basics/firstapp/starting-activity.jd

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


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<div id="tb-wrapper">
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<h2>This lesson teaches you to</h2>

<ol>
  <li><a href="#RespondToButton">Respond to the Send Button</a></li>
  <li><a href="#BuildIntent">Build an Intent</a></li>
  <li><a href="#StartActivity">Start the Second Activity</a></li>
  <li><a href="#CreateActivity">Create the Second Activity</a>
    <ol>
      <li><a href="#AddToManifest">Add it to the manifest</a></li>
    </ol>
  </li>
  <li><a href="#ReceiveIntent">Receive the Intent</a></li>
  <li><a href="#DisplayMessage">Display the Message</a></li>
</ol>

<h2>You should also read</h2>

<ul>
  <li><a href="{@docRoot}sdk/installing/index.html">Installing the
SDK</a></li>
</ul>


</div>
</div>


<p>After completing the <a href="building-ui.html">previous lesson</a>, you have an app that
shows an activity (a single screen) with a text field and a button. In this lesson, youll add some
code to <code>MainActivity</code> that
starts a new activity when the user clicks the Send button.</p>


<h2 id="RespondToButton">Respond to the Send Button</h2>

<p>To respond to the button's on-click event, open the <code>main.xml</code> layout file and add the
<a
href="{@docRoot}reference/android/view/View.html#attr_android:onClick">{@code android:onClick}</a>
attribute to the {@link android.widget.Button &lt;Button>} element:</p>

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

<p>The <a
href="{@docRoot}reference/android/view/View.html#attr_android:onClick">{@code
android:onClick}</a> attributes value, <code>"sendMessage"</code>, is the name of a method in your
activity that the system calls when the user clicks the button.</p>

<p>Open the <code>MainActivity</code> class and add the corresponding method:</p>

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

<p class="note"><strong>Tip:</strong> In Eclipse, press Ctrl + Shift + O to import missing classes
(Cmd + Shift + O on Mac).</p>

<p>In order for the system to match this method to the method name given to <a
href="{@docRoot}reference/android/view/View.html#attr_android:onClick">{@code android:onClick}</a>,
the signature must be exactly as shown. Specifically, the method must:</p>

<ul>
<li>Be public</li>
<li>Have a void return value</li>
<li>Have a {@link android.view.View} as the only parameter (this will be the {@link
android.view.View} that was clicked)</li>
</ul>

<p>Next, youll fill in this method to read the contents of the text field and deliver that text to
another activity.</p>


<h2 id="BuildIntent">Build an Intent</h2>

<p>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
apps "intent to do something." You can use intents for a wide
variety of tasks, but most often theyre used to start another activity.</p>

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

<pre>
Intent intent = new Intent(this, DisplayMessageActivity.class);
</pre>

<p>The constructor used here takes two parameters:</p>
<ul>
  <li>A {@link
android.content.Context} as its first parameter ({@code this} is used because the {@link
android.app.Activity} class is a subclass of {@link android.content.Context})
  <li>The {@link java.lang.Class} of the app component to which the system should deliver
the {@link android.content.Intent} (in this case, the activity that should be started)
</ul>

<div class="sidebox-wrapper">
<div class="sidebox">
  <h3>Sending an intent to other apps</h3>
  <p>The intent created in this lesson is what's considered an <em>explicit intent</em>, because the
{@link android.content.Intent}
specifies the exact app component to which the intent should be given. However, intents
can also be <em>implicit</em>, 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 <a
href="{@docRoot}training/basics/intents/index.html">Interacting with Other Apps</a>.</p>
</div>
</div>

<p class="note"><strong>Note:</strong> The reference to {@code DisplayMessageActivity}
will raise an error if youre using an IDE such as Eclipse because the class doesnt exist yet.
Ignore the error for now; youll create the class soon.</p>

<p>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:</p>

<pre>
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);
</pre>

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

<p>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:</p>

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

<p>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.</p>


<h2 id="StartActivity">Start the Second Activity</h2>

<p>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}.</p>

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

<pre>
/** 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);
}
</pre>

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


<h2 id="CreateActivity">Create the Second Activity</h2>

<div class="figure" style="width:400px">
<img src="{@docRoot}images/training/firstapp/adt-new-activity.png" alt="" />
<p class="img-caption"><strong>Figure 1.</strong> The new activity wizard in Eclipse.</p>
</div>

<p>To create a new activity using Eclipse:</p>

<ol>
  <li>Click New <img src="{@docRoot}images/tools/eclipse-new.png"
  style="vertical-align:baseline;margin:0" /> in the toolbar.</li>
  <li>In the window that appears, open the <strong>Android</strong> folder
  and select <strong>Android Activity</strong>. Click <strong>Next</strong>.</li>
  <li>Select <strong>BlankActivity</strong> and click <strong>Next</strong>.</li>
  <li>Fill in the activity details:
    <ul>
      <li><em>Project</em>: MyFirstApp</li>
      <li><em>Activity Name</em>: DisplayMessageActivity</li>
      <li><em>Layout Name</em>: activity_display_message</li>
      <li><em>Navigation Type</em>: None</li>
      <li><em>Hierarchial Parent</em>: com.example.myfirstapp.MainActivity</li>
      <li><em>Title</em>: My Message</li>
    </ul>
    <p>Click <strong>Finish</strong>.</p>
  </li>
</ol>

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

<p>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:</p>

<pre>
public class DisplayMessageActivity extends Activity {
    &#64;Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_display_message);
    }
}
</pre>

<p>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.</p>


<h3 id="AddToManifest">Add it to the manifest</h3>

<p>You must declare all activities in your manifest file, <code>AndroidManifest.xml</code>, using an
<a
href="{@docRoot}guide/topics/manifest/activity-element.html">{@code &lt;activity>}</a> element.</p>

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

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

<p>The <a href="{@docRoot}guide/topics/manifest/meta-data-element.html">{@code
  &lt;meta-data>}</a> element declares the name of this activity's parent activity
  within the app's logical hierarchy. The Android <a
href="{@docRoot}tools/extras/support-library.html">Support Library</a> uses this information
  to implement default navigation behaviors, such as <a
        href="{@docRoot}design/patterns/navigation.html">Up navigation</a>.</p>

<p class="note"><strong>Note:</strong> During <a
href="{@docRoot}sdk/installing/adding-packages.html">installation</a>, 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 <em>Android Dependencies</em>). If you're not using
Eclipse, you may need to manually add the library to your project&mdash;follow this guide for <a
href="{@docRoot}tools/extras/support-library.html#SettingUp">setting up the Support Library</a>.</p>

<p>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.</p>


<h2 id="ReceiveIntent">Receive the Intent</h2>

<p>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.</p>

<p>In the {@code DisplayMessageActivity} classs {@link android.app.Activity#onCreate onCreate()}
method, get the intent and extract the message delivered by {@code MainActivity}:</p>

<pre>
Intent intent = getIntent();
String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);
</pre>


<h2 id="DisplayMessage">Display the Message</h2>

<p>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 activitys layout by passing it to {@link
android.app.Activity#setContentView setContentView()}.</p>

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

<pre>
&#64;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);
}
</pre>

<p>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.</p>

<img src="{@docRoot}images/training/firstapp/firstapp.png" />
<p class="img-caption"><strong>Figure 2.</strong> Both activities in the final app, running
on Android 4.0.

<p>That's it, you've built your first Android app!</p>

<p>To learn more about building Android apps, continue to follow the
basic training classes. The next class is <a
href="{@docRoot}training/basics/activity-lifecycle/index.html">Managing the Activity
Lifecycle</a>.</p>