引用计数器

RefCounting.java

import static com.mceiba.util.Print.*;

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

public class RefCounting{
	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),
			new Composing(shared),
			new Composing(shared)
		};
		for(Composing c: composing) c.dispose();
	}
}