Starting Another Activity Android

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.

Inside the sendMessage() method, create an Intent to start an activity called DisplayMessageActivity:

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

This requires that you import the Intent class:

import android.content.Intent;

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

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)

Sending an intent to other apps

The intent created in this lesson is what’s considered an explicit intent, because the Intent specifies the exact app component to which the intent should be given. However, intents can also be implicit, in which case the 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 Intentparameters. For more information, see the class about Interacting with Other Apps.

Note: The reference to 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. Inside thesendMessage() method, use findViewById() to get theEditText element and add its text value to the intent:

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

Note: You now need an import statement forandroid.widget.EditText. You’ll define the EXTRA_MESSAGE constant in a moment.

An Intent can carry a collection of various 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.

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

publicclassMainActivityextendsActionBarActivity{
    publicfinalstaticString 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, call startActivity() and pass it your Intent. The system receives this call and starts an instance of the Activity specified by the Intent.

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

/** Called when the user clicks the Send button */
publicvoid sendMessage(View view){
    Intent intent =newIntent(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 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 theAndroid folder and select Android Activity. Click Next.
  3. Select BlankActivity and click Next.
  4. Fill in the activity details:
    • Project: MyFirstApp
    • Activity Name: DisplayMessageActivity
    • Layout Name: activity_display_message
    • Fragment Layout Name: fragment_display_message
    • Title: My Message
    • Hierarchial Parent: com.example.myfirstapp.MainActivity
    • Navigation Type: None

    Click Finish.

If you’re using a different IDE or the command line tools, create a new file namedDisplayMessageActivity.java in the project’s src/ directory, next to the original MainActivity.java file.

Open the DisplayMessageActivity.java file. If you used Eclipse to create this activity:

  • The class already includes an implementation of the required onCreate() method. You will update the implementation of this method later.
  • There’s also an implementation of the onCreateOptionsMenu() method, but you won’t need it for this app so you can remove it.
  • There’s also an implementation of onOptionsItemSelected() which handles the behavior for the action bar’s Up behavior. Keep this one the way it is.
  • There’s also a PlaceholderFragment class that extends Fragment. You will not need this class in the final version of this activity.

Fragments decompose application functionality and UI into reusable modules. For more information on fragments, see the Fragments API Guide. The final version of this activity does not use fragments.

Note: Your activity may look different if you did not use the latest version of the ADT plugin. Make sure you install the latest version of the ADT plugin to complete this tutorial.

The DisplayMessageActivity class should now look like this:

publicclassDisplayMessageActivityextendsActionBarActivity{

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

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

    @Override
    publicboolean 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){
            returntrue;
        }
        returnsuper.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    publicstaticclassPlaceholderFragmentextendsFragment{

        publicPlaceholderFragment(){}

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

If you used an IDE other than Eclipse, update your DisplayMessageActivity class with the above code.

All subclasses of Activity must implement the onCreate() method. The system calls this when creating a new instance of the activity. This method is where you must define the activity layout with thesetContentView() method and is where you should perform initial setup for the activity components.

Note: If you are using an IDE other than Eclipse, your project does not contain theactivity_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.

Add the title string

If you used Eclipse, you can skip to the next section, because the template provides the title string for the new activity.

If you’re using an IDE other than Eclipse, add the new activity’s title to the strings.xml file:

<resources>
    ...
    <stringname="title_activity_display_message">My Message</string>
</resources>

Add it to the manifest

All activities must be declared in your manifest file, AndroidManifest.xml, using an <activity> element.

When you use the Eclipse tools to create the activity, it creates a default entry. If you’re using a different IDE, you need to add the manifest entry yourself. It should look like this:

<application ... >
    ...
    <activity
        android:name="com.example.myfirstapp.DisplayMessageActivity"
        android:label="@string/title_activity_display_message"
        android:parentActivityName="com.example.myfirstapp.MainActivity">
        <meta-data
            android:name="android.support.PARENT_ACTIVITY"
            android:value="com.example.myfirstapp.MainActivity"/>
    </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. It’s included with the ADT Bundle but if you’re using a different IDE, you should have installed it during the Adding Platforms and Packages step. When using the templates in Eclipse, 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 Eclipse, 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 developing with Eclipse, 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, so if you’re using a different IDE, don’t worry that the app won’t yet compile.

Receive the Intent

Every Activity is invoked by an Intent, regardless of how the user navigated there. You can get the Intentthat started your activity by calling getIntent() and retrieve the data contained within it.

In the DisplayMessageActivity class’s onCreate() method, get the intent and extract the message delivered by MainActivity:

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

Display the Message

To show the message on the screen, create a TextView widget and set the text using setText(). Then add theTextView as the root view of the activity’s layout by passing it to setContentView().

The complete onCreate() method for DisplayMessageActivity now looks like this:

@Override
publicvoid 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 =newTextView(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.

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