Android Training/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 toMyActivity
that starts a new activity when the user clicks the Send button.
Respond to the Send Button
- In Android Studio, from the
res/layout
directory, edit thecontent_my.xml
file. - Add the
android:onClick
attribute to the<Button>
element.- res/layout/content_my.xmlThe
<Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="@string/button_send" android:onClick="sendMessage" />
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.
- res/layout/content_my.xml
- In the
java/com.mycompany.myfirstapp
directory, open theMyActivity.java
file. - Within the
MyActivity
class, add thesendMessage()
method stub shown below.- java/com.mycompany.myfirstapp/MyActivity.javaIn order for the system to match this method to the method name given to
/** Called when the user clicks the Send button */ public void sendMessage(View view) { // Do something in response to button }
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 theView
that was clicked)
- java/com.mycompany.myfirstapp/MyActivity.java
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
MyActivity.java
, inside thesendMessage()
method, create anIntent
to start an activity calledDisplayMessageActivity
with the following code:- java/com.mycompany.myfirstapp/MyActivity.java
public void sendMessage(View view) { Intent intent = new Intent(this, DisplayMessageActivity.class); }
- 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 theActivity
class is a subclass ofContext
) - The
Class
of the app component to which the system should deliver theIntent
(in this case, the activity that should be started) Android Studio indicates that you must import theIntent
class.
- java/com.mycompany.myfirstapp/MyActivity.java
- 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.
- java/com.mycompany.myfirstapp/MyActivity.java
- Inside the
sendMessage()
method, usefindViewById()
to get theEditText
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); }
- java/com.mycompany.myfirstapp/MyActivity.java
- At the top of the file, import the
EditText
class. In Android Studio, press Alt + Enter (option + return on Mac) to import missing classes. - Assign the text to a local
message
variable, and use theputExtra()
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. TheputExtra()
method takes the key name in the first parameter and the value in the second parameter.
- java/com.mycompany.myfirstapp/MyActivity.java
- At the top of the
MyActivity
class, add theEXTRA_MESSAGE
definition as follows:- java/com.mycompany.myfirstapp/MyActivity.java
public class MyActivity extends AppCompatActivity { 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.
- java/com.mycompany.myfirstapp/MyActivity.java
- In the
sendMessage()
method, to finish the intent, call thestartActivity()
method, passing it theIntent
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.
Portions of this page are modifications based on work created and shared by the Android Open Source Project and used according to terms described in the Creative Commons 2.5 Attribution License.