Determine if Instance of Class exists?

Question

Hello out there :)
could anyone tell me if there is a way to determine if an instance of a specific class exists.
btw my class design does not allow me to use static variables for instance counter or somehting - i am trying to prevent this.
here is my situation:
I have a class in which's constructor i would like to check if an instance of an inherited class exists - of THE inheritet class, that this constructor was called from (i could pass this information via constructor)
and now i only want the constructor to proceed, if there is no instance of this class yet.
thx to everyone who can help me :)

Answer 1

mlk wrote:
Imagist wrote:
Let's see, your root class would be:
public class ClassXY
     private static boolean instanceExists = false;
     public ClassXY() { instanceExists = true; }
     final public static boolean instanceExists() { return instanceExists; }
When a object gets GC'ed, should that still return exists? :)Ah! You're right! So the code would be:
public class ClassXY
     private static boolean instanceCount = 0;
     public ClassXY() { instanceCount++; }
     final public static boolean instanceExists() { return instanceCount > 0; }
     protected void finalize() { instanceCount--; }
}There might be a threading issue with that, too; it's been a while since I dealt with finalization.
One question for the original poster: do you want the constructor to fail if an instance already exists? If that's the case, the code above can be simplified further.