How can I use a java file without registering it as the main activity in the manifest?

[Mena]

I can use JNI to issue a notification using a java file. The only way I can do that is by replacing the activity line in the manifest like so:
From:

<activity android:name="org.qtproject.qt5.android.bindings.QtActivity" >

To the path for the java file:

<activity android:name="org.qtproject.example.notification.NotificationClient" >

or else the notifications won't show up!

What if I want to Use more than one Java file to do other stuff using JNI?
How can I add more than one activity in the manifest?