java - 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 gear, speed, seatHeight;
    public String color;

    //constructor
    public Bicycle(int gear, int speed, int seatHeight, String color) {
        gear = 0;
        speed = 0;
        seatHeight = 0;
        color ="Unknown";      
    }

    //getters and setters
    public int getGear() {
        return gear;
    }
    public void setGear(int Gear) {
        this.gear = Gear;
    }

    public int getSpeed() {
        return speed;
    }
    public void setSpeed(int Speed){
        this.speed = Speed;
    }

    public int getSeatHeight() {
        return seatHeight;
    }
    public void setSeatHeight(int SeatHeight) {
        this.seatHeight = SeatHeight;
    }

    public String getColor() {
        return color;
    }
    public void setColor(String Color) {
        this.color = Color;
    }

 }//end class



public class Variable extends Bicycle {

    public Variable(int gear, int speed, int seatHeight, String color) {
        super(gear, speed, seatHeight, color);

    }

}//end class


public class Tester {

    public static void main(String args[]){


       Bicycle bicycle1 = new Bicycle(0, 0, 0, null);
       bicycle1.setColor("red");
       System.out.println("Color: "+bicycle1.getColor());
       bicycle1.setSeatHeight(4);
       System.out.println("Seat Height: "+bicycle1.getSeatHeight());
       bicycle1.setSpeed(10);
       System.out.println("Speed: "+bicycle1.getSpeed());
       bicycle1.setGear(6);
       System.out.println("Gear: "+bicycle1.getGear());

       System.out.println("");//space

       Bicycle bicycle2 = new Bicycle(0, 0, 0, null);
       bicycle2.setColor("black");
       System.out.println("Color: "+bicycle2.getColor());
       bicycle2.setSeatHeight(6);
       System.out.println("Seat Height: "+bicycle2.getSeatHeight());
       bicycle2.setSpeed(12);
       System.out.println("Speed: "+bicycle2.getSpeed());
       bicycle2.setGear(6);
       System.out.println("Gear: "+bicycle2.getGear());

       System.out.println("");//space

    }//end method
 }//end class

The class variable is to be used to keep count of the number of instances of the Bicycle class created and the tester class creates a number of instances of the Bicycle class and demonstrates the workings of the Bicycle class and the class variable. I've looked all over the internet and I can't seem to find anything, could someone show me how to do it please, thanks in advance :)

Answer 1

Since static variables are initialized only once, and they're shared between all instances, you can:

class MyClass {

    private static int counter;

    public MyClass() {
        //...
        counter++;
    }

    public static int getNumOfInstances() {
        return counter;
    }
}

and to access the static field counter you can use MyClass.getNumOfInstances()

Read more about static fields in the JLS - 8.3.1.1. static Fields:

If a field is declared static, there exists exactly one incarnation of the field, no matter how many instances (possibly zero) of the class may eventually be created. A static field, sometimes called a class variable, is incarnated when the class is initialized (§12.4).

^{Note that counter is implicitly set to zero}