Winner of Garbage Collection Quiz

Congratulations to Evan Rosson for his correct answer which I have reposted here for posterity. The only real difference between my answer and Evan’s is that I didn’t pass in the current thread to the Object Tracker thread and I only used the non-timeout version of ReferenceQueue.remove(). I also didn’t use ReferenceQueue.poll in case you wanted to throw that in there somehow. How did my program still work? 5 and a half points for the first person to answer.

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

class WarmupQuestion {
    private static ArrayList list = track(new ArrayList());
    static public void main(String[] args) throws Exception {
        track(new WarmupQuestion());
        System.gc();
        Integer i = track(150);
        track(new int[1024 * 1024]);
        System.gc();
        track("Hello");
        i = null;
        System.gc();
    }

    public void finalize() {
        list.add(this);
    }

    public static <T> T track(T o) {
        return ObjectTracker.track(o);
    }

    static class TrackedObject {
        private StackTraceElement[] mStack = new Exception().getStackTrace();
        private Reference mPhantomRef;
        private Reference mWeakRef;
        private Class<?> mCls;
        private Date mCreated = new Date();

        public TrackedObject(Object obj, ReferenceQueue allrefs) {
            mPhantomRef = new PhantomReference(obj, allrefs);
            mWeakRef = new WeakReference(obj);
            mCls = obj.getClass();
        }

        public Reference getRef() {
            return mPhantomRef;
        }

        private void trace() {
            for (int i = 3; i < mStack.length; i++)
                System.out.println(mStack[i].toString());
        }

        public void die(Date killed) {
            System.out.println(String.format(
                    "object of type %s lived for %d ms", mCls, killed.getTime()
                            - mCreated.getTime()));
            trace();
        }

        public void die() {
            String format = (mWeakRef.get() == null)
                ? "object of type %s has a finalize that raises the dead."
                : "object of type %s lived until program exit.";
            System.out.println(String.format(format, mCls));
            trace();
        }
    }

    static class ObjectTracker extends Thread {
        private static ReferenceQueue mRefs = new ReferenceQueue();
        private static Map<Reference, TrackedObject> mTracked =
            new HashMap<Reference, TrackedObject>();
        private Thread mMain;

        public ObjectTracker(Thread thread) {
            mMain = thread;
        }

        public static <T> T track(T o) {
            TrackedObject tracked = new TrackedObject(o, mRefs);
            mTracked.put(tracked.getRef(), tracked);
            return o;
        }

        public void run() {
            try {
                Reference o = null;
                while (mMain.isAlive() || null != (o = mRefs.remove(1000))) {
                    if (o != null) {
                        mTracked.remove(o).die(new Date());
                    }
                }
                for (TrackedObject tracked : mTracked.values()) {
                    tracked.die();
                }
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        }
    }

    static {
        new ObjectTracker(Thread.currentThread()).start();
    }
}