page.title=Starting Another Activity parent.title=Building Your First App parent.link=index.html trainingnavtop=true page.tags=intents helpoutsWidget=true @jd:body

This lesson teaches you to

  1. Respond to the Send Button
  2. Build an Intent
  3. Create the Second Activity
  4. Receive the Intent
  5. 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 MyActivity that starts a new activity when the user clicks the Send button.

Respond to the Send Button

  1. In Android Studio, from the res/layout directory, edit the activity_my.xml file.
  2. To the {@link android.widget.Button <Button>} element, add the {@code android:onClick} attribute.

    res/layout/activity_my.xml

    <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.

  3. In the java/com.mycompany.myfirstapp directory, open the MyActivity.java file.
  4. Within the MyActivity class, add the {@code sendMessage()} method stub shown below.

    java/com.mycompany.myfirstapp/MyActivity.java

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

    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

  1. In MyActivity.java, inside the {@code sendMessage()} method, create an {@link android.content.Intent} to start an activity called {@code DisplayMessageActivity} with the following code:

    java/com.mycompany.myfirstapp/MyActivity.java

    public void sendMessage(View view) {
  Intent intent = new Intent(this, DisplayMessageActivity.class);
}

    Intents

    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. For more information, see Intents and Intent Filters.

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

    The constructor used here takes two parameters:

    Android Studio indicates that you must import the {@link android.content.Intent} class.

  2. At the top of the file, import the {@link android.content.Intent} class:

    java/com.mycompany.myfirstapp/MyActivity.java

    import android.content.Intent;

    Tip: In Android Studio, press Alt + Enter (option + return on Mac) to import missing classes.

  3. Inside the {@code sendMessage()} method, use {@link android.app.Activity#findViewById findViewById()} to get the {@link android.widget.EditText} element.

    java/com.mycompany.myfirstapp/MyActivity.java

    public void sendMessage(View view) {
  Intent intent = new Intent(this, DisplayMessageActivity.class);
  EditText editText = (EditText) findViewById(R.id.edit_message);
}
  4. At the top of the file, import the {@link android.widget.EditText} class.

    In Android Studio, press Alt + Enter (option + return on Mac) to import missing classes.

  5. Assign the text to a local message variable, and use the {@link android.content.Intent#putExtra putExtra()} method to add its text value to the intent.

    java/com.mycompany.myfirstapp/MyActivity.java

    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);
}

    An {@link android.content.Intent} can carry 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.

  6. At the top of the {@code MyActivity} class, add the {@code EXTRA_MESSAGE} definition as follows:

    java/com.mycompany.myfirstapp/MyActivity.java

    public class MyActivity extends ActionBarActivity {
    public final static String EXTRA_MESSAGE = "com.mycompany.myfirstapp.MESSAGE";
    ...
}

    For the next activity to query the extra data, you should define the key for your intent's extra using a public constant. It's generally a good practice to define keys for intent extras using your app's package name as a prefix. This ensures the keys are unique, in case your app interacts with other apps.

  7. In the {@code sendMessage()} method, to finish the intent, call the {@link android.app.Activity#startActivity startActivity()} method, passing it the {@link android.content.Intent} object created in step 1.

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

java/com.mycompany.myfirstapp/MyActivity.java

/** 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);
}

The system receives this call and starts an instance of the {@link android.app.Activity} specified by the {@link android.content.Intent}. Now you need to create the {@code DisplayMessageActivity} class in order for this to work.

Create the Second Activity

All subclasses of {@link android.app.Activity} must implement the {@link android.app.Activity#onCreate onCreate()} method. This method is where the activity receives the intent with the message, then renders the message. Also, the {@link android.app.Activity#onCreate onCreate()} method must define the activity layout with the {@link android.app.Activity#setContentView setContentView()} method. This is where the activity performs the initial setup of the activity components.

Create a new activity using Android Studio

Figure 1. The new activity wizard in Android Studio.

Android Studio includes a stub for the {@link android.app.Activity#onCreate onCreate()} method when you create a new activity.

  1. In Android Studio, in the java directory, select the package, com.mycompany.myfirstapp, right-click, and select New > Activity > Blank Activity.
  2. In the Choose options window, fill in the activity details:

    Click Finish.

  3. Open the {@code DisplayMessageActivity.java} file.

    The class already includes an implementation of the required {@link android.app.Activity#onCreate onCreate()} method. You will update the implementation of this method later. It also includes an implementation of {@link android.app.Activity#onOptionsItemSelected onOptionsItemSelected()}, which handles the action bar's Up behavior. Keep these two methods as they are for now.

  4. Remove the {@link android.app.Activity#onCreateOptionsMenu onCreateOptionsMenu()} method.

    You won't need it for this app.

If you're developing with Android Studio, you can run the app now, but not much happens. Clicking the Send button starts the second activity, but it uses a default "Hello world" layout provided by the template. You'll soon update the activity to instead display a custom text view.

Create the activity without Android Studio

If you're using a different IDE or the command line tools, do the following:

  1. Create a new file named {@code DisplayMessageActivity.java} in the project's src/ directory, next to the original {@code MyActivity.java} file.
  2. Add the following code to the file: 
    public class DisplayMessageActivity extends ActionBarActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_display_message);

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
        }
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() { }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                  Bundle savedInstanceState) {
              View rootView = inflater.inflate(R.layout.fragment_display_message,
                      container, false);
              return rootView;
        }
    }
}

    Note: If you are using an IDE other than Android Studio, your project does not contain the {@code activity_display_message} layout that's requested by {@link android.app.Activity#setContentView setContentView()}. That's OK because you will update this method later and won't be using that layout.

  3. To your {@code strings.xml} file, add the new activity's title as follows: 
    <resources>
    ...
    <string name="title_activity_display_message">My Message</string>
</resources>
  4. In your manifest file, AndroidManifest.xml, within the Application element, add the {@code <activity>} element for your {@code DisplayMessageActivity} class, as follows: 
    <application ... >
    ...
    <activity
        android:name="com.mycompany.myfirstapp.DisplayMessageActivity"
        android:label="@string/title_activity_display_message"
        android:parentActivityName="com.mycompany.myfirstapp.MyActivity" >
        <meta-data
            android:name="android.support.PARENT_ACTIVITY"
            android:value="com.mycompany.myfirstapp.MyActivity" />
    </activity>
</application>

The {@code android:parentActivityName} attribute declares the name of this activity's parent activity within the app's logical hierarchy. The system uses this value to implement default navigation behaviors, such as Up navigation on Android 4.1 (API level 16) and higher. You can provide the same navigation behaviors for older versions of Android by using the Support Library and adding the {@code <meta-data>} element as shown here.

Note: Your Android SDK should already include the latest Android Support Library, which you installed during the Adding SDK Packages step. When using the templates in Android Studio, the Support Library is automatically added to your app project (you can see the library's JAR file listed under Android Dependencies). If you're not using Android Studio, you need to manually add the library to your project—follow the guide for setting up the Support Library then return here.

If you're using a different IDE than Android Studio, don't worry that the app won't yet compile. You'll soon update the activity to display a custom text view.

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 the intent.

  1. In the java/com.mycompany.myfirstapp directory, edit the {@code DisplayMessageActivity.java} file.
  2. In the {@link android.app.Activity#onCreate onCreate()} method, remove the following line: 
      setContentView(R.layout.activity_display_message);
  3. Get the intent and assign it to a local variable. 
    Intent intent = getIntent();
  4. At the top of the file, import the {@link android.content.Intent} class.

    In Android Studio, press Alt + Enter (option + return on Mac) to import missing classes.

  5. Extract the message delivered by {@code MyActivity} with the {@link android.content.Intent#getStringExtra getStringExtra()} method. 
    String message = intent.getStringExtra(MyActivity.EXTRA_MESSAGE);

Display the Message

  1. In the {@link android.app.Activity#onCreate onCreate()} method, create a {@link android.widget.TextView} object. 
    TextView textView = new TextView(this);
  2. Set the text size and message with {@link android.widget.TextView#setText setText()}. 
    textView.setTextSize(40);
textView.setText(message);
  3. 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()}. 
    setContentView(textView);
  4. At the top of the file, import the {@link android.widget.TextView} class.

    In Android Studio, press Alt + Enter (option + return on Mac) to import missing classes.

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(MyActivity.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 4.4.

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

To learn more, follow the link below to the next class.