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, you’ll add some
code to <code>MyFirstActivity</code> that
starts a new activity when the user selects 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> attribute’s value, <code>sendMessage</code>, is the name of a method in your
activity that you want to call when the user selects the button.</p>

<p>Add the corresponding method inside the <code>MyFirstActivity</code> class:</p>

<pre>
/** Called when the user selects 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>Note that, 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, you’ll 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
app’s "intent to do something." You can use an {@link android.content.Intent} for a wide
variety of tasks, but most often they’re used to start another activity.</p>

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

<p>An intent not only allows you to start another activity, but 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 message 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 a
string as the key and the value in the second parameter.</p>

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

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

<p>It's generally a good practice to define keys for extras with your app's package name as a prefix
to ensure it's 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}.</p>

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

<pre>
/** Called when the user selects 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>

<p>In your project, create a new class file under the <code>src/&lt;package-name&gt;/</code>
directory called <code>DisplayMessageActivity.java</code>.</p>

<p class="note"><strong>Tip:</strong> In Eclipse, right-click the package name under the
<code>src/</code> directory and select <strong>New > Class</strong>.
Enter "DisplayMessageActivity" for the name and {@code android.app.Activity} for the superclass.</p>

<p>Inside the class, add the {@link android.app.Activity#onCreate onCreate()} callback method:</p>

<pre>
public class DisplayMessageActivity extends Activity {
    &#64;Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }
}
</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
initialize essential 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>Because {@code DisplayMessageActivity} is invoked using an explicit intent, it does not require
any intent filters (such as those you can see in the manifest for <code>MyFirstActivity</code>). So
the declaration for <code>DisplayMessageActivity</code> can be simply one line of code inside the <a
href="{@docRoot}guide/topics/manifest/application-element.html">{@code &lt;application>}</a>
element:</p>

<pre>
&lt;application ... >
    &lt;activity android:name="com.example.myapp.DisplayMessageActivity" />
    ...
&lt;/application>
</pre>

<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,
pressing the Send button starts the
second activity, but it doesn't show anything yet.</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 the retrieve data contained
within it.</p>

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

<pre>
Intent intent = getIntent();
String message = intent.getStringExtra(MyFirstActivity.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 activity’s 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(MyFirstActivity.EXTRA_MESSAGE);

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

    setContentView(textView);
}
</pre>

<p>You can now run the app, type a message in the text field, press Send, and view the message on
the second activity.</p>

<img src="{@docRoot}images/training/firstapp/firstapp.png" />
<p class="img-caption"><strong>Figure 1.</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>