OnJava8-Examples/polymorphism/ReferenceCounting.java

// polymorphism/ReferenceCounting.java
// <20>2016 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://mindviewinc.com/Books/OnJava/ for more book information.
// Cleaning up shared member objects.

class Shared {
  private int refcount = 0;
  private static long counter = 0;
  private final long id = counter++;
  public Shared() {
    System.out.println("Creating " + this);
  }
  public void addRef() { refcount++; }
  protected void dispose() {
    if(--refcount == 0)
      System.out.println("Disposing " + this);
  }
  @Override
  public String toString() { return "Shared " + id; }
}

class Composing {
  private Shared shared;
  private static long counter = 0;
  private final long id = counter++;
  public Composing(Shared shared) {
    System.out.println("Creating " + this);
    this.shared = shared;
    this.shared.addRef();
  }
  protected void dispose() {
    System.out.println("disposing " + this);
    shared.dispose();
  }
  @Override
  public String toString() { return "Composing " + id; }
}

public class ReferenceCounting {
  public static void main(String[] args) {
    Shared shared = new Shared();
    Composing[] composing = { new Composing(shared),
      new Composing(shared), new Composing(shared),
      new Composing(shared), new Composing(shared) };
    for(Composing c : composing)
      c.dispose();
  }
}
/* Output:
Creating Shared 0
Creating Composing 0
Creating Composing 1
Creating Composing 2
Creating Composing 3
Creating Composing 4
disposing Composing 0
disposing Composing 1
disposing Composing 2
disposing Composing 3
disposing Composing 4
Disposing Shared 0
*/