Create a new public class in the same location as the original activity subclass using the tool bar's New Java Class icon or the package's context menu from the Package Explorer, selecting New and then Class.

Name the class MySubActivity or something similar and make sure to complete the Superclass field as seen in the following screenshot:

Next, we need to declare our new activity in the manifest file. Open the AndroidManifest.xml file from the Package Explorer and select the Application tab.

Under Application Nodes, click on the Add... button and then select Activity.

In the panel on the right-hand side, fill in the Name field as .MySubActivity and the Label field as my sub activity.

Open the AndroidManifest.xml tab and check whether these changes are reflected in the XML, which should look similar to the following snippet:

<activity
  android:name=".MySubActivity"
  android:label="my sub activity">
</activity>

Next, we must add a button that the user can click on to switch activities. This is set up through the main.xml file which resides in the res/layout folder in the Package Explorer.

Open the main.xml file and click on the XML tab at the bottom so that the code can be edited.

Add the following <Button> element just after the <TextView> element that was generated automatically:

<Button
  android:text="click to switch activities"
  android:id="@+id/main_activity_button"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content">
</Button>

Now open the original Java activity class, ActivitySwitcher or whatever you called it.

Add the following code to the onCreate() method after the setContentView(R.layout.main); statement, making sure to replace the package and class parameters in the setClassName() call with your own, as they will most likely be different:

Button switchButton = (Button) findViewById(R.id.main_activity_button);

switchButton.setOnClickListener(new OnClickListener() {

  @Override
  public void onClick(View v) {
    Intent intent = new Intent();
    String packageName =
      "com.packtpub.android.activityswitcher";
    String className =
      "com.packtpub.android.activityswitcher.MySubActivity";
    intent.setClassName(packageName, className);
    startActivity(intent);
  }

});

Run the application on a device or emulator. Clicking on the button will now start the sub activity.