#Weak Object Pool

Problem

Did you ever want to track a bunch of objects without referencing them directly in order not to disturb their natural lifecycle using weak references? Have you ever tried to extend Java objects during run-time?

Solution

WeakObjectPool: A pool for holding and extending [weak referenced objects](http://download.oracle.com/javase/1,5.0/docs/api/java/lang/ref/WeakReference.html" title="WeakReference JavaDoc). How does this differ from Java's WeakHashMap? Several key differences are there, that effecitvely define WeakObjectPool:

1. WeakHashMap has a weak keys not values, like WeakObjectPool.
2. Its a pool not a map, otherwise also known as a multi-map.
3. WeakObjectPool allows you to store additional information about a weakly referenced object in the pool for the duration of the lifecycle of the object (a decorator).

#Usage:

No dependencies required.

new WeakObjectPool<KEY, VALUE, DECORATOR>()

Example:

/* Create the pool. 
     KEY type is String. 
	 VALUE type is Integer (tracked object).
	 DECORATOR type is String 
*/
WeakObjectPool<String, Integer, String> pool = new WeakObjectPool<>();

// make integer object x
Integer x = new Integer(314);
		
// store x in pool with extra info (String)
pool.add("x", x, "X was created on "+ new Date().getTime());
		
// get x out of the pool
System.out.println("Object x = "+ pool.get("x").get(0).getObject());
		System.out.println(pool.get("x").get(0).getInfo());

Console output:

Object x = 314
X was created on 1321807672613

Now if x is not referenced anymore and the object x was erased by the garbage collector the object will automatically disappear from the pool (that's what weak referenced means). To demonstrate:

// get rid of all x references
x=null;
// make sure it's completely gone
System.gc();
System.runFinalization();

// proof it's gone from the pool
System.out.println("References found to x = "+ pool.get("x").size());

Console output:

References found to x = 0. 

Background

This code is pretty old (written in 2007 with Java 5). I'd recommend having a look at Gauva to see if any of their collection types fit your needs. The decoration part of this code still seems to be unique though.