As a user navigates through, out of, and back to your app, the
[Activity](https://developer.android.com/reference/android/app/Activity) instances in your app transition through
different states in their lifecycle.
The `Activity` class provides a number of callbacks
that let the activity know when a state changes or that the
system is creating, stopping, or resuming an activity or destroying
the process the activity resides in.


Within the lifecycle callback methods, you can declare how your activity
behaves when the user leaves and re-enters the activity. For example, if you're
building a streaming video player, you might pause the video and terminate the
network connection when the user switches to another app. When the user returns,
you can reconnect to the network and let the user resume the video from the
same spot.

Each callback lets you perform specific work
that's appropriate to a given change of state. Doing the right work at the right
time and handling transitions properly make your app more robust and performant.
For example, good implementation of the lifecycle callbacks can help your app
avoid the following:

- Crashing if the user receives a phone call or switches to another app while using your app.
- Consuming valuable system resources when the user is not actively using it.
- Losing the user's progress if they leave your app and return to it at a later time.
- Crashing or losing the user's progress when the screen rotates between landscape and portrait orientation.


This document explains the activity lifecycle in detail. The document begins
by describing the lifecycle paradigm. Next, it explains each of the callbacks:
what happens internally while they execute and what you need to implement
during them.

It then briefly introduces the relationship between activity
state and a process's vulnerability to being killed by the system.
Finally, it discusses several topics related to transitions between
activity states.


For information about handling lifecycles, including guidance about
best practices, see [Handling Lifecycles with Lifecycle-Aware Components](https://developer.android.com/topic/libraries/architecture/lifecycle) and [Save UI states](https://developer.android.com/topic/libraries/architecture/saving-states).
To learn how to architect a robust, production-quality app using activities in
combination with architecture components, see
[Guide to app architecture](https://developer.android.com/topic/libraries/architecture/guide).

## Activity-lifecycle concepts


To navigate transitions between stages of the activity lifecycle, the
`Activity` class provides a core set of six callbacks:
[onCreate()](https://developer.android.com/reference/android/app/Activity#onCreate(android.os.Bundle)),
[onStart()](https://developer.android.com/reference/android/app/Activity#onStart()),
[onResume()](https://developer.android.com/reference/android/app/Activity#onResume()),
[onPause()](https://developer.android.com/reference/android/app/Activity#onPause()),
[onStop()](https://developer.android.com/reference/android/app/Activity#onStop()), and
[onDestroy()](https://developer.android.com/reference/android/app/Activity#onDestroy()). The system invokes
each of these callbacks as the activity enters a new state.


Figure 1 presents a visual representation of this paradigm.  
![](https://developer.android.com/guide/components/images/activity_lifecycle.png)

**Figure 1.** A simplified
illustration of the activity lifecycle.


As the user begins to leave the activity, the system calls methods
to dismantle the activity. In some cases, the activity is only partially
dismantled and still resides in memory, such as when the user switches to
another app. In these cases, the activity can still come back to the foreground.

If the user returns to the activity, it
resumes from where the user left off. With a few exceptions, apps are
[restricted from
starting activities when running in the background](https://developer.android.com/guide/components/activities/background-starts).

The system's likelihood of
killing a given process, along with the activities in it, depends on the state
of the activity at the time. For more information on the relationship between state and
vulnerability to ejection, see the section about [activity state and ejection from memory](https://developer.android.com/guide/components/activities/activity-lifecycle#asem).


Depending on the complexity of your activity, you probably don't need to
implement all the lifecycle methods. However, it's important that you
understand each one and implement those that make your app behave
the way users expect.

## Lifecycle callbacks


This section provides conceptual and implementation information about the
callback methods used during the activity lifecycle.


Some actions belong in the activity lifecycle methods. However, place code
that implements the actions of a dependent component in the
component, rather than the activity lifecycle method. To achieve this, you need
to make the dependent component lifecycle-aware. To learn how to make
your dependent components lifecycle-aware, see [Handling Lifecycles with Lifecycle-Aware Components](https://developer.android.com/topic/libraries/architecture/lifecycle).

### onCreate()


You must implement this callback, which fires when the system first creates the
activity. On activity creation, the activity enters the *Created* state.
In the [onCreate()](https://developer.android.com/reference/android/app/Activity#onCreate(android.os.Bundle))
method, perform basic application startup logic that
happens only once for the entire life of the activity.

For example, your
implementation of `onCreate()` might bind data to lists, associate the activity with a
[`ViewModel`](https://developer.android.com/reference/androidx/lifecycle/ViewModel),
and instantiate some class-scope variables. This method receives the
parameter `savedInstanceState`, which is a [Bundle](https://developer.android.com/reference/android/os/Bundle)
object containing the activity's previously saved state. If the activity has
never existed before, the value of the `Bundle` object is null.


If you have a lifecycle-aware component that is hooked up to the lifecycle of
your activity, it receives the
[`ON_CREATE`](https://developer.android.com/reference/androidx/lifecycle/Lifecycle.Event#ON_CREATE)
event. The method annotated with `@OnLifecycleEvent` is called so your lifecycle-aware
component can perform any setup code it needs for the created state.


The following example of the `onCreate()` method
shows fundamental setup for the activity, such as declaring the user interface
(defined in an XML layout file), defining member variables, and configuring
some of the UI. In this example, the XML layout file passes the
file's resource ID `R.layout.main_activity` to
[setContentView()](https://developer.android.com/reference/android/app/Activity#setContentView(android.view.View)).  

### Kotlin

```kotlin
lateinit var textView: TextView

// Some transient state for the activity instance.
var gameState: String? = null

override fun onCreate(savedInstanceState: Bundle?) {
    // Call the superclass onCreate to complete the creation of
    // the activity, like the view hierarchy.
    super.onCreate(savedInstanceState)

    // Recover the instance state.
    gameState = savedInstanceState?.getString(GAME_STATE_KEY)

    // Set the user interface layout for this activity.
    // The layout is defined in the project res/layout/main_activity.xml file.
    setContentView(R.layout.main_activity)

    // Initialize member TextView so it is available later.
    textView = findViewById(R.id.text_view)
}

// This callback is called only when there is a saved instance previously saved using
// onSaveInstanceState(). Some state is restored in onCreate(). Other state can optionally
// be restored here, possibly usable after onStart() has completed.
// The savedInstanceState Bundle is same as the one used in onCreate().
override fun onRestoreInstanceState(savedInstanceState: Bundle?) {
    textView.text = savedInstanceState?.getString(TEXT_VIEW_KEY)
}

// Invoked when the activity might be temporarily destroyed; save the instance state here.
override fun onSaveInstanceState(outState: Bundle?) {
    outState?.run {
        putString(GAME_STATE_KEY, gameState)
        putString(TEXT_VIEW_KEY, textView.text.toString())
    }
    // Call superclass to save any view hierarchy.
    super.onSaveInstanceState(outState)
}
```

### Java

```java
TextView textView;

// Some transient state for the activity instance.
String gameState;

@Override
public void onCreate(Bundle savedInstanceState) {
    // Call the superclass onCreate to complete the creation of
    // the activity, like the view hierarchy.
    super.onCreate(savedInstanceState);

    // Recover the instance state.
    if (savedInstanceState != null) {
        gameState = savedInstanceState.getString(GAME_STATE_KEY);
    }

    // Set the user interface layout for this activity.
    // The layout is defined in the project res/layout/main_activity.xml file.
    setContentView(R.layout.main_activity);

    // Initialize member TextView so it is available later.
    textView = (TextView) findViewById(R.id.text_view);
}

// This callback is called only when there is a saved instance previously saved using
// onSaveInstanceState(). Some state is restored in onCreate(). Other state can optionally
// be restored here, possibly usable after onStart() has completed.
// The savedInstanceState Bundle is same as the one used in onCreate().
@Override
public void onRestoreInstanceState(Bundle savedInstanceState) {
    textView.setText(savedInstanceState.getString(TEXT_VIEW_KEY));
}

// Invoked when the activity might be temporarily destroyed; save the instance state here.
@Override
public void onSaveInstanceState(Bundle outState) {
    outState.putString(GAME_STATE_KEY, gameState);
    outState.putString(TEXT_VIEW_KEY, textView.getText());

    // Call superclass to save any view hierarchy.
    super.onSaveInstanceState(outState);
}
```


As an alternative to defining the XML file and passing it to [setContentView()](https://developer.android.com/reference/android/app/Activity#setContentView(android.view.View)), you
can create new [View](https://developer.android.com/reference/android/view/View) objects in your activity code and build a
view hierarchy by inserting new `View` objects into a
[ViewGroup](https://developer.android.com/reference/android/view/ViewGroup). You then use that layout by passing the
root `ViewGroup` to `setContentView()`.
For more information about creating a user interface, see the
[user interface](https://developer.android.com/guide/topics/ui) documentation.


Your activity does not remain in the Created
state. After the `onCreate()` method finishes execution, the activity enters the *Started*
state and the system calls the [onStart()](https://developer.android.com/reference/android/app/Activity#onStart())
and [onResume()](https://developer.android.com/reference/android/app/Activity#onResume()) methods in quick
succession.

### onStart()


When the activity enters the Started state, the system
invokes [onStart()](https://developer.android.com/reference/android/app/Activity#onStart()).
This call makes the activity visible to the user as the
app prepares for the activity to enter the foreground and become interactive.
For example, this method is where the code that maintains
the UI is initialized.


When the activity moves to the Started state, any lifecycle-aware component tied
to the activity's lifecycle receives the
[`ON_START`](https://developer.android.com/reference/androidx/lifecycle/Lifecycle.Event#ON_START) event.


The `onStart()` method completes
quickly and, as with the Created state, the activity does not remain
in the Started state. Once this callback finishes, the activity enters the
*Resumed* state and the system invokes the
[onResume()](https://developer.android.com/reference/android/app/Activity#onResume()) method.

### onResume()


When the activity enters the Resumed state, it comes to the foreground, and
the system invokes the [onResume()](https://developer.android.com/reference/android/app/Activity#onResume())
callback. This is the state in which the app
interacts with the user. The app stays in this state until something happens to
take focus away from the app, such as the device receiving a phone call, the user
navigating to another activity, or the device screen turning off.


When the activity moves to the Resumed state, any lifecycle-aware component tied
to the activity's lifecycle receives the
[`ON_RESUME`](https://developer.android.com/reference/androidx/lifecycle/Lifecycle.Event#ON_RESUME)
event. This is where the lifecycle components can enable any functionality that needs to run while
the component is visible and in the foreground, such as starting a camera
preview.


When an interruptive event occurs, the activity enters the *Paused*
state and the system invokes the
[onPause()](https://developer.android.com/reference/android/app/Activity#onPause()) callback.


If the activity returns to
the Resumed state from the Paused state, the system once again calls the
`onResume()` method. For this reason, implement
`onResume()` to initialize components that you release during
`onPause()` and to perform any other
initializations that must occur each time the activity enters the Resumed
state.


Here is an example of a lifecycle-aware component that accesses the camera when
the component receives the `ON_RESUME` event:  

### Kotlin

```kotlin
class CameraComponent : LifecycleObserver {
    ...
    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun initializeCamera() {
        if (camera == null) {
            getCamera()
        }
    }
    ...
}
```

### Java

```java
public class CameraComponent implements LifecycleObserver {

    ...

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    public void initializeCamera() {
        if (camera == null) {
            getCamera();
        }
    }
    ...
}
```


The preceding code initializes the camera once the
[`LifecycleObserver`](https://developer.android.com/reference/androidx/lifecycle/LifecycleObserver)
receives the `ON_RESUME` event. In multi-window mode, however, your activity
might be fully visible even when it is in the Paused state. For example, when
the app is in multi-window mode and the user taps the window that does not
contain your activity, your activity moves to the Paused state.


If you
want the camera active only when the app is Resumed (visible
and active in the foreground), then initialize the camera after the
`ON_RESUME` event
demonstrated previously. If you want to keep the camera active while the activity
is Paused but visible, such as in multi-window mode, then
initialize the camera after the `ON_START` event.


However, having the camera
active while your activity is Paused might deny access to the camera to another
Resumed app in multi-window mode. Sometimes it is necessary to keep the
camera active while your activity is Paused, but it might actually degrade the
overall user experience if you do.


For this reason, think carefully about where in the
lifecycle it is most appropriate to take control of shared system resources in
the context of multi-window mode. To learn more about supporting multi-window
mode, see [Multi-window support](https://developer.android.com/guide/topics/ui/multi-window).


Regardless of which build-up event you choose to perform an
initialization operation in, make sure to use the corresponding lifecycle
event to release the resource. If you initialize something after
the `ON_START` event, release or terminate it after the
`ON_STOP` event. If you
initialize after the `ON_RESUME` event, release after the
`ON_PAUSE` event.


The preceding code snippet places camera initialization code in a
lifecycle-aware component. You can instead put this code directly into the activity
lifecycle callbacks, such as `onStart()` and
`onStop()`, but we don't recommend this. Adding this logic
to an independent, lifecycle-aware component lets you reuse the component
across multiple activities without having to duplicate code. To learn how to create a lifecycle-aware component, see
[Handling Lifecycles with Lifecycle-Aware Components](https://developer.android.com/topic/libraries/architecture/lifecycle).

### onPause()


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.
It indicates that the activity is no longer in the foreground, but it is
still visible if the user is in multi-window mode.
There are several reasons why an activity might enter
this state:

- An event that interrupts app execution, as described in the section about the [onResume()](https://developer.android.com/guide/components/activities/activity-lifecycle#onresume) callback, pauses the current activity. This is the most common case.
- In multi-window mode, only one app has focus at any time, and the system pauses all the other apps.
- The opening of a new, semi-transparent activity, such as a dialog, pauses the activity it covers. As long as the activity is partially visible but not in focus, it remains paused.


When an activity moves to the Paused state, any lifecycle-aware component tied
to the activity's lifecycle receives the
[`ON_PAUSE`](https://developer.android.com/reference/androidx/lifecycle/Lifecycle.Event#ON_PAUSE) event. This is where
the lifecycle components can stop any functionality that does not need to run
while the component is not in the foreground, such as stopping a camera
preview.

Use the [`onPause()`](https://developer.android.com/reference/android/app/Activity#onPause()) method to pause or
adjust operations that can't continue, or might continue in moderation,
while the `Activity` is in the Paused state, and that you
expect to resume shortly.


You can also use the `onPause()` method to
release system resources, handles to sensors (like GPS), or any resources that
affect battery life while your activity is Paused and the user does not
need them.


However, as mentioned in the section about `onResume()`, a Paused
activity might still be fully visible if the app is in multi-window mode.
Consider using `onStop()` instead of `onPause()` to fully release or adjust
UI-related resources and operations to better support multi-window mode.


The following example of a [`LifecycleObserver`](https://developer.android.com/reference/androidx/lifecycle/LifecycleObserver)
reacting to the `ON_PAUSE` event is the counterpart to the preceding
`ON_RESUME` event example, releasing the camera that initializes after
the `ON_RESUME` event is received:  

### Kotlin

```kotlin
class CameraComponent : LifecycleObserver {
    ...
    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    fun releaseCamera() {
        camera?.release()
        camera = null
    }
    ...
}
```

### Java

```java
public class JavaCameraComponent implements LifecycleObserver {
    ...
    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    public void releaseCamera() {
        if (camera != null) {
            camera.release();
            camera = null;
        }
    }
    ...
}
```

This example places the camera release code after the
`ON_PAUSE` event is received by the `LifecycleObserver`.


`onPause()` execution is very brief and
does not necessarily offer enough time to perform save operations. For this
reason, **don't** use `onPause()` to save application or user
data, make network calls, or execute database transactions. Such work might not
complete before the method completes.


Instead, perform heavy-load shutdown operations during
[onStop()](https://developer.android.com/reference/android/app/Activity#onStop()). For more information
about suitable operations to perform during
`onStop()`, see the next section. For more information about saving
data, see the section about [saving and restoring state](https://developer.android.com/guide/components/activities/activity-lifecycle#saras).

Completion of the `onPause()` method
does not mean that the activity leaves the Paused state. Rather, the activity
remains in this state until either the activity resumes or it becomes completely
invisible to the user. If the activity resumes, the system once again invokes
the `onResume()` callback.


If the
activity returns from the Paused state to the Resumed state, the system keeps
the `Activity` instance resident in memory, recalling
that instance when the system invokes `onResume()`.
In this scenario, you don't need to re-initialize components created during any of the
callback methods leading up to the Resumed state. If the activity becomes
completely invisible, the system calls
`onStop()`.

### onStop()


When your activity is no longer visible to the user, it enters the
*Stopped* state, and the system invokes the
[onStop()](https://developer.android.com/reference/android/app/Activity#onStop()) callback. This can occur when a newly launched activity covers the entire screen. The
system also calls `onStop()`
when the activity finishes running and is about to be terminated.


When the activity moves to the Stopped state, any lifecycle-aware component tied
to the activity's lifecycle receives the
[`ON_STOP`](https://developer.android.com/reference/androidx/lifecycle/Lifecycle.Event#ON_STOP) event. This is where
the lifecycle components can stop any functionality that does not need to run
while the component is not visible on the screen.


In the `onStop()` method, release or adjust
resources that are not needed while the app is not visible to the user. For example, your app might
pause animations or switch from fine-grained to coarse-grained location updates. Using
`onStop()` instead of [onPause()](https://developer.android.com/reference/android/app/Activity#onPause())
means that UI-related work continues, even when the user is viewing your activity in multi-window
mode.


Also, use `onStop()`
to perform relatively CPU-intensive shutdown operations. For example, if
you can't find a better time to save information to a database,
you might do so during `onStop()`. The
following example shows an implementation of
`onStop()` that saves the contents of a
draft note to persistent storage:  

### Kotlin

```kotlin
override fun onStop() {
    // Call the superclass method first.
    super.onStop()

    // Save the note's current draft, because the activity is stopping
    // and we want to be sure the current note progress isn't lost.
    val values = ContentValues().apply {
        put(NotePad.Notes.COLUMN_NAME_NOTE, getCurrentNoteText())
        put(NotePad.Notes.COLUMN_NAME_TITLE, getCurrentNoteTitle())
    }

    // Do this update in background on an AsyncQueryHandler or equivalent.
    asyncQueryHandler.startUpdate(
            token,     // int token to correlate calls
            null,      // cookie, not used here
            uri,       // The URI for the note to update.
            values,    // The map of column names and new values to apply to them.
            null,      // No SELECT criteria are used.
            null       // No WHERE columns are used.
    )
}
```

### Java

```java
@Override
protected void onStop() {
    // Call the superclass method first.
    super.onStop();

    // Save the note's current draft, because the activity is stopping
    // and we want to be sure the current note progress isn't lost.
    ContentValues values = new ContentValues();
    values.put(NotePad.Notes.COLUMN_NAME_NOTE, getCurrentNoteText());
    values.put(NotePad.Notes.COLUMN_NAME_TITLE, getCurrentNoteTitle());

    // Do this update in background on an AsyncQueryHandler or equivalent.
    asyncQueryHandler.startUpdate (
            mToken,  // int token to correlate calls
            null,    // cookie, not used here
            uri,    // The URI for the note to update.
            values,  // The map of column names and new values to apply to them.
            null,    // No SELECT criteria are used.
            null     // No WHERE columns are used.
    );
}
```


The preceding code sample uses SQLite directly. However, we recommend using Room,
a persistence library that provides an abstraction layer over SQLite. To learn
more about the benefits of using Room and how to implement Room in your app,
see the
[Room Persistence Library](https://developer.android.com/topic/libraries/architecture/room)
guide.


When your activity enters the Stopped state, the `Activity`
object is kept resident in memory: it maintains all state and member
information, but is not attached to the window manager. When the activity
resumes, it recalls this information.


You don't need to
re-initialize components created during any of the callback methods
leading up to the Resumed state. The system also keeps track of the current
state for each `View` object in the layout, so if the
user enters text into an [EditText](https://developer.android.com/reference/android/widget/EditText) widget, that
content is retained so you don't need to save and restore it.


**Note:** Once your activity is stopped, the system
might destroy the process that contains the activity if the system
needs to recover memory.
Even if the system destroys the process while the activity
is stopped, the system still retains the state of the `View`
objects, such as text in an `EditText` widget, in a
`Bundle`---a blob of key-value pairs---and restores them
if the user navigates back to the activity. For
more information about restoring an activity to which a user returns, see the
section about [saving and restoring state](https://developer.android.com/guide/components/activities/activity-lifecycle#saras).


From the Stopped state, the activity either comes back to interact with the
user, or the activity is finished running and goes away. If the activity comes
back, the system invokes [onRestart()](https://developer.android.com/reference/android/app/Activity#onRestart()).
If the `Activity` is finished running, the system calls
[onDestroy()](https://developer.android.com/reference/android/app/Activity#onDestroy()).

### onDestroy()


[`onDestroy()`](https://developer.android.com/reference/android/app/Activity#onDestroy()) is called before the
activity is destroyed. The system invokes this callback for one of two reasons:

1. The activity is finishing, due to the user completely dismissing the activity or due to [`finish()`](https://developer.android.com/reference/android/app/Activity#finish()) being called on the activity.
2. The system is temporarily destroying the activity due to a configuration change, such as device rotation or entering multi-window mode.


When the activity moves to the destroyed state, any lifecycle-aware component tied
to the activity's lifecycle receives the
[`ON_DESTROY`](https://developer.android.com/reference/androidx/lifecycle/Lifecycle.Event#ON_DESTROY) event. This is where
the lifecycle components can clean up anything they need to before the
`Activity` is destroyed.


Instead of putting logic in your `Activity` to determine why it is being destroyed,
use a [`ViewModel`](https://developer.android.com/reference/androidx/lifecycle/ViewModel) object to contain the
relevant view data for your `Activity`. If the `Activity` is recreated
due to a configuration change, the `ViewModel` does not have to do anything, since
it is preserved and given to the next `Activity` instance.


If the `Activity` isn't recreated, then the `ViewModel` has the
[`onCleared()`](https://developer.android.com/reference/androidx/lifecycle/ViewModel#onCleared()) method called, where
it can clean up any data it needs to before being destroyed. You can distinguish between these two scenarios with the
[`isFinishing()`](https://developer.android.com/reference/android/app/Activity#isFinishing()) method.


If the activity is finishing, `onDestroy()` is the final lifecycle callback the
activity receives. If `onDestroy()` is called as the result of a configuration
change, the system immediately creates a new activity instance and then calls
[`onCreate()`](https://developer.android.com/reference/android/app/Activity#onCreate(android.os.Bundle))
on that new instance in the new configuration.


The `onDestroy()` callback releases all resources not released by earlier
callbacks, such as [`onStop()`](https://developer.android.com/reference/android/app/Activity#onStop()).

## Activity state and ejection from memory


The system kills processes when it needs to free up RAM. The likelihood of the system
killing a given process depends on the state of the process at the time. Process state,
in turn, depends on the state of the activity running in the process.
Table 1 shows the correlations among process state, activity
state, and the likelihood of the system killing the process. This table only applies if a process is not running other types of
application components.

| Likelihood of being killed | Process state | Final activity state |
|---|---|---|
| Lowest | Foreground (having or about to get focus) | Resumed |
| Low | Visible (no focus) | Started/Paused |
| Higher | Background (invisible) | Stopped |
| Highest | Empty | Destroyed |


**Table 1.** Relationship between process lifecycle and activity state.


The system never kills an activity directly to free up memory. Instead, it
kills the process the activity runs in, destroying not only the activity
but everything else running in the process as well. To learn how to preserve
and restore your activity's UI state when system-initiated process death
occurs, see the section about [saving and restoring state](https://developer.android.com/guide/components/activities/activity-lifecycle#saras).


The user can also kill a process by using the Application Manager, under Settings,
to kill the corresponding app.


For more information about processes, see
[Processes and threads
overview](https://developer.android.com/guide/components/processes-and-threads).

## Saving and restoring transient UI state


A user expects an activity's UI state to remain the same throughout a
configuration change, such as rotation or switching into multi-window mode.
However, the system destroys the activity by default when such a configuration
change occurs, wiping away any UI state stored in the activity instance.


Similarly, a user expects UI state to remain the same if they temporarily
switch away from your app to a different app and then come back to your app
later. However, the system can destroy your application's process while the
user is away and your activity is stopped.


When system constraints destroy the activity, preserve the
user's transient UI state using a combination of
[`ViewModel`](https://developer.android.com/reference/androidx/lifecycle/ViewModel),
[`onSaveInstanceState()`](https://developer.android.com/reference/android/app/Activity#onSaveInstanceState(android.os.Bundle)),
and/or local storage. To learn more about user expectations compared to system
behavior and how to best preserve complex UI state data across
system-initiated activity and process death, see [Save UI states](https://developer.android.com/topic/libraries/architecture/saving-states).


This section outlines what instance state is and how to implement the
`onSaveInstance()` method, which is a callback on the activity itself. If your
UI data is lightweight, you can use `onSaveInstance()` alone to persist the UI
state across both configuration changes and system-initiated process death.
But because `onSaveInstance()` incurs serialization/deserialization costs, in
most cases you use both `ViewModel` and `onSaveInstance()`, as
outlined in [Save UI states](https://developer.android.com/topic/libraries/architecture/saving-states).


**Note:** To learn more about configuration changes, how to restrict Activity
recreation if needed, and how to react to those configuration changes from the
View system and Jetpack Compose, check out the
[Handle configuration changes](https://developer.android.com/guide/topics/resources/runtime-changes) page.

### Instance state


There are a few scenarios in which your activity is destroyed due to normal app
behavior, such as when the user presses the Back button or your activity
signals its own destruction by calling the
[`finish()`](https://developer.android.com/reference/android/app/Activity#finish())
method.


When your activity is destroyed because the user presses Back
or the activity finishes itself, both the system's and the user's concept of
that [`Activity`](https://developer.android.com/reference/android/app/Activity) instance is gone forever. In these
scenarios, the user's expectation matches the system's behavior, and you do not
have any extra work to do.


However, if the system destroys the activity due to system constraints (such as
a configuration change or memory pressure), then although the actual
`Activity`
instance is gone, the system remembers that it existed. If the user attempts to
navigate back to the activity, the system creates a new instance of that
activity using a set of saved data that describes the state of the activity
when it was destroyed.


The saved data that the system uses to restore the
previous state is called the *instance state* . It's a collection of
key-value pairs stored in a [`Bundle`](https://developer.android.com/reference/android/os/Bundle) object. By default, the
system uses the `Bundle` instance state to save information
about each [`View`](https://developer.android.com/reference/android/view/View) object in your activity layout, such as
the text value entered into an
[`EditText`](https://developer.android.com/reference/android/widget/EditText) widget.


So, if
your activity instance is destroyed and recreated, the state of the layout is
restored to its previous state with no code required by you. However, your
activity might have more state information that you'd like to restore, such as
member variables that track the user's progress in the activity.


**Note:** In order for the Android system to restore the state of
the views in your activity, each view must have a unique ID, supplied by
the `android:id` attribute.


A `Bundle` object isn't appropriate for preserving more than a
trivial amount of data, because it requires serialization on the main thread and consumes
system-process memory. To preserve more than a very small amount of data,
take a combined approach to preserving data, using persistent local
storage, the `onSaveInstanceState()` method, and the
`ViewModel` class, as outlined in [Save UI states](https://developer.android.com/topic/libraries/architecture/saving-states).

### Save simple, lightweight UI state using onSaveInstanceState()


As your activity begins to stop, the system calls the
[`onSaveInstanceState()`](https://developer.android.com/reference/android/app/Activity#onSaveInstanceState(android.os.Bundle))
method so your activity can save state information to an instance state
bundle. The default implementation of this method saves transient
information about the state of the activity's view hierarchy, such as the
text in an [`EditText`](https://developer.android.com/reference/android/widget/EditText) widget or the scroll position of a
[`ListView`](https://developer.android.com/reference/android/widget/ListView) widget.


To save additional instance state information for your activity, override
`onSaveInstanceState()`
and add key-value pairs to the `Bundle` object that is saved
in the event that your activity is destroyed unexpectedly. When you override
`onSaveInstanceState()`, you need to call the superclass implementation
if you want the default implementation to save the state of the view hierarchy.
This is shown in the following example:  

### Kotlin

```kotlin
override fun onSaveInstanceState(outState: Bundle?) {
    // Save the user's current game state.
    outState?.run {
        putInt(STATE_SCORE, currentScore)
        putInt(STATE_LEVEL, currentLevel)
    }

    // Always call the superclass so it can save the view hierarchy state.
    super.onSaveInstanceState(outState)
}

companion object {
    val STATE_SCORE = "playerScore"
    val STATE_LEVEL = "playerLevel"
}
```

### Java

```java
static final String STATE_SCORE = "playerScore";
static final String STATE_LEVEL = "playerLevel";
// ...


@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
    // Save the user's current game state.
    savedInstanceState.putInt(STATE_SCORE, currentScore);
    savedInstanceState.putInt(STATE_LEVEL, currentLevel);

    // Always call the superclass so it can save the view hierarchy state.
    super.onSaveInstanceState(savedInstanceState);
}
```

**Note:**
`onSaveInstanceState()` is not
called when the user explicitly closes the activity or in other cases when
`finish()` is called.

To save persistent data, such as user preferences or data for a database,
take appropriate opportunities when your activity is in the foreground.
If no such opportunity arises, save persistent data during the
[`onStop()`](https://developer.android.com/reference/android/app/Activity#onStop()) method.

### Restore activity UI state using saved instance state


When your activity is recreated after it was previously destroyed, you
can recover your saved instance state from the [`Bundle`](https://developer.android.com/reference/android/os/Bundle) that the
system passes to your activity. Both the
[`onCreate()`](https://developer.android.com/reference/android/app/Activity#onCreate(android.os.Bundle)) and
[`onRestoreInstanceState()`](https://developer.android.com/reference/android/app/Activity#onRestoreInstanceState(android.os.Bundle))
callback methods receive the same `Bundle` that contains the
instance state information.


Because the `onCreate()` method is
called whether the system is creating a new instance of your activity
or recreating a previous one, you need to check whether the state `Bundle`
is null before you attempt to read it. If it is null, then the system
is creating a new instance of the activity, instead of restoring a
previous one that was destroyed.

The following code snippet shows how you can restore some
state data in `onCreate()`:  

### Kotlin

```kotlin
override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState) // Always call the superclass first

    // Check whether we're recreating a previously destroyed instance.
    if (savedInstanceState != null) {
        with(savedInstanceState) {
            // Restore value of members from saved state.
            currentScore = getInt(STATE_SCORE)
            currentLevel = getInt(STATE_LEVEL)
        }
    } else {
        // Probably initialize members with default values for a new instance.
    }
    // ...
}
```

### Java

```java
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState); // Always call the superclass first

    // Check whether we're recreating a previously destroyed instance.
    if (savedInstanceState != null) {
        // Restore value of members from saved state.
        currentScore = savedInstanceState.getInt(STATE_SCORE);
        currentLevel = savedInstanceState.getInt(STATE_LEVEL);
    } else {
        // Probably initialize members with default values for a new instance.
    }
    // ...
}
```

Instead of restoring the state during
`onCreate()`, you can choose to implement
`onRestoreInstanceState()`, which the system calls after the
[`onStart()`](https://developer.android.com/reference/android/app/Activity#onStart()) method. The system calls
`onRestoreInstanceState()`
only if there is a saved state to restore, so you do
not need to check whether the `Bundle` is null.  

### Kotlin

```kotlin
override fun onRestoreInstanceState(savedInstanceState: Bundle?) {
    // Always call the superclass so it can restore the view hierarchy.
    super.onRestoreInstanceState(savedInstanceState)

    // Restore state members from saved instance.
    savedInstanceState?.run {
        currentScore = getInt(STATE_SCORE)
        currentLevel = getInt(STATE_LEVEL)
    }
}
```

### Java

```java
public void onRestoreInstanceState(Bundle savedInstanceState) {
    // Always call the superclass so it can restore the view hierarchy.
    super.onRestoreInstanceState(savedInstanceState);

    // Restore state members from saved instance.
    currentScore = savedInstanceState.getInt(STATE_SCORE);
    currentLevel = savedInstanceState.getInt(STATE_LEVEL);
}
```


Caution: Always call the superclass implementation of
`onRestoreInstanceState()`
so the default implementation can restore the state of the view hierarchy.

## Navigating between activities


An app is likely to enter and exit an activity, perhaps many times, during the
app's lifetime, such as when the user taps the device's Back button
or the activity launches a different activity.


This section
covers topics you need to know to implement successful activity transitions.
These topics include starting an activity from another activity, saving activity
state, and restoring activity state.

### Starting one activity from another


An activity often needs to start another activity at some point. This need
arises, for instance, when an app needs to move from the current screen to a
new one.


Depending on whether or not your activity wants a result back from the new activity
it's about to start, you start the new activity using either the
[startActivity()](https://developer.android.com/reference/android/app/Activity#startActivity(android.content.Intent, android.os.Bundle))
method or the
[startActivityForResult()](https://developer.android.com/reference/android/app/Activity#startActivityForResult(android.content.Intent, int))
method. In either case, you pass in an [Intent](https://developer.android.com/reference/android/content/Intent) object.


The `Intent` object specifies either the exact
activity you want to start or describes the type of action you want to perform.
The system selects the appropriate activity for you, which can even be
from a different application. An `Intent` object can
also carry small amounts of data to be used by the activity that is started.
For more information about the `Intent` class, see
[Intents and Intent
Filters](https://developer.android.com/guide/components/intents-filters).

#### startActivity()


If the newly started activity does not need to return a result, the current activity can start it
by calling the
[startActivity()](https://developer.android.com/reference/android/app/Activity#startActivity(android.content.Intent, android.os.Bundle))
method.


When working within your own application, you often need to simply launch a known activity.
For example, the following code snippet shows how to launch an activity called
`SignInActivity`.  

### Kotlin

```kotlin
val intent = Intent(this, SignInActivity::class.java)
startActivity(intent)
```

### Java

```java
Intent intent = new Intent(this, SignInActivity.class);
startActivity(intent);
```


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. 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 let the user send an
email message, you can create the following intent:  

### Kotlin

```kotlin
val intent = Intent(Intent.ACTION_SEND).apply {
    putExtra(Intent.EXTRA_EMAIL, recipientArray)
}
startActivity(intent)
```

### Java

```java
Intent intent = new Intent(Intent.ACTION_SEND);
intent.putExtra(Intent.EXTRA_EMAIL, recipientArray);
startActivity(intent);
```


The `EXTRA_EMAIL` extra added to the intent is a string array of
email addresses the email is to be sent to. When an email application
responds to this intent, it reads the string array provided in the extra and
places the addresses 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.

#### startActivityForResult()


Sometimes you want to get a result back from an activity when it ends. For example, you might start
an activity that lets the user pick a person in a list of contacts. When it ends, it returns the
person that was selected. To do this, you call the
[startActivityForResult(Intent, int)](https://developer.android.com/reference/android/app/Activity#startActivityForResult(android.content.Intent, int))
method, where
the integer parameter identifies the call.


This identifier is meant to distinguish between multiple calls to
`startActivityForResult(Intent, int)`
from the same activity. It's not a global identifier
and is not at risk of conflicting with other apps or activities. The result comes back through your
[onActivityResult(int, int, Intent)](https://developer.android.com/reference/android/app/Activity#onActivityResult(int, int, android.content.Intent))
method.


When a child activity exits, it can call `setResult(int)` to
return data to its parent.
The child activity must supply a result code, which can be the standard results
`RESULT_CANCELED`, `RESULT_OK`, or any custom values
starting at `RESULT_FIRST_USER`.


In addition,
the child activity can optionally return an `Intent`
object containing any additional data it wants. The parent activity uses the
`onActivityResult(int, int, Intent)`
method, along with the integer identifier the parent activity originally
supplied, to receive the information.


If a child activity fails for any reason, such as crashing, the parent
activity receives a result with the code `RESULT_CANCELED`.  

### Kotlin

```kotlin
class MyActivity : Activity() {
    // ...

    override fun onKeyDown(keyCode: Int, event: KeyEvent?): Boolean {
        if (keyCode == KeyEvent.KEYCODE_DPAD_CENTER) {
            // When the user center presses, let them pick a contact.
            startActivityForResult(
                    Intent(Intent.ACTION_PICK,Uri.parse("content://contacts")),
                    PICK_CONTACT_REQUEST)
            return true
        }
        return false
    }

    override fun onActivityResult(requestCode: Int, resultCode: Int, intent: Intent?) {
        when (requestCode) {
            PICK_CONTACT_REQUEST ->
                if (resultCode == RESULT_OK) {
                    // A contact was picked. Display it to the user.
                    startActivity(Intent(Intent.ACTION_VIEW, intent?.data))
                }
        }
    }

    companion object {
        internal val PICK_CONTACT_REQUEST = 0
    }
}
```

### Java

```java
public class MyActivity extends Activity {
     // ...

     static final int PICK_CONTACT_REQUEST = 0;

     public boolean onKeyDown(int keyCode, KeyEvent event) {
         if (keyCode == KeyEvent.KEYCODE_DPAD_CENTER) {
             // When the user center presses, let them pick a contact.
             startActivityForResult(
                 new Intent(Intent.ACTION_PICK,
                 new Uri("content://contacts")),
                 PICK_CONTACT_REQUEST);
            return true;
         }
         return false;
     }

     protected void onActivityResult(int requestCode, int resultCode,
             Intent data) {
         if (requestCode == PICK_CONTACT_REQUEST) {
             if (resultCode == RESULT_OK) {
                 // A contact was picked. Display it to the user.
                 startActivity(new Intent(Intent.ACTION_VIEW, data));
             }
         }
     }
 }
```

#### Coordinating activities


When one activity starts another, they both experience lifecycle transitions.
The first activity stops operating and enters the Paused or Stopped state, 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.


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

1. Activity A's [onPause()](https://developer.android.com/reference/android/app/Activity#onPause()) method executes.
2. Activity B's [onCreate()](https://developer.android.com/reference/android/app/Activity#onCreate(android.os.Bundle)), [onStart()](https://developer.android.com/reference/android/app/Activity#onStart()), and [onResume()](https://developer.android.com/reference/android/app/Activity#onResume()) methods execute in sequence. Activity B now has user focus.
3. If Activity A is no longer visible on screen, its [onStop()](https://developer.android.com/reference/android/app/Activity#onStop()) method executes.


This sequence of lifecycle callbacks lets you manage the transition of
information from one activity to another.