Introduction to Android Activities Tutorial

When you make an Android app, it’s pretty likely that the first thing you’ll do is plot and plan howactivity. These are where all the action happens, because they are the screens that allow the user to interact with your app.
you’ll take over the world. Just kidding. Actually, the first thing you do is create an
In short, activities are one of the basic building blocks of an Android application.
In this introduction to Android activities tutorial, you’ll dive into how to work with activities. You’ll work through creating a to-do list app named Forget Me Not and along the way learn:

• The process for creating, starting and stopping an activity and handle navigation between activities.
• The various stages in the lifecycle of an activity and how to handle each stage gracefully.
• The way to manage configuration changes and persist data within your activity.

This tutorial assumes you’re familiar with the basics of Android development. If you’re completely new to Java, XML or Android Studio, you should take a look at your Android Tutorial for Beginners Series before you start.

Getting Started

Download the starter project for this tutorial. Open Android Studio, and choose Open an Existing Android Studio Project.

Navigate to where you downloaded the sample project, and click Choose.

Your subject and new best friend for the rest of the tutorial is Forget Me Not, which is a simple app that lets you add and delete tasks from a list. It also displays the current date and time so you’re always on top of your game!
Run the project as it is now and you’ll see a very basic screen:

At this point, there isn’t much you can do. Your to-do list is empty and the add a task button does nothing when you tap it. Your task is to make this screen more exciting by making this a modifiable list.

Activity Lifecycle

Before firing up your fingers, indulge yourself in a bit of theory.
As mentioned earlier, activities are the foundations upon which you build screens for your app. They encompass multiple components the user can interact with, and it’s likely that your app will have multiple activities to handle the various screens you create.
Having all these activities doing different things makes it sound like developing an Android app is a complicated undertaking.
Fortunately, Android handles this with specific callback methods that initiate code only when it’s needed. This system is known as the activity lifecycle.
Handling the various lifecycle stages of your activities is crucial to creating a robust and reliable app. The lifecycle of an activity is best illustrated as a step pyramid of different stages linked by the core callback methods:

Following the diagram above, you can picture the lifecycle in action as it courses through your code. Take a closer look at each of the callbacks:

• onCreate(): When you first create an activity, you also call this method. This also where you initialize any UI elements or data objects. You also have the savedInstanceState of the activity that contains its previously saved state, and you can use it to recreate that state.
• onStart(): Just before presenting the user with an activity, this method is called. It’s always followed by onResume() and very rarely by onStop(). In here, you generally should start UI animations, audio based content or anything else that requires the activity’s contents to be on screen.
• onResume(): Before bringing an activity back to the foreground, you call this method. Here you have a good place to restart animations, update UI elements, restart camera previews, resume audio/video playback or initialize any components that you release during onPause().
• onPause(): Before sliding into the background, this method is called. Here you should stop any visuals or audio associated with the activity such as UI animations, music playback or the camera. This method is followed by onResume() if the activity returns to the foreground or by onStop() if it becomes hidden.
• onStop(): You call this method right after onPause(), but before the activity goes into the background, and it’s a good place to save data that you want to commit to the disk. It’s followed by either onRestart(), if this activity is coming back to the foreground, or onDestroy() if it’s being released from memory.
• onRestart(): After stopping an activity, but just before starting it again, you call this method. It’s always followed by onStart().
• onDestroy(): This is the final callback you’ll receive from the system before the activity is destroyed. A call to finish() is one way to destroy the activity, or it can be triggered by the system when it needs to recoup memory. If your activity includes any background threads or other long-running resources, destruction could lead to a memory leak if they’re not released, so you need to remember stop these processes here as well.

So many methods to remember! In the next section, you’ll see some of these lifecycle methods in action, and then it’ll be a lot easier to remember what everything does.

Creating an Activity

Keeping the activity lifecycle in mind, take a look at an activity in the sample project. Open MainActivity, you’ll see that onCreate looks like this:

@Override protected void onCreate(Bundle savedInstanceState) { // 1 super.onCreate(savedInstanceState);   // 2 getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);   // 3 setContentView(R.layout.activity_main);   // 4 mDateTimeTextView = (TextView) findViewById(R.id.dateTi meTextView); final Button addTaskBtn = (Button) findViewById(R.id.addTaskBtn); final ListView listview = (ListView) findViewById(R.id.taskListview); mList = new ArrayList<String>();   // 5 mAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, mList); listview.setAdapter(mAdapter);   // 6 listview.setOnItemClickListener(new AdapterView.OnItemClickListener() { @Override public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {   } }); }

Here’s a play-by-play of what’s happening above:

1. You call onCreate() on the superclass; remember that this is always the first thing you should do in a callback method.
2. You tell the WindowManager to make your activity’s window full screen.
3. You set the content view of your activity with the corresponding layout file resource.
4. Here you initialize all the UI and data variables. In this case, you’re using a TextView to show the current date and time, a button to add tasks to your list, a ListView to display your list, and an ArrayList to hold your data. You can find the implementation of all these UI elements in the activity_main.xml file.
5. Here you initialize and set the Adapter that will handle the data for your ListView.
6. You set an OnItemClickListener() for the ListView to capture the user’s tap on individual list entries.

Note: To learn more about ListViews and Adapters, refer to the Android Developer docs.

Now you see that the implementation follows the theory; you’re doing all the initialization for your activity during creation.

Starting an Activity

In its current stat, the app is a fairly useless lump of ones and zeros because you can’t add anything to the to-do list. You have the power to change that, and that’s exactly what you’ll do next.
In the MainActivity file you have open, add a field to the top of the class:

private final int ADD_TASK_REQUEST = 1;

You’ll use this variable to reference your request to add new tasks later on. You can assign any int value to ADD_TASK_REQUEST. If it returns the value you entered on the request, you can assume your request was handled successfully. More on this later.
Then add the following implementation for addTaskClicked:

Intent intent = new Intent(MainActivity.this, TaskDescriptionActivity.class); startActivityForResult(intent, ADD_TASK_REQUEST);

When the user taps the add a task button, you call addTaskClicked. Here you create an Intent to launch the TaskDescriptionActivity from MainActivity. It needs to produce a result from TaskDescriptionActivity, because the app needs to know if there is a new task to add to your list or not.
To accomplish this, you start the activity using startActivityForResult(...).

Note: In situations where a result is not a requirement, calling startActivity(...) will suffice.

When the TaskDescriptionActivity finishes, it returns a result in an intent to the onActivityResult(...) method. Implement this callback to receive the result by adding the following method to the bottom of MainActivity:
All appears to be well and good, but is it really?
Not quite. Forget Me Not needs to be able to move to another activity and extract task descriptions from it, so the next thing you’ll do is make one.

Creating an Activity

Android Studio makes it very easy to create an activity. All you need do is right-click on the package where you want to add the activity; in this case, the package is com.raywenderlich.todolist). Then navigate to New\Activity, and the choose Empty Activity, which is a basic template for an activity.

On the next screen, you enter the Activity Name and Android Studio will automatically fill the other fields based on that. Enter the activity name as TaskDescriptionActivity
Click Finish and put your hands in the air to celebrate. You’ve just created your first activity!

Android Studio will automatically generate the corresponding resources needed to create the activity. These are:

• Class: The class file is named TaskDescriptionActivity.java and located in your Java package. This is where you implement the activity’s behavior. This class must subclass the Activity class or an existing subclass of it.
• Layout: The layout file is located under res/layout and named activity_task_description.xml. It defines the placement of different UI elements on the screen when the activity is created.

In addition to this, you will see a new addition to your app’s AndroidManifest.xml file:

<activity <activity android:name=".TaskDescriptionActivity" > </activity>

The activity element declares the activity. Android apps have a strong sense of order, so all available activities must be declared in the manifest to ensure the app only has control of activities declared here.
You don’t want your app to accidentally use the wrong activity, or even worse, have it use activities that are used by other apps without explicit permission.
There are several attributes that you can include in this element to define properties for the activity, such as a label or icon, or a theme to style the activity’s UI.
android:name is the only required attribute. It specifies the activity’s class name.
Now build and run the app. When you tap on Add a Task, you’re presented with your newly generated new activity!

Looking good, except for the fact that it’s lacking substance. Now for a quick remedy to that!
In your newly generated TaskDescriptionActivity, paste the following into your class file, overwriting anything else except the class declaration and its brackets.

public static final String EXTRA_TASK_DESCRIPTION = "task";   private EditText mDescriptionView;   @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_task_description);   mDescriptionView = (EditText) findViewById(R.id.descriptionText); }   public void doneClicked(View view) {   }

Jump over to your associated layout in layout/activity_task_description and replace everything with the following:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" android:paddingBottom="@dimen/activity_vertical_margin" android:paddingLeft="@dimen/activity_horizontal_margin" android:paddingRight="@dimen/activity_horizontal_margin" android:paddingTop="@dimen/activity_vertical_margin" tools:context="com.raywenderlich.todolist.TaskDescriptionActivity">   <TextView android:id="@+id/descriptionLabel" android:text="@string/description" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_alignParentTop="true" android:padding="20dp"/>   <EditText android:id="@+id/descriptionText" android:layout_width="match_parent" android:layout_height="100dp" android:padding="20dp" android:layout_below="@+id/descriptionLabel"/>   <Button android:id="@+id/doneBtn" android:text="@string/done" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_below="@+id/descriptionText" android:padding="20dp" android:onClick="doneClicked"/> </RelativeLayout>

Run the app again, tap Add a Task and your new screen will look a lot more interesting.

Stopping an Activity

Just as important as starting an activity with all the right methods is properly stopping it.
In TaskDescriptionActivity, add the following to doneClicked, which is called when the Done button is clicked on this screen:

// 1 String taskDescription = mDescriptionView.getText().toString();   if (!taskDescription.isEmpty()) { // 2 Intent result = new Intent(); result.putExtra(EXTRA_TASK_DESCRIPTION, taskDescription); setResult(RESULT_OK, result); } else { // 3 setResult(RESULT_CANCELED); } // 4 finish();

You can see a few things are happening here:

1. You retrieve the task description from the TextView.
2. You create a result intent to pass back to MainActivity if the task description retrieved in step one is not empty. Then you bundle the task description with the intent and set the result to RESULT_OK, indicating that the user successfully entered a task.
3. You set the result to RESULT_CANCELED, indicating there was no task entered by the user if the task description retrieved in step one is empty.
4. Here you close the activity.

Once you call finish() in step four, the callback onActivityResult(...) needs to be executed in MainActivity; in turn, it adds the task to the list.
To do that you need to add the following method to the bottom of MainActivity.

@Override protected void onActivityResult(int requestCode, int resultCode, Intent data) { // 1 - Check which request you're responding to if (requestCode == ADD_TASK_REQUEST) { // 2 - Make sure the request was successful if (resultCode == RESULT_OK) { // 3 - The user entered a task. Add a task to the list. String task = data.getStringExtra(TaskDescriptionActivity.EXTRA_TASK_DESCRIPTION); mList.add(task); // 4 mAdapter.notifyDataSetChanged(); } } }

Lets take this step-by-step:

1. You check the requestCode to ensure the returned result is indeed for your add task request you started with TaskDescriptionActivity.
2. You make sure the resultCode is RESULT_OK — the standard activity result for a successful operation.
3. Here you extract the task description from the result intent and add it to your list.
4. Finally, you call notifyDataSetChanged() on your list adapter. In turn, it notifies the listView about changes in your data model so it can trigger a refresh of its view.

Build and run the project to see it in action. After the app starts, press Add a Task. This will bring up a new screen that lets you enter a task. Now add a description and press Done. The screen will close and the new task is on your list:

Implementing Callbacks

Every to-do list needs to have a good grasp on date and time, so they should be the next thing you add to your app. Open MainActivity and add the following after the existing member variable declarations at the top:

private BroadcastReceiver mTickReceiver;

Now initialize the BroadcastReceiver by adding the following to the bottom of onCreate():

mTickReceiver = new BroadcastReceiver() { @Override public void onReceive(Context context, Intent intent) { if (intent.getAction().equals(Intent.ACTION_TIME_TICK)) { mDateTimeTextView.setText(getCurrentTimeStamp()); } } };

Here, you create a BroadcastReceiver that sets the date and time on the screen if it receives a time change broadcast from the system. You use getCurrentTimeStamp(), which is a utility method in your activity, to return the current date and time.

Note: If you’re not familiar with BroadcastReceivers, you should refer to the Android Developer documentation.

Next add the following methods to MainActivity underneath onCreate()

@Override protected void onResume() { // 1 super.onResume(); // 2 mDateTimeTextView.setText(getCurrentTimeStamp()); // 3 registerReceiver(mTickReceiver, new IntentFilter(Intent.ACTION_TIME_TICK)); }   @Override protected void onPause() { // 4 super.onPause(); // 5 if (mTickReceiver != null) { try { unregisterReceiver(mTickReceiver); } catch (IllegalArgumentException e) { Log.e(TAG, "Timetick Receiver not registered", e); } } }

Here you do a few things:

1. You call onResume() on the superclass.
2. You set the date and time TextView with the current value.
3. You register the broadcast receiver in onResume(). This ensures it will receive the broadcasts for ACTION_TIME_TICK. These are sent every minute after the time changes.
4. You call onPause() on the superclass.
5. You unregister the broadcast receiver in onPause(), so the app no longer receives the time change broadcasts. This cuts down unnecessary system overhead.

Build and run the app. Now you should now see the current date and time at the top of the screen. Even if you navigate to the add task screen and come back, the time still gets updated.

Persisting State

Every to-do list is good at remembering what you need to do, except for your friend Forget Me Not. Unfortunately, the app is quite forgetful at the moment. See it for yourself.
Open the app and follow these steps.

1. Press Add a Task.
2. Enter “Replace regular with decaf in the breakroom” as the task description and press Done. You’ll see your new task in the list.
3. Close the app from the recent apps.
4. Open the app again.

You can see that it forgot about your evil plans.

Persisting Data Between Launches

Open MainActivity, and add the following variables to the top of the class:

private final String PREFS_TASKS = "prefs_tasks"; private final String KEY_TASKS_LIST = "list";

And add the following underneath the rest of your activity lifecycle methods.

@Override protected void onStop() { super.onStop();   // Save all data which you want to persist. StringBuilder savedList = new StringBuilder(); for (String s : mList) { savedList.append(s); savedList.append(","); } getSharedPreferences(PREFS_TASKS, MODE_PRIVATE).edit() .putString(KEY_TASKS_LIST, savedList.toString()).commit(); }

Here you build a comma separated string with all the task descriptions in your list, and then you save the string to SharedPreferences in the onStop() callback. As mentioned earlier, implementing this should be a best practice because it commits any unsaved changes.
Add the following to onCreate() below the existing mList initialization code:

String savedList = getSharedPreferences(PREFS_TASKS, MODE_PRIVATE).getString(KEY_TASKS_LIST, null); if (savedList != null) { String[] items = savedList.split(","); mList = new ArrayList<String>(Arrays.asList(items)); }

Here you read the saved list from the SharedPreferences and initialize mList by converting the retrieved comma separated string to an ArrayList.

Note: Here you used SharedPreferences since you were only saving primitive data types. For more complex data you can use a variety of storage options available on Android.

Now, build and run the app. Add a task, close and reopen the app. Do you see a difference? You are now able to retain tasks in your list!

Configuration Changes

You need the ability to delete entries from Forget Me Not.
Open MainActivity, and at the bottom of the class add:

private void taskSelected(final int position) { // 1 AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(MainActivity.this);   // 2 alertDialogBuilder.setTitle(R.string.alert_title);   // 3 alertDialogBuilder .setMessage(mList.get(position)) .setPositiveButton(R.string.delete, new DialogInterface.OnClickListener() { public void onClick(DialogInterface dialog, int id) { mList.remove(position); mAdapter.notifyDataSetChanged(); } }) .setNegativeButton(R.string.cancel, new DialogInterface.OnClickListener() { public void onClick(DialogInterface dialog, int id) { dialog.cancel(); } });   // 4 AlertDialog alertDialog = alertDialogBuilder.create();   // 5 alertDialog.show(); }

In a nutshell, you’re creating and showing an alert dialog when you select a task from the list. Here is the step-by-step explanation:

1. You create an AlertDialog.Builder which facilitates the creation of an AlertDialog.
2. You set the alert dialog title.
3. You set the alert dialog message to be the description of the selected task.Then you also implement the PositiveButton to remove the item from the list and refresh it, and the NegativeButton to dismiss the dialog.
4. You create the alert dialog from the AlertDialog.Builder object.
5. You display the alert dialog to the user.

Add the following to onItemClick(...) in the OnItemClickListener of the listView in onCreate().

taskSelected(i);

It’s good practice to keep text you want in your app separate from the code. The reason is so that you can easily change it, which is especially useful for text that you use in multiple places. It’s also handy for those times you need to translate your app into another language.
To do this open values/strings.xml and within the resources element add:

<string name="alert_title">Task</string> <string name="delete">Delete</string> <string name="cancel">Cancel</string>

Build and run the app. Tap on one of the tasks. You’ll see an alert dialog with options to CANCEL or DELETE the task from the list:

Open the app and tap on a task to trigger the alert dialog. Now rotate the device, and make sure you have rotation in the device settings set to auto-rotate.
As soon as you rotate the device, the alert dialog is dismissed. This makes for an unreliable, undesirable user experience. Users don’t like it when things just vanish from their screen without reason, so you need to make it so the user must explicitly dismiss the dialog.

Handling Configuration Changes

Configuration changes, such as rotation, keyboard visibility and so on, cause an activity to shut down and restart. You can find the full list of system events that cause an activity to be recreated here.
There are a couple of ways you can handle a configuration change.
One way is to add the following line to the AndroidManifest.xml, just below MainActivity’s android:name in the activity element. It should now look like this:

android:configChanges="orientation|screenSize">

Here, you declare that your MainActivity will handle any configuration changes that arise from a change in orientation or screen size. This simple line prevents a restart of your activity by the system, and it passes the work to MainActivity.
There’s one more place to modify the code so the app will handle the configuration change in MainActivity. Add the following method at the bottom of the class:

@Override public void onConfigurationChanged(Configuration newConfig) { super.onConfigurationChanged(newConfig); }

Here you’re just calling the superclass’s onConfigurationChanged() method since you’re not updating or resetting any elements based on screen rotation.
onConfigurationChanged() is passed into a configuration object that contains the updated device configuration.
You can determine the new configuration and make appropriate changes to update the resources used in your interface by reading fields in the configuration.
Now, build and run the app. Follow steps 1-2 described above again. This time, that dialog stays in place until you dismiss it.

An alternative way to handle configuration changes is to retain a stateful object that’s carried forward to the recreated instance of your activity. You can accomplish this by implementing the onSaveInstanceState() callback.
When you do this, the system saves your activity’s state in a bundle, and you can restore it when you initiate the corresponding onRestoreInstanceState() callback. However, the bundle is not designed to hold large data sets such as bitmaps, and it can only store data that is serializable.
The downside of the stateful object solution is that serializing and deserializing the data during a configuration change can come at a high cost. It can consume a lot of memory and slow down the activity restart process.
In such instances, retaining a fragment is currently the most preferable way to handle a configuration change. You can learn more about fragments and how to use them to retain information when your activity is restarted in our Android Fragments Tutorial .

Where To Go From Here?

Congratulations! You have just learned the basics of using activities in Android and now have an excellent understanding of the ever-important activity lifestyle.
You covered quite a few concepts, including:

• How to create an activity
• How to stop an activity
• How to persist data when an activity stops
• How to work around a configuration changes

You can download the completed project here. If you’re still hungry for more, check out Google’s documentation.
I hope you enjoyed this introduction to Android activities tutorial, and if you have any questions, comments or awesome modifications to this project app please join the forum discussion and comment below!