static variables

[mark juszczec]

HI folks

I had a bit of an epiphany and want to double check I got it right.

All objects created in:

public static void main(String...args){

}


are themselves static, right?

Mark

[John Stark]

Do you mean like this:

class Main
{
	public static void main(String[] args)
	{
		Main m = new Main();	  
	}
}

?

I would say the variable m is a local variable in main(). The created object is eligible to garbage collection after the method has finished if no other references have been created which are still alive. I think this is the same in a non-static method so there is nothing particular to objects created in a static method.

John

[mark juszczec]

My question came from this problem in Sierra and Bates:

class Beta { }

class Alpha {
   static Beta b1;
   Beta b2;
}

public class Tester {
   public static void main(String[] args) {
        Beta b1 = new Beta();
        Beta b2 = new Beta();
        Alpha a1 = new Alpha();
        Alpha a2 = new Alpha();
        a1.b1 = b1;
        a1.b2 = b1;
        a2.b2 = b2;
        a1 = null; b1 = null; b2 = null;
    }
}

The correct answer says, in part:

What might be less clear is that you can still access the other Beta object through the static variable a2.b1 because it's static.

and my question is what the heck makes a2.b1 static?

[John Stark]

The irritating thing here is that the instance variable a2 is used to access the static variable a2.b1. The compiler will replace the name of the instance variable a2 by the name of the class (Alpha). Therefore a2.b1 could be written as Alpha.b1. So b1 is a static variable of the class Alpha. The expression a2.b1 has nothing to do with the object referenced by a2. In fact you could even do a2 = null; and a2.b1 will still work.

John

[Paul Clapham]

mark juszczec wrote:All objects created in:

public static void main(String...args){

}


are themselves static, right?

No. There is no such thing as a "static object". Only variables and methods can be declared as static. Did you have some particular feature in mind when you made that statement?

[Paul Clapham]

mark juszczec wrote:and my question is what the heck makes a2.b1 static?

This declaration makes the "b1" variable static:

static Beta b1;