Starting Another Activity

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 <Button> element, add the 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 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 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 android:onClick, the signature must be exactly as shown. Specifically, the method must:

   • Be public
   • Have a void return value
   • Have a View as the only parameter (this will be the View that was clicked)

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 sendMessage() method, create an Intent to start an activity called 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 Intent is an object that provides runtime binding between separate components (such as two activities). The 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 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:

   • A Context as its first parameter (this is used because the Activity class is a subclass of Context)
   • The Class of the app component to which the system should deliver the Intent (in this case, the activity that should be started)

   Android Studio indicates that you must import the Intent class.

2. At the top of the file, import the 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 sendMessage() method, use findViewById() to get the 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 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 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 Intent can carry data types as key-value pairs called extras. The putExtra() method takes the key name in the first parameter and the value in the second parameter.

6. At the top of the MyActivity class, add the 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 sendMessage() method, to finish the intent, call the startActivity() method, passing it the Intent object created in step 1.

With this new code, the complete 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 Activity specified by the Intent. Now you need to create the DisplayMessageActivity class in order for this to work.

Create the Second Activity

All subclasses of Activity must implement the onCreate() method. This method is where the activity receives the intent with the message, then renders the message. Also, the onCreate() method must define the activity layout with the 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 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:
   • Activity Name: DisplayMessageActivity
   • Layout Name: activity_display_message
   • Title: My Message
   • Hierarchical Parent: com.mycompany.myfirstapp.MyActivity
   • Package name: com.mycompany.myfirstapp

   Click Finish.

3. Open the DisplayMessageActivity.java file.

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

4. Remove the 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 DisplayMessageActivity.java in the project's src/ directory, next to the original 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 activity_display_message layout that's requested by setContentView(). That's OK because you will update this method later and won't be using that layout.

3. To your 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 <activity> element for your 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 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 <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 Activity is invoked by an Intent, regardless of how the user navigated there. You can get the Intent that started your activity by calling getIntent() and retrieve the data contained within the intent.

1. In the java/com.mycompany.myfirstapp directory, edit the DisplayMessageActivity.java file.
2. In the 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 Intent class.

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

5. Extract the message delivered by MyActivity with the getStringExtra() method.

   String message = intent.getStringExtra(MyActivity.EXTRA_MESSAGE);
   

Display the Message

1. In the onCreate() method, create a TextView object.

   TextView textView = new TextView(this);
   

2. Set the text size and message with setText().

   textView.setTextSize(40);
   textView.setText(message);
   

3. Then add the TextView as the root view of the activity’s layout by passing it to setContentView().

   setContentView(textView);
   

4. At the top of the file, import the TextView class.

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

The complete onCreate() method for 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.