public class Main {

    public static void main(String[] args) throws InterruptedException {
        ReferenceQueue<Bar> objectReferenceQueue = new ReferenceQueue<>();
        PhantomReference<Bar> phantomReference = new PhantomReference<Bar>(new Bar(), objectReferenceQueue);
        while (true){
            System.out.println(objectReferenceQueue.poll());
            System.out.println(phantomReference.get());
            Thread.sleep(500);
        }
    }
}

class Bar {
    int counter = 0;

    @Override
    public void finalize() throws Throwable {
        System.out.println("finalize");
        super.finalize();
    }
}

public class Main {

    public static void main(String[] args) throws InterruptedException {
        ReferenceQueue<Bar> objectReferenceQueue = new ReferenceQueue<>();
        PhantomReference<Bar> phantomReference = new PhantomReference<Bar>(new Bar(), objectReferenceQueue);
        while (true) {
            Reference<? extends Bar> reference = objectReferenceQueue.poll();
            System.out.println("poll(): " + reference);
            if (reference != null) {
                System.out.println("poll().get(): " + reference.get());
                return;
            }

            System.gc();
            Thread.sleep(500);
        }
    }
}

class Bar {
    int counter = 777;

    @Override
    public void finalize() throws Throwable {
        System.out.println("finalize");
        super.finalize();
    }
}

PhantomReference<Bar> phantomReference = new PhantomReference<Bar>(new Bar(), objectReferenceQueue);

objectReferenceQueue.poll().clear() 

 WeakReference<Bar> ref = new WeakReference<Bar>(new Bar(),weakReferenceQueue);

class Registry {
     private Set registeredObjects = new HashSet();

     public void register(Object object) {
         registeredObjects.add( new WeakReference(object) );
     }
 }

public class SimpleGCExample {
    public static void main(String[] args) throws InterruptedException {
        ReferenceQueue<Object> queue = new ReferenceQueue<>();
        SimpleGCExample e = new SimpleGCExample();
        Reference<Object> pRef = new PhantomReference<>(e, queue),
                wRef = new WeakReference<>(e, queue);
        e = null;
        for (int count = 0, collected = 0; collected < 2; ) {
            Reference ref = queue.remove(100);
            if (ref == null) {
                System.gc();
                count++;
            } else {
                collected++;
                System.out.println((ref == wRef ? "weak" : "phantom")
                        + " reference enqueued after " + count + " gc polls");
            }
        }
    }

    @Override
    protected void finalize() throws Throwable {
        System.out.println("finalizing the object in " + Thread.currentThread());
        Thread.sleep(100);
        System.out.println("done finalizing.");
    }
}