staticDemo5.java

package Basics;

// https://www.youtube.com/watch?v=U_ho3BRDwsE&list=PLmOn9nNkQxJH0qBIrtV6otI0Ep4o2q67A&index=321

/** static attr can be shared by ALL instances */
public class staticDemo5 {
  public static void main(String[] args) {

    // run
    Circle c1 = new Circle();
    Circle c2 = new Circle();

    System.out.println("c1.id = " + c1.getId()); // 1001
    System.out.println("c2.id = " + c2.getId()); // 1002

    System.out.println("total instances = " + Circle.getTotal());

    Circle c3 = new Circle();

    System.out.println("total instances = " + Circle.getTotal());

    Circle c4 = new Circle(9.99);
    System.out.println("c4's area = " + c4.findArea());

    System.out.println("total instances = " + Circle.getTotal());
  }
}

class Circle {
  // static attr
  private static int total; // how many instances created
  private static int init = 1001; // multiple instances share this attr
  // attr
  private double radius;
  private int id;

  // constructor
  public Circle() {
    // first time : 1001
    // second time : 1002
    // ....
    id = init++;

    // first time : 0
    // second time : 1
    // ....
    total++;
  }

  public Circle(double radius) {
    // don't forget below
    // V1
    // this();
    // V2
    id = id++;
    total++;
    this.radius = radius;
  }

  /** Note : getter for total is static method (since total is static attr) */
  public static int getTotal() {
    return total;
  }

  public double getRadius() {
    return radius;
  }

  // getter, setter
  public void setRadius(double radius) {
    this.radius = radius;
  }

  public int getId() {
    return id;
  }

  public void setId(int id) {
    this.id = id;
  }

  // method
  public double findArea() {
    return 3.14 * radius * radius;
  }
}