Why is android.app.Activity not abstract by design?

Question

Why is android.app.Activity not abstract by design if documentation/tutorials say that at least its method onCreate() must be implemented.

From http://developer.android.com/guide/components/activities.html

You must implement this method. The system calls this when creating your activity.

Even if it was not mentioned explicitly, without the implementation there is no point in having android.app.Activity object, or? I understand that parrent needs probably to execute the code in own implementation of onCreate() but surely there can be design when part of parent onCreate() would a a call to an abstract call-back method which needs to be implemented by the developer.

Example of this design:

public abstract class Activity {


    public abstract void implementThis(Object o);

    public void onCreate(Object o){
        //Do stuff
        implementThis(o);
        //Do other stuff or end.
    }

}

Am I missing something here? Why is there non-abstract Activity if developer must and needs to subclass it and provide custom implementation?

Answer 1

Read here: http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/1.5_r4/android/app/Activity.java#Activity.onCreate%28android.os.Bundle%29

"The default implementation of this method performs a restore of any view state that had previously been frozen by onSaveInstanceState(android.os.Bundle)."