How to count the number of all instances of a class and their respective subclasses?

Question

I have a class A, where A extends into B and in C. I have a counter of instances in A.

public Class A {

...

   private int nInstancias = 0;

   ...

   public A(){
      nInstancias+=1;
      ...
   }

   ...

   public int getnInstancias() {
       return nInstancias;
   }

   ...

}

However, whenever I print the nInstancias with its getNInstancias, always gives 0.

What I’m doing wrong and how can I increase the instacing nInstancias A or any subclass of A?

It is also necessary to change the nInstances in the subclass builder?

Answer 1

Try to change the variable to static, so its value will be independent of the instance to which it is linked:

private static int nInstancias = 0;

and within the builder of A:

public A(){
   nInstancias++;
   ...
}

Note: Since the other classes extend from A, the counter will increment each time an instance of B and C is created as well. See the test:

public  class ClasseA {

    private static int instances = 0;

    public ClasseA() {
        instances++;
    }

    public static int getNumberInstances(){
        return instances;
    }

    public static void main(String[] args) {

        System.out.println("Instanciando ClasseA...");
        ClasseA a = new ClasseA();
        System.out.println("No. de instancias: " + ClasseA.getNumberInstances());

        System.out.println("Instanciando ClasseB...");
        ClasseB b = new ClasseB();
        System.out.println("No. de instancias: " + ClasseA.getNumberInstances());

        System.out.println("Instanciando ClasseC...");
        ClasseC c = new ClasseC();
        System.out.println("No. de instancias: " + ClasseA.getNumberInstances());
    }
}


class ClasseB extends ClasseA {

}

class ClasseC extends ClasseA{

}

Upshot:

Instanciando ClasseA...
No. de instancias: 1
Instanciando ClasseB...
No. de instancias: 2
Instanciando ClasseC...
No. de instancias: 3

Working: https://ideone.com/gPyfaD

Reference:

• How to Count Number of Instances of a Class

• What is the use of a static or final variable in java?