View source for Android Training/Starting Another Activity/en

<languages />
After completing the [[Spezial:MyLanguage/Android Training/Building a Simple User Interface|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 <code>MyActivity</code> that starts a new activity when the user clicks the Send button.

== Respond to the Send Button ==
# In Android Studio, from the <code>res/layout</code> directory, edit the <code>content_my.xml</code> file.
# Add the <code>[http://developer.android.com/reference/android/view/View.html#attr_android:onClick android:onClick]</code> attribute to the <code>[http://developer.android.com/reference/android/widget/Button.html <Button>]</code> element.
#: ''res/layout/content_my.xml''<syntaxhighlight lang="java">
<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/button_send"
    android:onClick="sendMessage" />
</syntaxhighlight>
#: The <code>android:onClick</code> attribute’s value, <code>"sendMessage"</code>, is the name of a method in your activity that the system calls when the user clicks the button.
# In the <code>java/com.mycompany.myfirstapp</code> directory, open the <code>MyActivity.java</code> file.
# Within the <code>MyActivity</code> class, add the <code>sendMessage()</code> method stub shown below.
#: ''java/com.mycompany.myfirstapp/MyActivity.java''<syntaxhighlight lang="java">
/** Called when the user clicks the Send button */
public void sendMessage(View view) {
    // Do something in response to button
}
</syntaxhighlight>
#: In order for the system to match this method to the method name given to <code>android:onClick</code>, the signature must be exactly as shown. Specifically, the method must:
#* Be public
#* Have a void return value
#* Have a <code>[http://developer.android.com/reference/android/view/View.html View]</code> as the only parameter (this will be the <code>View</code> 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 ==
# In <code>MyActivity.java</code>, inside the <code>sendMessage()</code> method, create an <code>Intent</code> to start an activity called <code>DisplayMessageActivity</code> with the following code:
#: ''java/com.mycompany.myfirstapp/MyActivity.java''<syntaxhighlight lang="java">
public void sendMessage(View view) {
    Intent intent = new Intent(this, DisplayMessageActivity.class);
}
</syntaxhighlight>
#: '''Note:''' The reference to <code>DisplayMessageActivity</code> 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 <code>Context</code> as its first parameter (<code>this</code> is used because the <code>Activity</code> class is a subclass of <code>Context</code>)
#* The <code>Class</code> of the app component to which the system should deliver the <code>Intent</code> (in this case, the activity that should be started) Android Studio indicates that you must import the <code>Intent</code> class.
# At the top of the file, import the <code>Intent</code> class:
#: ''java/com.mycompany.myfirstapp/MyActivity.java''<syntaxhighlight lang="java">
import android.content.Intent;
</syntaxhighlight>
#: '''Tip:''' In Android Studio, press Alt + Enter (option + return on Mac) to import missing classes.
# Inside the <code>sendMessage()</code> method, use <code>findViewById()</code> to get the <code>EditText</code> element.
#: ''java/com.mycompany.myfirstapp/MyActivity.java''<syntaxhighlight lang="java">
public void sendMessage(View view) {
    Intent intent = new Intent(this, DisplayMessageActivity.class);
    EditText editText = (EditText) findViewById(R.id.edit_message);
}
</syntaxhighlight>
# At the top of the file, import the <code>EditText</code> class. In Android Studio, press Alt + Enter (option + return on Mac) to import missing classes.
# Assign the text to a local <code>message</code> variable, and use the <code>putExtra()</code> method to add its text value to the intent.
#: ''java/com.mycompany.myfirstapp/MyActivity.java''<syntaxhighlight lang="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);
}
</syntaxhighlight>
#: An <code>Intent</code> can carry data types as key-value pairs called ''extras''. The <code>putExtra()</code> method takes the key name in the first parameter and the value in the second parameter.
# At the top of the <code>MyActivity</code> class, add the <code>EXTRA_MESSAGE</code> definition as follows:
#: ''java/com.mycompany.myfirstapp/MyActivity.java''<syntaxhighlight lang="java">
public class MyActivity extends AppCompatActivity {
    public final static String EXTRA_MESSAGE = "com.mycompany.myfirstapp.MESSAGE";
    ...
}
</syntaxhighlight>
#: 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.
# In the <code>sendMessage()</code> method, to finish the intent, call the <code>startActivity()</code> method, passing it the <code>Intent</code> object created in step 1.

With this new code, the complete <code>sendMessage()</code> method that's invoked by the Send button now looks like this:
''java/com.mycompany.myfirstapp/MyActivity.java''<syntaxhighlight lang="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);
}
</syntaxhighlight>
The system receives this call and starts an instance of the <code>Activity</code> specified by the <code>Intent</code>. Now you need to create the <code>DisplayMessageActivity</code> class in order for this to work.

== Create the Second Activity ==
All subclasses of <code>[http://developer.android.com/reference/android/app/Activity.html Activity]</code> must implement the <code>[http://developer.android.com/reference/android/app/Activity.html#onCreate(android.os.Bundle) onCreate()]</code> method. This method is where the activity receives the intent with the message, then renders the message. Also, the <code>onCreate()</code> method must define the activity layout with the <code>setContentView()</code> method. This is where the activity performs the initial setup of the activity components.

=== Create a new activity using Android Studio ===
Android Studio includes a stub for the <code>onCreate()</code> method when you create a new activity. The ''New Android Activity'' window appears.
# In Android Studio, in the <code>java</code> directory, select the package, '''com.mycompany.myfirstapp''', right-click, and select '''New > Activity > Blank Activity'''.
# 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'''.
# Open the <code>DisplayMessageActivity.java</code> file. The class already includes an implementation of the required <code>onCreate()</code> method. You update the implementation of this method later.
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:
# Create a new file named <code>DisplayMessageActivity.java</code> in the project's <code>src/</code> directory, next to the original<code>MyActivity.java</code> file.
# Add the following code to the file:
#: <syntaxhighlight lang="java">
public class DisplayMessageActivity extends AppCompatActivity {

    @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 app bar item clicks here. The app bar
        // automatically handles 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;
        }
    }
}
</syntaxhighlight>
#: '''Note:''' If you are using an IDE other than Android Studio, your project does not contain the <code>activity_display_message</code> layout that's requested by <code>[http://developer.android.com/reference/android/app/Activity.html#setContentView(android.view.View) setContentView()]</code>. That's OK because you will update this method later and won't be using that layout.
# To your <code>strings.xml</code> file, add the new activity's title as follows:
#: <syntaxhighlight lang="xml">
<resources>
    ...
    <string name="title_activity_display_message">My Message</string>
</resources>
</syntaxhighlight>
# In your manifest file, <code>AndroidManifest.xml</code>, within the <code>Application</code> element, add the <code>[http://developer.android.com/guide/topics/manifest/activity-element.html <activity>]</code> element for your <code>DisplayMessageActivity</code> class, as follows:
#: <syntaxhighlight lang="xml">
<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>
</syntaxhighlight>
The <code>[http://developer.android.com/guide/topics/manifest/activity-element.html#parent android:parentActivityName]</code> 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 navigationon Android 4.1 (API level 16) and higher. You can provide the same navigation behaviors for older versions of Android by using the [http://developer.android.com/tools/support-library/index.html Support Library] and adding the <code>[http://developer.android.com/guide/topics/manifest/meta-data-element.html <meta-data>]</code> element as shown here.

'''Note:''' Your Android SDK should already include the latest Android Support Library, which you installed during the [http://developer.android.com/sdk/installing/adding-packages.html 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 [http://developer.android.com/tools/support-library/setup.html 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 <code>Activity</code> is invoked by an <code>[[Intent]]</code>, regardless of how the user navigated there. You can get the <code>Intent</code>that started your activity by calling <code>[http://developer.android.com/reference/android/app/Activity.html#getIntent() getIntent()]</code> and retrieve the data contained within the intent.
# In the <code>java/com.mycompany.myfirstapp</code> directory, edit the <code>DisplayMessageActivity.java</code> file.
# Get the intent and assign it to a local variable.
#: <syntaxhighlight lang="java">
Intent intent = getIntent();
</syntaxhighlight>
# At the top of the file, import the <code>[http://developer.android.com/reference/android/content/Intent.html Intent]</code> class.  In Android Studio, press Alt + Enter (option + return on Mac) to import missing classes.
# Extract the message delivered by <code>MyActivity</code> with the <code>[http://developer.android.com/reference/android/content/Intent.html#getStringExtra(java.lang.String) getStringExtra()]</code> method.
#: <syntaxhighlight lang="java">
String message = intent.getStringExtra(MyActivity.EXTRA_MESSAGE);
</syntaxhighlight>
== Display the Message ==
# In the res/layout directory, edit the <code>content_display_message.xml</code> file.
# Add an <code>android:id</code> attribute to the <code>RelativeLayout</code>. You need this attribute to reference the object from your app code.
#: <syntaxhighlight lang="xml">
< RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
...
android:id="@+id/content">
</RelativeLayout>
</syntaxhighlight>
# Switch back to editing <code>DisplayMessageActivity.java</code>.
# In the <code>onCreate()</code> method, create a <code>TextView</code> object.
#: <syntaxhighlight lang="java">
TextView textView = new TextView(this);
</syntaxhighlight>
# Set the text size and message with <code>[http://developer.android.com/reference/android/widget/TextView.html#setText(char&#x5B;&#x5D;,&#x20;int,&#x20;int) setText()]</code>.
#: <syntaxhighlight lang="java">
textView.setTextSize(40);
textView.setText(message);
</syntaxhighlight>
# Add the <code>TextView</code> to the <code>[http://developer.android.com/reference/android/widget/RelativeLayout.html RelativeLayout]</code> identified by <code>R.id.content</code>.
#: <syntaxhighlight lang="java">
RelativeLayout layout = (RelativeLayout) findViewById(R.id.content);
layout.addView(textView);
</syntaxhighlight>
# At the top of the file, import the <code>[http://developer.android.com/reference/android/widget/TextView.html TextView]</code> class. In Android Studio, press Alt + Enter (option + return on Mac) to import missing classes.

The complete <code>onCreate()</code> method for <code>DisplayMessageActivity</code> now looks like this:
<syntaxhighlight lang="java">
@Override
protected void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   setContentView(R.layout.activity_display_message);
   Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
   setSupportActionBar(toolbar);

   FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
   fab.setOnClickListener(new View.OnClickListener() {
       @Override
       public void onClick(View view) {
           Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                   .setAction("Action", null)
                   .show();
       }
   });
   getSupportActionBar().setDisplayHomeAsUpEnabled(true);

   Intent intent = getIntent();
   String message = intent.getStringExtra(MyActivity.EXTRA_MESSAGE);
   TextView textView = new TextView(this);
   textView.setTextSize(40);
   textView.setText(message);

   RelativeLayout layout = (RelativeLayout) findViewById(R.id.content);
   layout.addView(textView);
</syntaxhighlight>

You can now run the app. When it opens, type a message in the text field, and click '''Send'''. The second activity replaces the first one on the screen, showing the message you entered in the first activity.

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

{{TNT|Android_Training/Attribution}}