How can Android source code not have a main method

Question

I've seen this in a few tutorials now... but how in the world can Android source code not have a main method and still run.

For example (from http://developer.android.com/guide/tutorials/hello-world.html):

public class HelloAndroid extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }
}

That runs but there is no main!!!

I've also thought that using things like onCreate (or formLoad, etc.) was bad becuase a constructor should do that work and such built-in methods can be smelly sometimes. But onCreate is an entry point? Even without a main?

What if there is more than one activity... is there a hierarchy to these built in event handlers? OnCreate trumps everything else? Otherwise, how would the app know what to run or where to enter the program?

Thanks!

Answer 1

I think that Jonathon's answer is going in the right direction. He says the OS expects a certain structure. There's a name for that structure which is a "state machine". In this case Android calls it the "activity lifecycle". Rob gives a link to the documentation which contains an important diagram of that state machine though the text is a bit dry. A quick search also found me the following link that explains it fairly clearly: http://www.android-app-market.com/android-activity-lifecycle.html