Start another activity

When you complete the previous lesson, you have an app that shows an activity that consists of a single screen with a text field and a Send button. In this lesson, you add some code to the MainActivity that starts a new activity to display a message when the user taps the Send button.

Note: This lesson assumes that you use Android Studio v3.0 or higher.

Respond to the Send button

Follow these steps to add a method to the MainActivity class that's called when the Send button is tapped:

1. In the file app > java > com.example.myfirstapp > MainActivity, add the following sendMessage() method stub:

   Kotlin

   class MainActivity : AppCompatActivity() {
       override fun onCreate(savedInstanceState: Bundle?) {
           super.onCreate(savedInstanceState)
           setContentView(R.layout.activity_main)
       }
   
       /** Called when the user taps the Send button */
       fun sendMessage(view: View) {
           // Do something in response to button
       }
   }
   

   Java

   public class MainActivity extends AppCompatActivity {
       @Override
       protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.activity_main);
       }
   
       /** Called when the user taps the Send button */
       public void sendMessage(View view) {
           // Do something in response to button
       }
   }
   

   You might see an error because Android Studio cannot resolve the View class used as the method argument. To clear the error, click the View declaration, place your cursor on it, and then press Alt+Enter, or Option+Enter on a Mac, to perform a Quick Fix. If a menu appears, select Import class.

2. Return to the activity_main.xml file to call the method from the button:
   1. Select the button in the Layout Editor.
   2. In the Attributes window, locate the onClick property and select sendMessage [MainActivity] from its drop-down list.

   Now when the button is tapped, the system calls the sendMessage() method.

   Take note of the details in this method. They're required for the system to recognize the method as compatible with the android:onClick attribute. Specifically, the method has the following characteristics:

   • Public access.
   • A void or, in Kotlin, an implicit unit return value.
   • A View as the only parameter. This is the View object you clicked at the end of Step 1.
3. Next, fill in this method to read the contents of the text field and deliver that text to another activity.

Build an intent

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 in this lesson, your intent starts another activity.

In MainActivity, add the EXTRA_MESSAGE constant and the sendMessage() code, as shown:

Kotlin

const val EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE"

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
    }

    /** Called when the user taps the Send button */
    fun sendMessage(view: View) {
        val editText = findViewById<EditText>(R.id.editText)
        val message = editText.text.toString()
        val intent = Intent(this, DisplayMessageActivity::class.java).apply {
            putExtra(EXTRA_MESSAGE, message)
        }
        startActivity(intent)
    }
}

Java

public class MainActivity extends AppCompatActivity {
    public static final String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    /** Called when the user taps the Send button */
    public void sendMessage(View view) {
        Intent intent = new Intent(this, DisplayMessageActivity.class);
        EditText editText = (EditText) findViewById(R.id.editText);
        String message = editText.getText().toString();
        intent.putExtra(EXTRA_MESSAGE, message);
        startActivity(intent);
    }
}

Expect Android Studio to encounter Cannot resolve symbol errors again. To clear the errors, press Alt+Enter, or Option+Return on a Mac. Your should end up with the following imports:

Kotlin

import androidx.appcompat.app.AppCompatActivity
import android.content.Intent
import android.os.Bundle
import android.view.View
import android.widget.EditText

Java

import androidx.appcompat.app.AppCompatActivity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;

An error still remains for DisplayMessageActivity, but that's okay. You fix it in the next section.

Here's what's going on in sendMessage():

• The Intent constructor takes two parameters, a Context and a Class.

  The Context parameter is used first because the Activity class is a subclass of Context.

  The Class parameter of the app component, to which the system delivers the Intent, is, in this case, the activity to start.

• The putExtra() method adds the value of EditText to the intent. An Intent can carry data types as key-value pairs called extras.

  Your key is a public constant EXTRA_MESSAGE because the next activity uses the key to retrieve the text value. It's a good practice to define keys for intent extras with your app's package name as a prefix. This ensures that the keys are unique, in case your app interacts with other apps.

• The startActivity() method starts an instance of the DisplayMessageActivity that's specified by the Intent. Next, you need to create that class.

Note: The Navigation Architecture Component allows you to use the Navigation Editor to associate one activity with another. Once the relationship is made, you can use the API to start the second activity when the user triggers the associated action, such as when the user clicks a button. To learn more, see Navigation.

Create the second activity

To create the second activity, follow these steps:

1. In the Project window, right-click the app folder and select New > Activity > Empty Activity.
2. In the Configure Activity window, enter "DisplayMessageActivity" for Activity Name. Leave all other properties set to their defaults and click Finish.

Android Studio automatically does three things:

• Creates the DisplayMessageActivity file.
• Creates the layout file activity_display_message.xml, which corresponds with the DisplayMessageActivity file.
• Adds the required <activity> element in AndroidManifest.xml.

If you run the app and tap the button on the first activity, the second activity starts but is empty. This is because the second activity uses the empty layout provided by the template.

Add a text view

Figure 1. The text view centered at the top of the layout.

The new activity includes a blank layout file. Follow these steps to add a text view to where the message appears:

1. Open the file app > res > layout > activity_display_message.xml.
2. Click Enable Autoconnection to Parent in the toolbar. This enables Autoconnect. See figure 1.
3. In the Palette panel, click Text, drag a TextView into the layout, and drop it near the top-center of the layout so that it snaps to the vertical line that appears. Autoconnect adds left and right constraints in order to place the view in the horizontal center.
4. Create one more constraint from the top of the text view to the top of the layout, so that it appears as shown in figure 1.

Optionally, you can make some adjustments to the text style if you expand textAppearance in the Common Attributes panel of the Attributes window, and change attributes such as textSize and textColor.

Display the message

In this step, you modify the second activity to display the message that was passed by the first activity.

1. In DisplayMessageActivity, add the following code to the onCreate() method:

   Kotlin

   override fun onCreate(savedInstanceState: Bundle?) {
       super.onCreate(savedInstanceState)
       setContentView(R.layout.activity_display_message)
       
       // Get the Intent that started this activity and extract the string
       val message = intent.getStringExtra(EXTRA_MESSAGE)
   
       // Capture the layout's TextView and set the string as its text
       val textView = findViewById<TextView>(R.id.textView).apply {
           text = message
       }
   }
   

   Java

   @Override
   protected void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.activity_display_message);
       
       // Get the Intent that started this activity and extract the string
       Intent intent = getIntent();
       String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);
   
       // Capture the layout's TextView and set the string as its text
       TextView textView = findViewById(R.id.textView);
       textView.setText(message);
   }
   

2. Press Alt+Enter, or Option+Return on a Mac, to import these other needed classes:

   Kotlin

   import androidx.appcompat.app.AppCompatActivity
   import android.content.Intent
   import android.os.Bundle
   import android.widget.TextView
   

   Java

   import androidx.appcompat.app.AppCompatActivity;
   import android.content.Intent;
   import android.os.Bundle;
   import android.widget.TextView;
   

Add upward navigation

Each screen in your app that's not the main entry point, which are all the screens that aren't the home screen, must provide navigation that directs the user to the logical parent screen in the app's hierarchy. To do this, add an Up button in the app bar.

To add an Up button, you need to declare which activity is the logical parent in the AndroidManifest.xml file. Open the file at app > manifests > AndroidManifest.xml, locate the <activity> tag for DisplayMessageActivity, and replace it with the following:

<activity android:name=".DisplayMessageActivity"
          android:parentActivityName=".MainActivity">
    <!-- The meta-data tag is required if you support API level 15 and lower -->
    <meta-data
        android:name="android.support.PARENT_ACTIVITY"
        android:value=".MainActivity" />
</activity>

The Android system now automatically adds the Up button to the app bar.

Run the app

Click Apply Changes in the toolbar to run the app. When it opens, type a message in the text field and tap Send to see the message appear in the second activity.

Figure 2. App opened, with text entered on the left screen and displayed on the right.

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

To continue to learn the basics about Android app development, go back to Build your first app and follow the other links provided there.