Figure 2. The lifecycle of a fragment (while its activity is running).
A {@link android.app.Fragment} represents a behavior or a portion of user interface in an {@link android.app.Activity}. You can combine multiple fragments in a single activity to build a multi-pane UI and reuse a fragment in multiple activities. You can think of a fragment as a modular section of an activity, which has its own lifecycle, receives its own input events, and which you can add or remove while the activity is running (sort of like a "sub activity" that you can reuse in different activities).
A fragment must always be embedded in an activity and the fragment's lifecycle is directly affected by the host activity's lifecycle. For example, when the activity is paused, so are all fragments in it, and when the activity is destroyed, so are all fragments. However, while an activity is running (it is in the resumed lifecycle state), you can manipulate each fragment independently, such as add or remove them. When you perform such a fragment transaction, you can also add it to a back stack that's managed by the activity—each back stack entry in the activity is a record of the fragment transaction that occurred. The back stack allows the user to reverse a fragment transaction (navigate backwards), by pressing the Back button.
When you add a fragment as a part of your activity layout, it lives in a {@link android.view.ViewGroup} inside the activity's view hierarchy and the fragment defines its own view layout. You can insert a fragment into your activity layout by declaring the fragment in the activity's layout file, as a {@code <fragment>} element, or from your application code by adding it to an existing {@link android.view.ViewGroup}. However, a fragment is not required to be a part of the activity layout; you may also use a fragment without its own UI as an invisible worker for the activity.
This document describes how to build your application to use fragments, including how fragments can maintain their state when added to the activity's back stack, share events with the activity and other fragments in the activity, contribute to the activity's action bar, and more.
Android introduced fragments in Android 3.0 (API level 11), primarily to support more dynamic and flexible UI designs on large screens, such as tablets. Because a tablet's screen is much larger than that of a handset, there's more room to combine and interchange UI components. Fragments allow such designs without the need for you to manage complex changes to the view hierarchy. By dividing the layout of an activity into fragments, you become able to modify the activity's appearance at runtime and preserve those changes in a back stack that's managed by the activity.
For example, a news application can use one fragment to show a list of articles on the left and another fragment to display an article on the right—both fragments appear in one activity, side by side, and each fragment has its own set of lifecycle callback methods and handle their own user input events. Thus, instead of using one activity to select an article and another activity to read the article, the user can select an article and read it all within the same activity, as illustrated in the tablet layout in figure 1.
You should design each fragment as a modular and reusable activity component. That is, because each fragment defines its own layout and its own behavior with its own lifecycle callbacks, you can include one fragment in multiple activities, so you should design for reuse and avoid directly manipulating one fragment from another fragment. This is especially important because a modular fragment allows you to change your fragment combinations for different screen sizes. When designing your application to support both tablets and handsets, you can reuse your fragments in different layout configurations to optimize the user experience based on the available screen space. For example, on a handset, it might be necessary to separate fragments to provide a single-pane UI when more than one cannot fit within the same activity.
Figure 1. An example of how two UI modules defined by fragments can be combined into one activity for a tablet design, but separated for a handset design.
For example—to continue with the news application example—the application can embed two fragments in Activity A, when running on a tablet-sized device. However, on a handset-sized screen, there's not enough room for both fragments, so Activity A includes only the fragment for the list of articles, and when the user selects an article, it starts Activity B, which includes the second fragment to read the article. Thus, the application supports both tablets and handsets by reusing fragments in different combinations, as illustrated in figure 1.
For more information about designing your application with different fragment combinations for different screen configurations, see the guide to Supporting Tablets and Handsets.
Figure 2. The lifecycle of a fragment (while its activity is running).
To create a fragment, you must create a subclass of {@link android.app.Fragment} (or an existing subclass of it). The {@link android.app.Fragment} class has code that looks a lot like an {@link android.app.Activity}. It contains callback methods similar to an activity, such as {@link android.app.Fragment#onCreate onCreate()}, {@link android.app.Fragment#onStart onStart()}, {@link android.app.Fragment#onPause onPause()}, and {@link android.app.Fragment#onStop onStop()}. In fact, if you're converting an existing Android application to use fragments, you might simply move code from your activity's callback methods into the respective callback methods of your fragment.
Usually, you should implement at least the following lifecycle methods:
Most applications should implement at least these three methods for every fragment, but there are several other callback methods you should also use to handle various stages of the fragment lifecycle. All the lifecycle callback methods are discussed more later, in the section about Handling the Fragment Lifecycle.
There are also a few subclasses that you might want to extend, instead of the base {@link android.app.Fragment} class:
A fragment is usually used as part of an activity's user interface and contributes its own layout to the activity.
To provide a layout for a fragment, you must implement the {@link android.app.Fragment#onCreateView onCreateView()} callback method, which the Android system calls when it's time for the fragment to draw its layout. Your implementation of this method must return a {@link android.view.View} that is the root of your fragment's layout.
Note: If your fragment is a subclass of {@link android.app.ListFragment}, the default implementation returns a {@link android.widget.ListView} from {@link android.app.Fragment#onCreateView onCreateView()}, so you don't need to implement it.
To return a layout from {@link android.app.Fragment#onCreateView onCreateView()}, you can inflate it from a layout resource defined in XML. To help you do so, {@link android.app.Fragment#onCreateView onCreateView()} provides a {@link android.view.LayoutInflater} object.
For example, here's a subclass of {@link android.app.Fragment} that loads a layout from the {@code example_fragment.xml} file:
public static class ExampleFragment extends Fragment {
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
return inflater.inflate(R.layout.example_fragment, container, false);
}
}
In the sample above, {@code R.layout.example_fragment} is a reference to a layout resource named {@code example_fragment.xml} saved in the application resources. For information about how to create a layout in XML, see the User Interface documentation.
The {@code container} parameter passed to {@link android.app.Fragment#onCreateView onCreateView()} is the parent {@link android.view.ViewGroup} (from the activity's layout) in which your fragment layout will be inserted. The {@code savedInstanceState} parameter is a {@link android.os.Bundle} that provides data about the previous instance of the fragment, if the fragment is being resumed (restoring state is discussed more in the section about Handling the Fragment Lifecycle).
The {@link android.view.LayoutInflater#inflate(int,ViewGroup,boolean) inflate()} method takes three arguments:
Now you've seen how to create a fragment that provides a layout. Next, you need to add the fragment to your activity.
Usually, a fragment contributes a portion of UI to the host activity, which is embedded as a part of the activity's overall view hierarchy. There are two ways you can add a fragment to the activity layout:
In this case, you can specify layout properties for the fragment as if it were a view. For example, here's the layout file for an activity with two fragments:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="horizontal"
android:layout_width="match_parent"
android:layout_height="match_parent">
<fragment android:name="com.example.news.ArticleListFragment"
android:id="@+id/list"
android:layout_weight="1"
android:layout_width="0dp"
android:layout_height="match_parent" />
<fragment android:name="com.example.news.ArticleReaderFragment"
android:id="@+id/viewer"
android:layout_weight="2"
android:layout_width="0dp"
android:layout_height="match_parent" />
</LinearLayout>
The {@code android:name} attribute in the {@code <fragment>} specifies the {@link android.app.Fragment} class to instantiate in the layout.
When the system creates this activity layout, it instantiates each fragment specified in the layout and calls the {@link android.app.Fragment#onCreateView onCreateView()} method for each one, to retrieve each fragment's layout. The system inserts the {@link android.view.View} returned by the fragment directly in place of the {@code <fragment>} element.
Note: Each fragment requires a unique identifier that the system can use to restore the fragment if the activity is restarted (and which you can use to capture the fragment to perform transactions, such as remove it). There are three ways to provide an ID for a fragment:
At any time while your activity is running, you can add fragments to your activity layout. You simply need to specify a {@link android.view.ViewGroup} in which to place the fragment.
To make fragment transactions in your activity (such as add, remove, or replace a fragment), you must use APIs from {@link android.app.FragmentTransaction}. You can get an instance of {@link android.app.FragmentTransaction} from your {@link android.app.Activity} like this:
FragmentManager fragmentManager = {@link android.app.Activity#getFragmentManager()}
FragmentTransaction fragmentTransaction = fragmentManager.{@link android.app.FragmentManager#beginTransaction()};
You can then add a fragment using the {@link android.app.FragmentTransaction#add(int,Fragment) add()} method, specifying the fragment to add and the view in which to insert it. For example:
ExampleFragment fragment = new ExampleFragment(); fragmentTransaction.add(R.id.fragment_container, fragment); fragmentTransaction.commit();
The first argument passed to {@link android.app.FragmentTransaction#add(int,Fragment) add()} is the {@link android.view.ViewGroup} in which the fragment should be placed, specified by resource ID, and the second parameter is the fragment to add.
Once you've made your changes with {@link android.app.FragmentTransaction}, you must call {@link android.app.FragmentTransaction#commit} for the changes to take effect.
The examples above show how to add a fragment to your activity in order to provide a UI. However, you can also use a fragment to provide a background behavior for the activity without presenting additional UI.
To add a fragment without a UI, add the fragment from the activity using {@link android.app.FragmentTransaction#add(Fragment,String)} (supplying a unique string "tag" for the fragment, rather than a view ID). This adds the fragment, but, because it's not associated with a view in the activity layout, it does not receive a call to {@link android.app.Fragment#onCreateView onCreateView()}. So you don't need to implement that method.
Supplying a string tag for the fragment isn't strictly for non-UI fragments—you can also supply string tags to fragments that do have a UI—but if the fragment does not have a UI, then the string tag is the only way to identify it. If you want to get the fragment from the activity later, you need to use {@link android.app.FragmentManager#findFragmentByTag findFragmentByTag()}.
For an example activity that uses a fragment as a background worker, without a UI, see the {@code FragmentRetainInstance.java} sample.
To manage the fragments in your activity, you need to use {@link android.app.FragmentManager}. To get it, call {@link android.app.Activity#getFragmentManager()} from your activity.
Some things that you can do with {@link android.app.FragmentManager} include:
For more information about these methods and others, refer to the {@link android.app.FragmentManager} class documentation.
As demonstrated in the previous section, you can also use {@link android.app.FragmentManager} to open a {@link android.app.FragmentTransaction}, which allows you to perform transactions, such as add and remove fragments.
A great feature about using fragments in your activity is the ability to add, remove, replace, and perform other actions with them, in response to user interaction. Each set of changes that you commit to the activity is called a transaction and you can perform one using APIs in {@link android.app.FragmentTransaction}. You can also save each transaction to a back stack managed by the activity, allowing the user to navigate backward through the fragment changes (similar to navigating backward through activities).
You can acquire an instance of {@link android.app.FragmentTransaction} from the {@link android.app.FragmentManager} like this:
FragmentManager fragmentManager = {@link android.app.Activity#getFragmentManager()};
FragmentTransaction fragmentTransaction = fragmentManager.{@link android.app.FragmentManager#beginTransaction()};
Each transaction is a set of changes that you want to perform at the same time. You can set up all the changes you want to perform for a given transaction using methods such as {@link android.app.FragmentTransaction#add add()}, {@link android.app.FragmentTransaction#remove remove()}, and {@link android.app.FragmentTransaction#replace replace()}. Then, to apply the transaction to the activity, you must call {@link android.app.FragmentTransaction#commit()}.
Before you call {@link android.app.FragmentTransaction#commit()}, however, you might want to call {@link android.app.FragmentTransaction#addToBackStack addToBackStack()}, in order to add the transaction to a back stack of fragment transactions. This back stack is managed by the activity and allows the user to return to the previous fragment state, by pressing the Back button.
For example, here's how you can replace one fragment with another, and preserve the previous state in the back stack:
// Create new fragment and transaction Fragment newFragment = new ExampleFragment(); FragmentTransaction transaction = getFragmentManager().beginTransaction(); // Replace whatever is in the fragment_container view with this fragment, // and add the transaction to the back stack transaction.replace(R.id.fragment_container, newFragment); transaction.addToBackStack(null); // Commit the transaction transaction.commit();
In this example, {@code newFragment} replaces whatever fragment (if any) is currently in the layout container identified by the {@code R.id.fragment_container} ID. By calling {@link android.app.FragmentTransaction#addToBackStack addToBackStack()}, the replace transaction is saved to the back stack so the user can reverse the transaction and bring back the previous fragment by pressing the Back button.
If you add multiple changes to the transaction (such as another {@link android.app.FragmentTransaction#add add()} or {@link android.app.FragmentTransaction#remove remove()}) and call {@link android.app.FragmentTransaction#addToBackStack addToBackStack()}, then all changes applied before you call {@link android.app.FragmentTransaction#commit commit()} are added to the back stack as a single transaction and the Back button will reverse them all together.
The order in which you add changes to a {@link android.app.FragmentTransaction} doesn't matter, except:
If you do not call {@link android.app.FragmentTransaction#addToBackStack(String) addToBackStack()} when you perform a transaction that removes a fragment, then that fragment is destroyed when the transaction is committed and the user cannot navigate back to it. Whereas, if you do call {@link android.app.FragmentTransaction#addToBackStack(String) addToBackStack()} when removing a fragment, then the fragment is stopped and will be resumed if the user navigates back.
Tip: For each fragment transaction, you can apply a transition animation, by calling {@link android.app.FragmentTransaction#setTransition setTransition()} before you commit.
Calling {@link android.app.FragmentTransaction#commit()} does not perform the transaction immediately. Rather, it schedules it to run on the activity's UI thread (the "main" thread) as soon as the thread is able to do so. If necessary, however, you may call {@link android.app.FragmentManager#executePendingTransactions()} from your UI thread to immediately execute transactions submitted by {@link android.app.FragmentTransaction#commit()}. Doing so is usually not necessary unless the transaction is a dependency for jobs in other threads.
Caution: You can commit a transaction using {@link android.app.FragmentTransaction#commit commit()} only prior to the activity saving its state (when the user leaves the activity). If you attempt to commit after that point, an exception will be thrown. This is because the state after the commit can be lost if the activity needs to be restored. For situations in which its okay that you lose the commit, use {@link android.app.FragmentTransaction#commitAllowingStateLoss()}.
Although a {@link android.app.Fragment} is implemented as an object that's independent from an {@link android.app.Activity} and can be used inside multiple activities, a given instance of a fragment is directly tied to the activity that contains it.
Specifically, the fragment can access the {@link android.app.Activity} instance with {@link android.app.Fragment#getActivity()} and easily perform tasks such as find a view in the activity layout:
View listView = {@link android.app.Fragment#getActivity()}.{@link android.app.Activity#findViewById findViewById}(R.id.list);
Likewise, your activity can call methods in the fragment by acquiring a reference to the {@link android.app.Fragment} from {@link android.app.FragmentManager}, using {@link android.app.FragmentManager#findFragmentById findFragmentById()} or {@link android.app.FragmentManager#findFragmentByTag findFragmentByTag()}. For example:
ExampleFragment fragment = (ExampleFragment) getFragmentManager().findFragmentById(R.id.example_fragment);
In some cases, you might need a fragment to share events with the activity. A good way to do that is to define a callback interface inside the fragment and require that the host activity implement it. When the activity receives a callback through the interface, it can share the information with other fragments in the layout as necessary.
For example, if a news application has two fragments in an activity—one to show a list of articles (fragment A) and another to display an article (fragment B)—then fragment A must tell the activity when a list item is selected so that it can tell fragment B to display the article. In this case, the {@code OnArticleSelectedListener} interface is declared inside fragment A:
public static class FragmentA extends ListFragment {
...
// Container Activity must implement this interface
public interface OnArticleSelectedListener {
public void onArticleSelected(Uri articleUri);
}
...
}
Then the activity that hosts the fragment implements the {@code OnArticleSelectedListener} interface and overrides {@code onArticleSelected()} to notify fragment B of the event from fragment A. To ensure that the host activity implements this interface, fragment A's {@link android.app.Fragment#onAttach onAttach()} callback method (which the system calls when adding the fragment to the activity) instantiates an instance of {@code OnArticleSelectedListener} by casting the {@link android.app.Activity} that is passed into {@link android.app.Fragment#onAttach onAttach()}:
public static class FragmentA extends ListFragment {
OnArticleSelectedListener mListener;
...
@Override
public void onAttach(Activity activity) {
super.onAttach(activity);
try {
mListener = (OnArticleSelectedListener) activity;
} catch (ClassCastException e) {
throw new ClassCastException(activity.toString() + " must implement OnArticleSelectedListener");
}
}
...
}
If the activity has not implemented the interface, then the fragment throws a {@link java.lang.ClassCastException}. On success, the {@code mListener} member holds a reference to activity's implementation of {@code OnArticleSelectedListener}, so that fragment A can share events with the activity by calling methods defined by the {@code OnArticleSelectedListener} interface. For example, if fragment A is an extension of {@link android.app.ListFragment}, each time the user clicks a list item, the system calls {@link android.app.ListFragment#onListItemClick onListItemClick()} in the fragment, which then calls {@code onArticleSelected()} to share the event with the activity:
public static class FragmentA extends ListFragment {
OnArticleSelectedListener mListener;
...
@Override
public void onListItemClick(ListView l, View v, int position, long id) {
// Append the clicked item's row ID with the content provider Uri
Uri noteUri = ContentUris.{@link android.content.ContentUris#withAppendedId withAppendedId}(ArticleColumns.CONTENT_URI, id);
// Send the event and Uri to the host activity
mListener.onArticleSelected(noteUri);
}
...
}
The {@code id} parameter passed to {@link android.app.ListFragment#onListItemClick onListItemClick()} is the row ID of the clicked item, which the activity (or other fragment) uses to fetch the article from the application's {@link android.content.ContentProvider}.
More information about using a content provider is available in the Content Providers document.
Your fragments can contribute menu items to the activity's Options Menu (and, consequently, the Action Bar) by implementing {@link android.app.Fragment#onCreateOptionsMenu(Menu,MenuInflater) onCreateOptionsMenu()}. In order for this method to receive calls, however, you must call {@link android.app.Fragment#setHasOptionsMenu(boolean) setHasOptionsMenu()} during {@link android.app.Fragment#onCreate(Bundle) onCreate()}, to indicate that the fragment would like to add items to the Options Menu (otherwise, the fragment will not receive a call to {@link android.app.Fragment#onCreateOptionsMenu onCreateOptionsMenu()}).
Any items that you then add to the Options Menu from the fragment are appended to the existing menu items. The fragment also receives callbacks to {@link android.app.Fragment#onOptionsItemSelected(MenuItem) onOptionsItemSelected()} when a menu item is selected.
You can also register a view in your fragment layout to provide a context menu by calling {@link android.app.Fragment#registerForContextMenu(View) registerForContextMenu()}. When the user opens the context menu, the fragment receives a call to {@link android.app.Fragment#onCreateContextMenu(ContextMenu,View,ContextMenu.ContextMenuInfo) onCreateContextMenu()}. When the user selects an item, the fragment receives a call to {@link android.app.Fragment#onContextItemSelected(MenuItem) onContextItemSelected()}.
Note: Although your fragment receives an on-item-selected callback for each menu item it adds, the activity is first to receive the respective callback when the user selects a menu item. If the activity's implementation of the on-item-selected callback does not handle the selected item, then the event is passed to the fragment's callback. This is true for the Options Menu and context menus.
For more information about menus, see the Menus and Action Bar developer guides.
Figure 3. The effect of the activity lifecycle on the fragment lifecycle.
Managing the lifecycle of a fragment is a lot like managing the lifecycle of an activity. Like an activity, a fragment can exist in three states:
Also like an activity, you can retain the state of a fragment using a {@link android.os.Bundle}, in case the activity's process is killed and you need to restore the fragment state when the activity is recreated. You can save the state during the fragment's {@link android.app.Fragment#onSaveInstanceState onSaveInstanceState()} callback and restore it during either {@link android.app.Fragment#onCreate onCreate()}, {@link android.app.Fragment#onCreateView onCreateView()}, or {@link android.app.Fragment#onActivityCreated onActivityCreated()}. For more information about saving state, see the Activities document.
The most significant difference in lifecycle between an activity and a fragment is how one is stored in its respective back stack. An activity is placed into a back stack of activities that's managed by the system when it's stopped, by default (so that the user can navigate back to it with the Back button, as discussed in Tasks and Back Stack). However, a fragment is placed into a back stack managed by the host activity only when you explicitly request that the instance be saved by calling {@link android.app.FragmentTransaction#addToBackStack(String) addToBackStack()} during a transaction that removes the fragment.
Otherwise, managing the fragment lifecycle is very similar to managing the activity lifecycle. So, the same practices for managing the activity lifecycle also apply to fragments. What you also need to understand, though, is how the life of the activity affects the life of the fragment.
The lifecycle of the activity in which the fragment lives directly affects the lifecycle of the fragment, such that each lifecycle callback for the activity results in a similar callback for each fragment. For example, when the activity receives {@link android.app.Activity#onPause}, each fragment in the activity receives {@link android.app.Fragment#onPause}.
Fragments have a few extra lifecycle callbacks, however, that handle unique interaction with the activity in order to perform actions such as build and destroy the fragment's UI. These additional callback methods are:
The flow of a fragment's lifecycle, as it is affected by its host activity, is illustrated by figure 3. In this figure, you can see how each successive state of the activity determines which callback methods a fragment may receive. For example, when the activity has received its {@link android.app.Activity#onCreate onCreate()} callback, a fragment in the activity receives no more than the {@link android.app.Fragment#onActivityCreated onActivityCreated()} callback.
Once the activity reaches the resumed state, you can freely add and remove fragments to the activity. Thus, only while the activity is in the resumed state can the lifecycle of a fragment change independently.
However, when the activity leaves the resumed state, the fragment again is pushed through its lifecycle by the activity.
To bring everything discussed in this document together, here's an example of an activity using two fragments to create a two-pane layout. The activity below includes one fragment to show a list of Shakespeare play titles and another to show a summary of the play when selected from the list. It also demonstrates how to provide different configurations of the fragments, based on the screen configuration.
Note: The complete source code for this activity is available in {@code FragmentLayout.java}.
The main activity applies a layout in the usual way, during {@link android.app.Activity#onCreate onCreate()}:
{@sample development/samples/ApiDemos/src/com/example/android/apis/app/FragmentLayout.java main}The layout applied is {@code fragment_layout.xml}:
{@sample development/samples/ApiDemos/res/layout-land/fragment_layout.xml layout}Using this layout, the system instantiates the {@code TitlesFragment} (which lists the play titles) as soon as the activity loads the layout, while the {@link android.widget.FrameLayout} (where the fragment for showing the play summary will go) consumes space on the right side of the screen, but remains empty at first. As you'll see below, it's not until the user selects an item from the list that a fragment is placed into the {@link android.widget.FrameLayout}.
However, not all screen configurations are wide enough to show both the list of plays and the summary, side by side. So, the layout above is used only for the landscape screen configuration, by saving it at {@code res/layout-land/fragment_layout.xml}.
Thus, when the screen is in portrait orientation, the system applies the following layout, which is saved at {@code res/layout/fragment_layout.xml}:
{@sample development/samples/ApiDemos/res/layout/fragment_layout.xml layout}This layout includes only {@code TitlesFragment}. This means that, when the device is in portrait orientation, only the list of play titles is visible. So, when the user clicks a list item in this configuration, the application will start a new activity to show the summary, instead of loading a second fragment.
Next, you can see how this is accomplished in the fragment classes. First is {@code TitlesFragment}, which shows the list of Shakespeare play titles. This fragment extends {@link android.app.ListFragment} and relies on it to handle most of the list view work.
As you inspect this code, notice that there are two possible behaviors when the user clicks a list item: depending on which of the two layouts is active, it can either create and display a new fragment to show the details in the same activity (adding the fragment to the {@link android.widget.FrameLayout}), or start a new activity (where the fragment can be shown).
{@sample development/samples/ApiDemos/src/com/example/android/apis/app/FragmentLayout.java titles}The second fragment, {@code DetailsFragment} shows the play summary for the item selected from the list from {@code TitlesFragment}:
{@sample development/samples/ApiDemos/src/com/example/android/apis/app/FragmentLayout.java details}Recall from the {@code TitlesFragment} class, that, if the user clicks a list item and the current layout does not include the {@code R.id.details} view (which is where the {@code DetailsFragment} belongs), then the application starts the {@code DetailsActivity} activity to display the content of the item.
Here is the {@code DetailsActivity}, which simply embeds the {@code DetailsFragment} to display the selected play summary when the screen is in portrait orientation:
{@sample development/samples/ApiDemos/src/com/example/android/apis/app/FragmentLayout.java details_activity}Notice that this activity finishes itself if the configuration is landscape, so that the main activity can take over and display the {@code DetailsFragment} alongside the {@code TitlesFragment}. This can happen if the user begins the {@code DetailsActivity} while in portrait orientation, but then rotates to landscape (which restarts the current activity).
For more samples using fragments (and complete source files for this example), see the sample code available in ApiDemos (available for download from the Samples SDK component).