Question

A.java:

public class A {
    public static StartActivity startActivity;
}

B.java:

public class StartActivity extends Activity  
    public void onCreate(Bundle savedInstanceState) {
        A.activity = this;
    }
}

From the 2 java files above, I would like to ask what is the meaning of a.activity = this;?
Do that mean create a new instance of activity?

Answer 1

Do that mean create a new instance of activity?

No. activity is a field, not an class, and you can’t create an “instance” of a variable, just of classes. All that code is doing is assigning an already existing object (the current b object) to a’s static field, activity, that’s all.

Edit
Correction, I suppose activity is also a class since you have both a type and a variable given the same name, activity? Your naming is non-standard which is confusing the issue and your question greatly. I suggest you change your question and make the naming standard and non-confusing. All types including classes, interfaces and enums should begin with an uppercase letter and all variables with a lowercase letter.

Edit 2
OK, after reviewing your latest code in your edited question, we see that A has a static StartActivity field, also known as a static variable, called Activity. Inside of class B’s onCreate(...) method it assigns this, the reference to the current B object itself, to A’s activity field. So if activity was assigned to a prior StartActivity object, that assignment has now been changed to the current B object if its onCreate(...) method is called.