How to Count Number of Instances of a Class

Question

Can anyone tell me how to count the number of instances of a class?

Here\'s my code

public class Bicycle {

    //instance variables
    public int          


        

Answer 1

In addition, you should override finalize method to decrement the counter

public class Bicycle {
...
    public static int instances = 0;

    {
        ++instances; //separate counting from constructor
    }
...
    public Bicycle(int gear, int speed, int seatHeight, String color) {
        gear = 0;
        speed = 0;
        seatHeight = 0;
        color ="Unknown";
    }

    @Override
    protected void finalize() {
        super.finalize();
        --instances;
    }

}

You should have in mind that static variables are CLASS scoped (there is no one for each instance, only one per class)

Then, you could demonstrate instance decrement with:

...
System.out.println("Count:" + Bicycle.getNumOfInstances()); // 2
bicycle1 = null;
bicycle2 = null;
System.gc(); // not guaranteed to collect but it will in this case
Thread.sleep(2000); // you expect to check again after some time
System.out.println("Count again:" + Bicycle.getNumOfInstances()); // 0