page.title=Starting Another Activity parent.title=Building Your First App parent.link=index.html trainingnavtop=true previous.title=Building a Simpler User Interface previous.link=building-ui.html @jd:body

This lesson teaches you to

  1. Respond to the Send Button
  2. Build an Intent
  3. Start the Second Activity
  4. Create the Second Activity
    1. Add it to the manifest
  5. Receive the Intent
  6. Display the Message

You should also read

After completing the previous lesson, you have an app that shows an activity (a single screen) with a text field and a button. In this lesson, you’ll add some code to MainActivity that starts a new activity when the user clicks the Send button.

Respond to the Send Button

To respond to the button's on-click event, open the main.xml layout file and add the {@code android:onClick} attribute to the {@link android.widget.Button <Button>} element:

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/button_send"
    android:onClick="sendMessage" />

The {@code android:onClick} attribute’s value, "sendMessage", is the name of a method in your activity that the system calls when the user clicks the button.

Open the MainActivity class and add the corresponding method:

/** Called when the user clicks the Send button */
public void sendMessage(View view) {
    // Do something in response to button
}

Tip: In Eclipse, press Ctrl + Shift + O to import missing classes (Cmd + Shift + O on Mac).

In order for the system to match this method to the method name given to {@code android:onClick}, the signature must be exactly as shown. Specifically, the method must:

Next, you’ll fill in this method to read the contents of the text field and deliver that text to another activity.

Build an Intent

An {@link android.content.Intent} is an object that provides runtime binding between separate components (such as two activities). The {@link android.content.Intent} represents an app’s "intent to do something." You can use intents for a wide variety of tasks, but most often they’re used to start another activity.

Inside the {@code sendMessage()} method, create an {@link android.content.Intent} to start an activity called {@code DisplayMessageActivity}:

Intent intent = new Intent(this, DisplayMessageActivity.class);

The constructor used here takes two parameters:

Sending an intent to other apps

The intent created in this lesson is what's considered an explicit intent, because the {@link android.content.Intent} specifies the exact app component to which the intent should be given. However, intents can also be implicit, in which case the {@link android.content.Intent} does not specify the desired component, but allows any app installed on the device to respond to the intent as long as it satisfies the meta-data specifications for the action that's specified in various {@link android.content.Intent} parameters. For more information, see the class about Interacting with Other Apps.

Note: The reference to {@code DisplayMessageActivity} will raise an error if you’re using an IDE such as Eclipse because the class doesn’t exist yet. Ignore the error for now; you’ll create the class soon.

An intent not only allows you to start another activity, but it can carry a bundle of data to the activity as well. So, use {@link android.app.Activity#findViewById findViewById()} to get the {@link android.widget.EditText} element and add its text value to the intent:

Intent intent = new Intent(this, DisplayMessageActivity.class);
EditText editText = (EditText) findViewById(R.id.edit_message);
String message = editText.getText().toString();
intent.putExtra(EXTRA_MESSAGE, message);

An {@link android.content.Intent} can carry a collection of various data types as key-value pairs called extras. The {@link android.content.Intent#putExtra putExtra()} method takes the key name in the first parameter and the value in the second parameter.

In order for the next activity to query the extra data, you should define your key using a public constant. So add the {@code EXTRA_MESSAGE} definition to the top of the {@code MainActivity} class:

public class MainActivity extends Activity {
    public final static String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE";
    ...
}

It's generally a good practice to define keys for intent extras using your app's package name as a prefix. This ensures they are unique, in case your app interacts with other apps.

Start the Second Activity

To start an activity, you simply need to call {@link android.app.Activity#startActivity startActivity()} and pass it your {@link android.content.Intent}. The system receives this call and starts an instance of the {@link android.app.Activity} specified by the {@link android.content.Intent}.

With this new code, the complete {@code sendMessage()} method that's invoked by the Send button now looks like this:

/** Called when the user clicks the Send button */
public void sendMessage(View view) {
    Intent intent = new Intent(this, DisplayMessageActivity.class);
    EditText editText = (EditText) findViewById(R.id.edit_message);
    String message = editText.getText().toString();
    intent.putExtra(EXTRA_MESSAGE, message);
    startActivity(intent);
}

Now you need to create the {@code DisplayMessageActivity} class in order for this to work.

Create the Second Activity

Figure 1. The new activity wizard in Eclipse.

To create a new activity using Eclipse:

  1. Click New in the toolbar.
  2. In the window that appears, open the Android folder and select Android Activity. Click Next.
  3. Select BlankActivity and click Next.
  4. Fill in the activity details:

    Click Finish.

If you're using a different IDE or the command line tools, create a new file named {@code DisplayMessageActivity.java} in the project's src/ directory, next to the original {@code MainActivity.java} file.

Open the {@code DisplayMessageActivity.java} file. If you used Eclipse to create it, the class already includes an implementation of the required {@link android.app.Activity#onCreate onCreate()} method. There's also an implementation of the {@link android.app.Activity#onCreateOptionsMenu onCreateOptionsMenu()} method, but you won't need it for this app so you can remove it. The class should look like this:

public class DisplayMessageActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_display_message);
    }
}

All subclasses of {@link android.app.Activity} must implement the {@link android.app.Activity#onCreate onCreate()} method. The system calls this when creating a new instance of the activity. It is where you must define the activity layout and where you should perform initial setup for the activity components.

Add it to the manifest

You must declare all activities in your manifest file, AndroidManifest.xml, using an {@code <activity>} element.

When you use the Eclipse tools to create the activity, it creates a default entry. It should look like this:

<application ... >
    ...
    <activity
        android:name=".DisplayMessageActivity"
        android:label="@string/title_activity_display_message" >
        <meta-data
            android:name="android.support.PARENT_ACTIVITY"
            android:value="com.example.myfirstapp.MainActivity" />
    </activity>
</application>

The {@code <meta-data>} element declares the name of this activity's parent activity within the app's logical hierarchy. The Android Support Library uses this information to implement default navigation behaviors, such as Up navigation.

Note: During installation, you should have downloaded the latest Support Library. Eclipse automatically includes this library in your app project (you can see the library's JAR file listed under Android Dependencies). If you're not using Eclipse, you may need to manually add the library to your project—follow this guide for setting up the Support Library.

The app is now runnable because the {@link android.content.Intent} in the first activity now resolves to the {@code DisplayMessageActivity} class. If you run the app now, clicking the Send button starts the second activity, but it's still using the default "Hello world" layout.

Receive the Intent

Every {@link android.app.Activity} is invoked by an {@link android.content.Intent}, regardless of how the user navigated there. You can get the {@link android.content.Intent} that started your activity by calling {@link android.app.Activity#getIntent()} and retrieve the data contained within it.

In the {@code DisplayMessageActivity} class’s {@link android.app.Activity#onCreate onCreate()} method, get the intent and extract the message delivered by {@code MainActivity}:

Intent intent = getIntent();
String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);

Display the Message

To show the message on the screen, create a {@link android.widget.TextView} widget and set the text using {@link android.widget.TextView#setText setText()}. Then add the {@link android.widget.TextView} as the root view of the activity’s layout by passing it to {@link android.app.Activity#setContentView setContentView()}.

The complete {@link android.app.Activity#onCreate onCreate()} method for {@code DisplayMessageActivity} now looks like this:

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    // Get the message from the intent
    Intent intent = getIntent();
    String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);

    // Create the text view
    TextView textView = new TextView(this);
    textView.setTextSize(40);
    textView.setText(message);

    // Set the text view as the activity layout
    setContentView(textView);
}

You can now run the app. When it opens, type a message in the text field, click Send, and the message appears on the second activity.

Figure 2. Both activities in the final app, running on Android 4.0.

That's it, you've built your first Android app!

To learn more about building Android apps, continue to follow the basic training classes. The next class is Managing the Activity Lifecycle.