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

public class WeakHash {

    public static void main(String[] args) {
	try {
	    ReferenceQueue aReferenceQueue = new ReferenceQueue();
	    Object anObject = new Object();
	    String extraData = new String("Extra Data");
	    WeakHashMap aWeakHashMap = new WeakHashMap();;

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

//Check whether the key is in the hash map
	    System.out.println("aWeakHashMap key: " + aWeakHashMap.containsKey(anObject));
	    System.out.println();

//Check that the key maps to its value
            System.out.println("aWeakhashMap Value: " + aWeakHashMap.get(anObject));

//Clear the strong reference to the key 
//To make it weakly reachable
            anObject = null;
	    System.out.println();

//Run the garbage collector
	    System.out.println("*** running gc...");
	    System.gc();
	    System.out.println();


//Check whether the Key is in the hash map 
            System.out.println("aWeakHashMap key: " + aWeakHashMap.containsKey(anObject));
            System.out.println();

//Check whether the key maps to its value 
	    System.out.println("aWeakHashMap Value: " + aWeakHashMap.get(anObject));


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