<ol>
  <li><a href="#Creating">Creating an Activity</a>
    <ol>
      <li><a href="#UI">Implementing a user interface</a></li>
      <li><a href="#Declaring">Declaring the activity in the manifest</a></li>
    </ol>
  </li>
  <li><a href="#StartingAnActivity">Starting an Activity</a>
    <ol>
      <li><a href="#StartingAnActivityForResult">Starting an activity for a result</a></li>
    </ol>
  </li>
  <li><a href="#ShuttingDown">Shutting Down an Activity</a></li>
  <li><a href="#Lifecycle">Managing the Activity Lifecycle</a>
    <ol>
      <li><a href="#ImplementingLifecycleCallbacks">Implementing the lifecycle callbacks</a></li>
      <li><a href="#SavingActivityState">Saving activity state</a></li>
      <li><a href="#ConfigurationChanges">Handling configuration changes</a></li>
      <li><a href="#CoordinatingActivities">Coordinating activities</a></li>
    </ol>
  </li>
</ol>

<h2>Key classes</h2>
<ol>
  <li>{@link android.app.Activity}</li>
</ol>

<h2>See also</h2>
<ol>
  <li><a href="{@docRoot}guide/components/tasks-and-back-stack.html">Tasks and Back
Stack</a></li>
</ol>

</div>
</div>



<p>An {@link android.app.Activity} is an application component that provides a screen with which
users can interact in order to do something, such as dial the phone, take a photo, send an email, or
view a map. Each activity is given a window in which to draw its user interface. The window
typically fills the screen, but may be smaller than the screen and float on top of other
windows.</p>

<p> An application usually consists of multiple activities that are loosely bound
to each other. Typically, one activity in an application is specified as the "main" activity, which
is presented to the user when launching the application for the first time. Each
activity can then start another activity in order to perform different actions. Each time a new
activity starts, the previous activity is stopped, but the system preserves the activity
in a stack (the "back stack"). When a new activity starts, it is pushed onto the back stack and
takes user focus. The back stack abides to the basic "last in, first out" stack mechanism,
so, when the user is done with the current activity and presses the <em>Back</em> button, it
is popped from the stack (and destroyed) and the previous activity resumes. (The back stack is
discussed more in the <a href="{@docRoot}guide/components/tasks-and-back-stack.html">Tasks
and Back Stack</a> document.)</p>

<p>When an activity is stopped because a new activity starts, it is notified of this change in state
through the activity's lifecycle callback methods.
There are several callback methods that an activity might receive, due to a change in its
state&mdash;whether the system is creating it, stopping it, resuming it, or destroying it&mdash;and
each callback provides you the opportunity to perform specific work that's
appropriate to that state change. For instance, when stopped, your activity should release any
large objects, such as network or database connections. When the activity resumes, you can
reacquire the necessary resources and resume actions that were interrupted. These state transitions
are all part of the activity lifecycle.</p>

<p>The rest of this document discusses the basics of how to build and use an activity,
including a complete discussion of how the activity lifecycle works, so you can properly manage
the transition between various activity states.</p>



<h2 id="Creating">Creating an Activity</h2>

<p>To create an activity, you must create a subclass of {@link android.app.Activity} (or
an existing subclass of it). In your subclass, you need to implement callback methods that the
system calls when the activity transitions between various states of its lifecycle, such as when
the activity is being created, stopped, resumed, or destroyed. The two most important callback
methods are:</p>

<dl>
  <dt>{@link android.app.Activity#onCreate onCreate()}</dt>
  <dd>You must implement this method. The system calls this when creating your
    activity. Within your implementation, you should initialize the essential components of your
activity.
    Most importantly, this is where you must call {@link android.app.Activity#setContentView
    setContentView()} to define the layout for the activity's user interface.</dd>
  <dt>{@link android.app.Activity#onPause onPause()}</dt>
  <dd>The system calls this method as the first indication that the user is leaving your
activity (though it does not always mean the activity is being destroyed). This is usually where you
should commit any changes that should be persisted beyond the current user session (because
the user might not come back).</dd>
</dl>

<p>There are several other lifecycle callback methods that you should use in order to provide a
fluid user experience between activities and handle unexpected interuptions that cause your activity
to be stopped and even destroyed. All of the lifecycle callback methods are discussed later, in
the section about <a href="#Lifecycle">Managing the Activity Lifecycle</a>.</p>



<h3 id="UI">Implementing a user interface</h3>

<p> The user interface for an activity is provided by a hierarchy of views&mdash;objects derived
from the {@link android.view.View} class.  Each view controls a particular rectangular space
within the activity's window and can respond to user interaction. For example, a view might be a
button that initiates an action when the user touches it.</p>

<p>Android provides a number of ready-made views that you can use to design and organize your
layout. "Widgets" are views that provide a visual (and interactive) elements for the screen, such
as a button, text field, checkbox, or just an image. "Layouts" are views derived from {@link
android.view.ViewGroup} that provide a unique layout model for its child views, such as a linear
layout, a grid layout, or relative layout. You can also subclass the {@link android.view.View} and
{@link android.view.ViewGroup} classes (or existing subclasses) to create your own widgets and
layouts and apply them to your activity layout.</p>

<p>The most common way to define a layout using views is with an XML layout file saved in your
application resources. This way, you can maintain the design of your user interface separately from
the source code that defines the activity's behavior. You can set the layout as the UI for your
activity with {@link android.app.Activity#setContentView(int) setContentView()}, passing the
resource ID for the layout. However, you can also create new {@link android.view.View}s in your
activity code and build a view hierarchy by inserting new {@link
android.view.View}s into a {@link android.view.ViewGroup}, then use that layout by passing the root
{@link android.view.ViewGroup} to {@link android.app.Activity#setContentView(View)
setContentView()}.</p>

<p>For information about creating a user interface, see the <a
href="{@docRoot}guide/topics/ui/index.html">User Interface</a> documentation.</p>



<h3 id="Declaring">Declaring the activity in the manifest</h3>

<p>You must declare your activity in the manifest file in order for it to
be accessible to the system. To declare your activity, open your manifest file and add an <a
href="{@docRoot}guide/topics/manifest/activity-element.html">{@code &lt;activity&gt;}</a> element
as a child of the <a
href="{@docRoot}guide/topics/manifest/application-element.html">{@code &lt;application&gt;}</a>
element. For example:</p>

<pre>
&lt;manifest ... &gt;
  &lt;application ... &gt;
      &lt;activity android:name=".ExampleActivity" /&gt;
      ...
  &lt;/application ... &gt;
  ...
&lt;/manifest &gt;
</pre>

<p>There are several other attributes that you can include in this element, to define properties
such as the label for the activity, an icon for the activity, or a theme to style the activity's
UI. The <a href="{@docRoot}guide/topics/manifest/activity-element.html#nm">{@code android:name}</a>
attribute is the only required attribute&mdash;it specifies the class name of the activity. Once
you publish your application, you should not change this name, because if you do, you might break
some functionality, such as application shortcuts (read the blog post, <a
href="http://android-developers.blogspot.com/2011/06/things-that-cannot-change.html">Things
That Cannot Change</a>).</p>

<p>See the <a
href="{@docRoot}guide/topics/manifest/activity-element.html">{@code &lt;activity&gt;}</a> element
reference for more information about declaring your activity in the manifest.</p>


<h4>Using intent filters</h4>

<p>An <a href="{@docRoot}guide/topics/manifest/activity-element.html">{@code
&lt;activity&gt;}</a> element can also specify various intent filters&mdash;using the <a
href="{@docRoot}guide/topics/manifest/intent-filter-element.html">{@code
&lt;intent-filter&gt;}</a> element&mdash;in order to declare how other application components may
activate it.</p>

<p>When you create a new application using the Android SDK tools, the stub activity
that's created for you automatically includes an intent filter that declares the activity
responds to the "main" action and should be placed in the "launcher" category. The intent filter
looks like this:</p>

<pre>
&lt;activity android:name=".ExampleActivity" android:icon="@drawable/app_icon"&gt;
    &lt;intent-filter&gt;
        &lt;action android:name="android.intent.action.MAIN" /&gt;
        &lt;category android:name="android.intent.category.LAUNCHER" /&gt;
    &lt;/intent-filter&gt;
&lt;/activity&gt;
</pre>

<p>The <a href="{@docRoot}guide/topics/manifest/action-element.html">{@code
&lt;action&gt;}</a> element specifies that this is the "main" entry point to the application. The <a
href="{@docRoot}guide/topics/manifest/category-element.html">{@code
&lt;category&gt;}</a> element specifies that this activity should be listed in the
system's application launcher (to allow users to launch this activity).</p>

<p>If you intend for your application to be self-contained and not allow other applications to
activate its activities, then you don't need any other intent filters. Only one activity should
have the "main" action and "launcher" category, as in the previous example. Activities that
you don't want to make available to other applications should have no intent filters and you can
start them yourself using explicit intents (as discussed in the following section).</p>

<p>However, if you want your activity to respond to implicit intents that are delivered from
other applications (and your own), then you must define additional intent filters for your
activity. For each type of intent to which you want to respond, you must include an <a
href="{@docRoot}guide/topics/manifest/intent-filter-element.html">{@code
&lt;intent-filter&gt;}</a> that includes an
<a href="{@docRoot}guide/topics/manifest/action-element.html">{@code
&lt;action&gt;}</a> element and, optionally, a <a
href="{@docRoot}guide/topics/manifest/category-element.html">{@code
&lt;category&gt;}</a> element and/or a <a
href="{@docRoot}guide/topics/manifest/data-element.html">{@code
&lt;data&gt;}</a> element. These elements specify the type of intent to which your activity can
respond.</p>

<p>For more information about how your activities can respond to intents, see the <a
href="{@docRoot}guide/components/intents-filters.html">Intents and Intent Filters</a>
document.</p>



<h2 id="StartingAnActivity">Starting an Activity</h2>

<p>You can start another activity by calling {@link android.app.Activity#startActivity
  startActivity()}, passing it an {@link android.content.Intent} that describes the activity you
  want to start. The intent specifies either the exact activity you want to start or describes the
  type of action you want to perform (and the system selects the appropriate activity for you,
which
  can even be from a different application). An intent can also carry small amounts of data to be
  used by the activity that is started.</p>

<p>When working within your own application, you'll often need to simply launch a known activity.
 You can do so by creating an intent that explicitly defines the activity you want to start,
using the class name. For example, here's how one activity starts another activity named {@code
SignInActivity}:</p>

<pre>
Intent intent = new Intent(this, SignInActivity.class);
startActivity(intent);
</pre>

<p>However, your application might also want to perform some action, such as send an email, text
  message, or status update, using data from your activity. In this case, your application might
 not have its own activities to perform such actions, so you can instead leverage the activities
  provided by other applications on the device, which can perform the actions for you. This is where
intents are really valuable&mdash;you can create an intent that describes an action you want to
perform and the system
  launches the appropriate activity from another application. If there are
  multiple activities that can handle the intent, then the user can select which one to use. For
  example, if you want to allow the user to send an email message, you can create the
  following intent:</p>

<pre>
Intent intent = new Intent(Intent.ACTION_SEND);
intent.putExtra(Intent.EXTRA_EMAIL, recipientArray);
startActivity(intent);
</pre>

<p>The {@link android.content.Intent#EXTRA_EMAIL} extra added to the intent is a string array of
  email addresses to which the email should be sent. When an email application responds to this
  intent, it reads the string array provided in the extra and places them in the "to" field of the
  email composition form. In this situation, the email application's activity starts and when the
  user is done, your activity resumes.</p>




<h3 id="StartingAnActivityForResult">Starting an activity for a result</h3>

<p>Sometimes, you might want to receive a result from the activity that you start. In that case,
  start the activity by calling {@link android.app.Activity#startActivityForResult
  startActivityForResult()} (instead of {@link android.app.Activity#startActivity
  startActivity()}). To then receive the result from the subsequent
activity, implement the {@link android.app.Activity#onActivityResult onActivityResult()} callback
  method. When the subsequent activity is done, it returns a result in an {@link
android.content.Intent} to your {@link android.app.Activity#onActivityResult onActivityResult()}
method.</p>

<p>For example, perhaps you want the user to pick one of their contacts, so your activity can
do something with the information in that contact. Here's how you can create such an intent and
handle the result:</p>

<pre>
private void pickContact() {
    // Create an intent to "pick" a contact, as defined by the content provider URI
    Intent intent = new Intent(Intent.ACTION_PICK, Contacts.CONTENT_URI);
    startActivityForResult(intent, PICK_CONTACT_REQUEST);
}

&#64;Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    // If the request went well (OK) and the request was PICK_CONTACT_REQUEST
    if (resultCode == Activity.RESULT_OK &amp;&amp; requestCode == PICK_CONTACT_REQUEST) {
        // Perform a query to the contact's content provider for the contact's name
        Cursor cursor = getContentResolver().query(data.getData(),
        new String[] {Contacts.DISPLAY_NAME}, null, null, null);
        if (cursor.moveToFirst()) { // True if the cursor is not empty
            int columnIndex = cursor.getColumnIndex(Contacts.DISPLAY_NAME);
            String name = cursor.getString(columnIndex);
            // Do something with the selected contact's name...
        }
    }
}
</pre>

<p>This example shows the basic logic you should use in your {@link
android.app.Activity#onActivityResult onActivityResult()} method in order to handle an
activity result. The first condition checks whether the request was successful&mdash;if it was, then
the {@code resultCode} will be {@link android.app.Activity#RESULT_OK}&mdash;and whether the request
to which this result is responding is known&mdash;in this case, the {@code requestCode} matches the
second parameter sent with {@link android.app.Activity#startActivityForResult
startActivityForResult()}. From there, the code handles the activity result by querying the
data returned in an {@link android.content.Intent} (the {@code data} parameter).</p>

<p>What happens is, a {@link
android.content.ContentResolver} performs a query against a content provider, which returns a
{@link android.database.Cursor} that allows the queried data to be read. For more information, see
the <a
href="{@docRoot}guide/topics/providers/content-providers.html">Content Providers</a> document.</p>

<p>For more information about using intents, see the <a
href="{@docRoot}guide/components/intents-filters.html">Intents and Intent
Filters</a> document.</p>


<h2 id="ShuttingDown">Shutting Down an Activity</h2>

<p>You can shut down an activity by calling its {@link android.app.Activity#finish
finish()} method. You can also shut down a separate activity that you previously started by calling
{@link android.app.Activity#finishActivity finishActivity()}.</p>

<p class="note"><strong>Note:</strong> In most cases, you should not explicitly finish an activity
using these methods. As discussed in the following section about the activity lifecycle, the
Android system manages the life of an activity for you, so you do not need to finish your own
activities. Calling these methods could adversely affect the expected user
experience and should only be used when you absolutely do not want the user to return to this
instance of the activity.</p>


<h2 id="Lifecycle">Managing the Activity Lifecycle</h2>

<p>Managing the lifecycle of your activities by implementing callback methods is
crucial to developing a strong
and flexible application. The lifecycle of an activity is directly affected by its association with
other activities, its task and back stack.</p>

<p>An activity can exist in essentially three states:</p>

<dl>
  <dt><i>Resumed</i></dt>
    <dd>The activity is in the foreground of the screen and has user focus. (This state is
also sometimes referred to as "running".)</dd>

  <dt><i>Paused</i></dt>
    <dd>Another activity is in the foreground and has focus, but this one is still visible. That is,
another activity is visible on top of this one and that activity is partially transparent or doesn't
cover the entire screen. A paused activity is completely alive (the {@link android.app.Activity}
object is retained in memory, it maintains all state and member information, and remains attached to
the window manager), but can be killed by the system in extremely low memory situations.</dd>

  <dt><i>Stopped</i></dt>
    <dd>The activity is completely obscured by another activity (the activity is now in the
"background"). A stopped activity is also still alive (the {@link android.app.Activity}
object is retained in memory, it maintains all state and member information, but is <em>not</em>
attached to the window manager). However, it is no longer visible to the user and it
can be killed by the system when memory is needed elsewhere.</dd>
</dl>

<p>If an activity is paused or stopped, the system can drop it from memory either by asking it to
finish (calling its {@link android.app.Activity#finish finish()} method), or simply killing its
process.  When the activity is opened again (after being finished or killed), it must be created all
over.</p>



<h3 id="ImplementingLifecycleCallbacks">Implementing the lifecycle callbacks</h3>

<p>When an activity transitions into and out of the different states described above, it is notified
through various callback methods. All of the callback methods are hooks that you
can override to do appropriate work when the state of your activity changes. The following skeleton
activity includes each of the fundamental lifecycle methods:</p>


<pre>
public class ExampleActivity extends Activity {
    &#64;Override
    public void {@link android.app.Activity#onCreate onCreate}(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // The activity is being created.
    }
    &#64;Override
    protected void {@link android.app.Activity#onStart onStart()} {
        super.onStart();
        // The activity is about to become visible.
    }
    &#64;Override
    protected void {@link android.app.Activity#onResume onResume()} {
        super.onResume();
        // The activity has become visible (it is now "resumed").
    }
    &#64;Override
    protected void {@link android.app.Activity#onPause onPause()} {
        super.onPause();
        // Another activity is taking focus (this activity is about to be "paused").
    }
    &#64;Override
    protected void {@link android.app.Activity#onStop onStop()} {
        super.onStop();
        // The activity is no longer visible (it is now "stopped")
    }
    &#64;Override
    protected void {@link android.app.Activity#onDestroy onDestroy()} {
        super.onDestroy();
        // The activity is about to be destroyed.
    }
}
</pre>

<p class="note"><strong>Note:</strong> Your implementation of these lifecycle methods must
always call the superclass implementation before doing any work, as shown in the examples above.</p>

<p>Taken together, these methods define the entire lifecycle of an activity. By implementing these
methods, you can monitor three nested loops in the activity lifecycle: </p>

<ul>
<li>The <b>entire lifetime</b> of an activity happens between the call to {@link
android.app.Activity#onCreate onCreate()} and the call to {@link
android.app.Activity#onDestroy}. Your activity should perform setup of
"global" state (such as defining layout) in {@link android.app.Activity#onCreate onCreate()}, and
release all remaining resources in {@link android.app.Activity#onDestroy}. For example, if your
activity has a thread running in the background to download data from the network, it might create
that thread in {@link android.app.Activity#onCreate onCreate()} and then stop the thread in {@link
android.app.Activity#onDestroy}.</li>

<li><p>The <b>visible lifetime</b> of an activity happens between the call to {@link
android.app.Activity#onStart onStart()} and the call to {@link
android.app.Activity#onStop onStop()}. During this time, the user can see the activity
on-screen and interact with it. For example, {@link android.app.Activity#onStop onStop()} is called
when a new activity starts and this one is no longer visible. Between these two methods, you can
maintain resources that are needed to show the activity to the user. For example, you can register a
{@link android.content.BroadcastReceiver} in {@link
android.app.Activity#onStart onStart()} to monitor changes that impact your UI, and unregister
it in {@link android.app.Activity#onStop onStop()} when the user can no longer see what you are
displaying. The system might call {@link android.app.Activity#onStart onStart()} and {@link
android.app.Activity#onStop onStop()} multiple times during the entire lifetime of the activity, as
the activity alternates between being visible and hidden to the user.</p></li>

<li><p>The <b>foreground lifetime</b> of an activity happens between the call to {@link
android.app.Activity#onResume onResume()} and the call to {@link android.app.Activity#onPause
onPause()}. During this time, the activity is in front of all other activities on screen and has
user input focus.  An activity can frequently transition in and out of the foreground&mdash;for
example, {@link android.app.Activity#onPause onPause()} is called when the device goes to sleep or
when a dialog appears. Because this state can transition often, the code in these two methods should
be fairly lightweight in order to avoid slow transitions that make the user wait.</p></li>
</ul>

<p>Figure 1 illustrates these loops and the paths an activity might take between states.
The rectangles represent the callback methods you can implement to perform operations when
the activity transitions between states. <p>

<img src="{@docRoot}images/activity_lifecycle.png" alt="" />
<p class="img-caption"><strong>Figure 1.</strong> The activity lifecycle.</p>

<p>The same lifecycle callback methods are listed in table 1, which describes each of the callback
methods in more detail and locates each one within the
activity's overall lifecycle, including whether the system can kill the activity after the
callback method completes.</p>

<p class="table-caption"><strong>Table 1.</strong> A summary of the activity lifecycle's
callback methods.</p>

<table border="2" width="85%" frame="hsides" rules="rows">
<colgroup align="left" span="3"></colgroup>
<colgroup align="left"></colgroup>
<colgroup align="center"></colgroup>
<colgroup align="center"></colgroup>

<thead>
<tr><th colspan="3">Method</th> <th>Description</th> <th>Killable after?</th> <th>Next</th></tr>
</thead>

<tbody>
<tr>
  <td colspan="3" align="left"><code>{@link android.app.Activity#onCreate onCreate()}</code></td>
  <td>Called when the activity is first created.
      This is where you should do all of your normal static set up &mdash;
      create views, bind data to lists, and so on.  This method is passed
      a Bundle object containing the activity's previous state, if that
      state was captured (see <a href="#actstate">Saving Activity State</a>,
      later).
      <p>Always followed by {@code onStart()}.</p></td>
  <td align="center">No</td>
      <td align="center">{@code onStart()}</td>
</tr>

<tr>
   <td rowspan="5" style="border-left: none; border-right: none;">&nbsp;&nbsp;&nbsp;&nbsp;</td>
   <td colspan="2" align="left"><code>{@link android.app.Activity#onRestart
onRestart()}</code></td>
   <td>Called after the activity has been stopped, just prior to it being
       started again.
       <p>Always followed by {@code onStart()}</p></td>
   <td align="center">No</td>
   <td align="center">{@code onStart()}</td>
</tr>

<tr>
   <td colspan="2" align="left"><code>{@link android.app.Activity#onStart onStart()}</code></td>
   <td>Called just before the activity becomes visible to the user.
       <p>Followed by {@code onResume()} if the activity comes
       to the foreground, or {@code onStop()} if it becomes hidden.</p></td>
    <td align="center">No</td>
    <td align="center">{@code onResume()} <br/>or<br/> {@code onStop()}</td>
</tr>

<tr>
   <td rowspan="2" style="border-left: none;">&nbsp;&nbsp;&nbsp;&nbsp;</td>
   <td align="left"><code>{@link android.app.Activity#onResume onResume()}</code></td>
   <td>Called just before the activity starts
       interacting with the user.  At this point the activity is at
       the top of the activity stack, with user input going to it.
       <p>Always followed by {@code onPause()}.</p></td>
   <td align="center">No</td>
   <td align="center">{@code onPause()}</td>
</tr>

<tr>
   <td align="left"><code>{@link android.app.Activity#onPause onPause()}</code></td>
   <td>Called when the system is about to start resuming another
       activity.  This method is typically used to commit unsaved changes to
       persistent data, stop animations and other things that may be consuming
       CPU, and so on.  It should do whatever it does very quickly, because
       the next activity will not be resumed until it returns.
       <p>Followed either by {@code onResume()} if the activity
       returns back to the front, or by {@code onStop()} if it becomes
       invisible to the user.</td>
   <td align="center"><strong style="color:#800000">Yes</strong></td>
   <td align="center">{@code onResume()} <br/>or<br/> {@code onStop()}</td>
</tr>

<tr>
   <td colspan="2" align="left"><code>{@link android.app.Activity#onStop onStop()}</code></td>
   <td>Called when the activity is no longer visible to the user.  This
       may happen because it is being destroyed, or because another activity
       (either an existing one or a new one) has been resumed and is covering it.
       <p>Followed either by {@code onRestart()} if
       the activity is coming back to interact with the user, or by
       {@code onDestroy()} if this activity is going away.</p></td>
   <td align="center"><strong style="color:#800000">Yes</strong></td>
   <td align="center">{@code onRestart()} <br/>or<br/> {@code onDestroy()}</td>
</tr>

<tr>
   <td colspan="3" align="left"><code>{@link android.app.Activity#onDestroy
onDestroy()}</code></td>
   <td>Called before the activity is destroyed.  This is the final call
       that the activity will receive.  It could be called either because the
       activity is finishing (someone called <code>{@link android.app.Activity#finish
       finish()}</code> on it), or because the system is temporarily destroying this
       instance of the activity to save space.  You can distinguish
       between these two scenarios with the <code>{@link
       android.app.Activity#isFinishing isFinishing()}</code> method.</td>
   <td align="center"><strong style="color:#800000">Yes</strong></td>
   <td align="center"><em>nothing</em></td>
</tr>
</tbody>
</table>

<p>The column labeled "Killable after?" indicates whether or not the system can
kill the process hosting the activity at any time <em>after the method returns</em>, without
executing another line of the activity's code.  Three methods are marked "yes": ({@link
android.app.Activity#onPause
onPause()}, {@link android.app.Activity#onStop onStop()}, and {@link android.app.Activity#onDestroy
onDestroy()}). Because {@link android.app.Activity#onPause onPause()} is the first
of the three, once the activity is created, {@link android.app.Activity#onPause onPause()} is the
last method that's guaranteed to be called before the process <em>can</em> be killed&mdash;if
the system must recover memory in an emergency, then {@link
android.app.Activity#onStop onStop()} and {@link android.app.Activity#onDestroy onDestroy()} might
not be called. Therefore, you should use {@link android.app.Activity#onPause onPause()} to write
crucial persistent data (such as user edits) to storage. However, you should be selective about
what information must be retained during {@link android.app.Activity#onPause onPause()}, because any
blocking procedures in this method block the transition to the next activity and slow the user
experience.</p>

<p> Methods that are marked "No" in the <b>Killable</b> column protect the process hosting the
activity from being killed from the moment they are called.  Thus, an activity is killable
from the time {@link android.app.Activity#onPause onPause()} returns to the time
{@link android.app.Activity#onResume onResume()} is called. It will not again be killable until
{@link android.app.Activity#onPause onPause()} is again called and returns. </p>

<p class="note"><strong>Note:</strong> An activity that's not technically "killable" by this
definition in table 1 might still be killed by the system&mdash;but that would happen only in
extreme circumstances when there is no other recourse. When an activity might be killed is
discussed more in the <a
href="{@docRoot}guide/components/processes-and-threads.html">Processes and
Threading</a> document.</p>


<h3 id="SavingActivityState">Saving activity state</h3>

<p>The introduction to <a href="#Lifecycle">Managing the Activity Lifecycle</a> briefly mentions
that
when an activity is paused or stopped, the state of the activity is retained. This is true because
the {@link android.app.Activity} object is still held in memory when it is paused or
stopped&mdash;all information about its members and current state is still alive. Thus, any changes
the user made within the activity are retained so that when the activity returns to the
foreground (when it "resumes"), those changes are still there.</p>

<p>However, when the system destroys an activity in order to recover memory, the {@link
android.app.Activity} object is destroyed, so the system cannot simply resume it with its state
intact. Instead, the system must recreate the {@link android.app.Activity} object if the user
navigates back to it. Yet, the user is unaware
that the system destroyed the activity and recreated it and, thus, probably
expects the activity to be exactly as it was. In this situation, you can ensure that
important information about the activity state is preserved by implementing an additional
callback method that allows you to save information about the state of your activity: {@link
android.app.Activity#onSaveInstanceState onSaveInstanceState()}.</p>

<p>The system calls {@link android.app.Activity#onSaveInstanceState onSaveInstanceState()}
before making the activity vulnerable to destruction. The system passes this method
a {@link android.os.Bundle} in which you can save 
state information about the activity as name-value pairs, using methods such as {@link
android.os.Bundle#putString putString()} and {@link
android.os.Bundle#putInt putInt()}. Then, if the system kills your application
process and the user navigates back to your activity, the system recreates the activity and passes
the {@link android.os.Bundle} to both {@link android.app.Activity#onCreate onCreate()} and {@link
android.app.Activity#onRestoreInstanceState onRestoreInstanceState()}. Using either of these
methods, you can extract your saved state from the {@link android.os.Bundle} and restore the
activity state. If there is no state information to restore, then the {@link
android.os.Bundle} passed to you is null (which is the case when the activity is created for
the first time).</p>

<img src="{@docRoot}images/fundamentals/restore_instance.png" alt="" />
<p class="img-caption"><strong>Figure 2.</strong> The two ways in which an activity returns to user
focus with its state intact: either the activity is destroyed, then recreated and the activity must restore
the previously saved state, or the activity is stopped, then resumed and the activity state
remains intact.</p>

<p class="note"><strong>Note:</strong> There's no guarantee that {@link
android.app.Activity#onSaveInstanceState onSaveInstanceState()} will be called before your
activity is destroyed, because there are cases in which it won't be necessary to save the state
(such as when the user leaves your activity using the <em>Back</em> button, because the user is
explicitly
closing the activity). If the system calls {@link android.app.Activity#onSaveInstanceState
onSaveInstanceState()}, it does so before {@link
android.app.Activity#onStop onStop()} and possibly before {@link android.app.Activity#onPause
onPause()}.</p>

<p>However, even if you do nothing and do not implement {@link
android.app.Activity#onSaveInstanceState onSaveInstanceState()}, some of the activity state is
restored by the {@link android.app.Activity} class's default implementation of {@link
android.app.Activity#onSaveInstanceState onSaveInstanceState()}. Specifically, the default
implementation calls the corresponding {@link
android.view.View#onSaveInstanceState onSaveInstanceState()} method for every {@link
android.view.View} in the layout, which allows each view to provide information about itself
that should be saved. Almost every widget in the Android framework implements this method as
appropriate, such that any visible changes to the UI are automatically saved and restored when your
activity is recreated. For example, the {@link android.widget.EditText} widget saves any text
entered by the user and the {@link android.widget.CheckBox} widget saves whether it's checked or
not. The only work required by you is to provide a unique ID (with the <a
href="{@docRoot}guide/topics/resources/layout-resource.html#idvalue">{@code android:id}</a>
attribute) for each widget you want to save its state. If a widget does not have an ID, then the
system cannot save its state.</p>

<div class="sidebox-wrapper">
<div class="sidebox">
<p>You can also explicitly stop a view in your layout from saving its state by setting the
{@link android.R.attr#saveEnabled android:saveEnabled} attribute to {@code "false"} or by calling
the {@link android.view.View#setSaveEnabled setSaveEnabled()} method. Usually, you should not
disable this, but you might if you want to restore the state of the activity UI differently.</p>
</div>
</div>

<p>Although the default implementation of {@link
android.app.Activity#onSaveInstanceState onSaveInstanceState()} saves useful information about
your activity's UI, you still might need to override it to save additional information.
For example, you might need to save member values that changed during the activity's life (which
might correlate to values restored in the UI, but the members that hold those UI values are not
restored, by default).</p>

<p>Because the default implementation of {@link
android.app.Activity#onSaveInstanceState onSaveInstanceState()} helps save the state of the UI, if
you override the method in order to save additional state information, you should always call the
superclass implementation of {@link android.app.Activity#onSaveInstanceState onSaveInstanceState()}
before doing any work. Likewise, you should also call the superclass implementation of {@link
android.app.Activity#onRestoreInstanceState onRestoreInstanceState()} if you override it, so the
default implementation can restore view states.</p>

<p class="note"><strong>Note:</strong> Because {@link android.app.Activity#onSaveInstanceState
onSaveInstanceState()} is not guaranteed
to be called, you should use it only to record the transient state of the activity (the state of
the UI)&mdash;you should never use it to store persistent data.  Instead, you should use  {@link
android.app.Activity#onPause onPause()} to store persistent data (such as data that should be saved
to a database) when the user leaves the activity.</p>

<p>A good way to test your application's ability to restore its state is to simply rotate the
device so that the screen orientation changes. When the screen orientation changes, the system
destroys and recreates the activity in order to apply alternative resources that might be available
for the new screen configuration. For this reason alone, it's very important that your activity
completely restores its state when it is recreated, because users regularly rotate the screen while
using applications.</p>


<h3 id="ConfigurationChanges">Handling configuration changes</h3>

<p>Some device configurations can change during runtime (such as screen orientation, keyboard
availability, and language). When such a change occurs, Android recreates the running activity
(the system calls {@link android.app.Activity#onDestroy}, then immediately calls {@link
android.app.Activity#onCreate onCreate()}). This behavior is
designed to help your application adapt to new configurations by automatically reloading your
application with alternative resources that you've provided (such as different layouts for
different screen orientations and sizes).</p>

<p>If you properly design your activity to handle a restart due to a screen orientation change and
restore the activity state as described above, your application will be more resilient to other
unexpected events in the activity lifecycle.</p>

<p>The best way to handle such a restart is
  to save and restore the state of your activity using {@link
  android.app.Activity#onSaveInstanceState onSaveInstanceState()} and {@link
android.app.Activity#onRestoreInstanceState onRestoreInstanceState()} (or {@link
android.app.Activity#onCreate onCreate()}), as discussed in the previous section.</p>

<p>For more information about configuration changes that happen at runtime and how you can handle
them, read the guide to <a href="{@docRoot}guide/topics/resources/runtime-changes.html">Handling
Runtime Changes</a>.</p>



<h3 id="CoordinatingActivities">Coordinating activities</h3>

 <p>When one activity starts another, they both experience lifecycle transitions. The first activity
pauses and stops (though, it won't stop if it's still visible in the background), while the other
activity is created. In case these activities share data saved to disc or elsewhere, it's important
to understand that the first activity is not completely stopped before the second one is created.
Rather, the process of starting the second one overlaps with the process of stopping the first
one.</p>

<p>The order of lifecycle callbacks is well defined, particularly when the two activities are in the
same process and one is starting the other. Here's the order of operations that occur when Activity
A starts Acivity B: </p>

<ol>
<li>Activity A's {@link android.app.Activity#onPause onPause()} method executes.</li>

<li>Activity B's {@link android.app.Activity#onCreate onCreate()}, {@link
android.app.Activity#onStart onStart()}, and {@link android.app.Activity#onResume onResume()}
methods execute in sequence. (Activity B now has user focus.)</li>

<li>Then, if Activity A is no longer visible on screen, its {@link
android.app.Activity#onStop onStop()} method executes.</li>
</ol>

 <p>This predictable sequence of lifecycle callbacks allows you to manage the transition of
information from one activity to another. For example, if you must write to a database when the
first activity stops so that the following activity can read it, then you should write to the
database during {@link android.app.Activity#onPause onPause()} instead of during {@link
android.app.Activity#onStop onStop()}.</p>

<!--
<h2>Beginner's Path</h2>

<p>For more information about how Android maintains a history of activities and
enables user multitasking, continue with the <b><a
href="{@docRoot}guide/components/tasks-and-back-stack.html">Tasks and Back
Stack</a></b> document.</p>
-->