Start Another Activity

Start 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 to MainActivitythat starts a new activity to display the message when the user taps Send.

Note: This lesson expects you are using Android Studio 2.3 or higher.

Respond to the send button

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Add a method in MainActivity.java that's called by the button as follows:

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

   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 may see an error because Android Studio cannot resolve the View class used as the method argument. So click to place your cursor on the Viewdeclaration, and then perform a Quick Fix by pressing Alt + Enter (or Option + Enter on Mac). (If a menu appears, select Import class.)
2. Now return to the activity_main.xml file to call this method from the button:
   1. Click to select the button in the Layout Editor.
   2. In the Properties window, locate the onClick property and select sendMessage [MainActivity] from the drop-down list.

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

Take note of the details in this method that are required in order for the system to recognize it as compatible with the android:onClick attribute. Specifically, the method must declare the following:

• Public access
• A void return value
• A View as the only parameter (it is the View object 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

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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.java, add the EXTRA_MESSAGE constant and the sendMessage() code, as shown here:

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);
    }
}

Android Studio will again encounter Cannot resolve symbol errors, so press Alt + Enter (or Option + Return on Mac). Your imports should end up as the following:

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;

An error remains for DisplayMessageActivity, but that's okay; you'll fix that in the next section.

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

• The Intent constructor takes two parameters:
  • A Context as its first parameter (this is used because the Activity class is a subclass of Context)
  • The Class of the app component to which the system should deliver the Intent (in this case, the activity that should be started).
• The putExtra() method adds the EditText's value 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 using your app's package name as a prefix. This ensures the keys are unique, in case your app interacts with other apps.
• The startActivity() method starts an instance of the DisplayMessageActivity specified by the Intent. Now you need to create that class