import java.lang.ref.*;
import java.util.*;

public class WeakObj {

    public static void main(String[] args) {
	try {
	    ReferenceQueue aReferenceQueue = new ReferenceQueue();
	    Object anObject = new Object();
	    WeakReference ref = new WeakReference(anObject, aReferenceQueue);
	    String extraData = new String("Extra Data");
	    HashMap aHashMap = new HashMap();;

//Associate extraData (value) with anObject (key) in aHashMap
            aHashMap.put(ref, extraData);

//Check that a reference to an object was created 
	    System.out.println("*** created ref to some object");
	    System.out.println();
	    System.out.println("contents of ref: " + ref.get());
	    System.out.println();

//Check whether the Reference Object is enqueued
	    System.out.println("ref.isEnqueued(): " + ref.isEnqueued());
	    System.out.println();

//Clear the strong reference to anObject 
//To make it weakly reachable
            anObject = null;

//Clear the strong reference to extraData
	    if(anObject == null){
		extraData = null;
	    }

//Run the garbage collector, and
//Check the Reference object's referent 
	    System.out.println("*** running gc...");
	    System.gc();
	    System.out.println();
	    System.out.println("contents of ref: " + ref.get());
	    System.out.println();

//Check whether the Reference Object is enqueued
	    System.out.println("ref.isEnqueued(): " + ref.isEnqueued());
	    System.out.println();

//Enqueue the Reference Object. 
//This method returns false if the Reference Object is already enqueued.
	    System.out.println("enqueued="+ref.enqueue());

	} catch (Exception e) {
	    System.err.println("An exception occurred:");
	    e.printStackTrace();
	}
    }
}