CountDownTimer with WeakReferences

CountDownTimer.java

package codekata.observer;

import java.lang.ref.WeakReference;
import java.util.LinkedList;
import java.util.List;

public class CountDownTimer {
    private static long ONE_SECOND = 1000L;
    private long countDownMillis;
    private long elapsedTimeSinceLastTick;
    private long totalElapsedTime;
    private List<WeakReference<CountDownObserver>> observers = new LinkedList<>();

    public CountDownTimer(long initialValue) {
        this.countDownMillis = initialValue;
    }

    //Called every frame
    public void update(long deltaTime) {
        if (isFinished()) {
            return;
        }

        if (totalElapsedTime == 0) {
            notifyStart();
        }

        totalElapsedTime += deltaTime;
        elapsedTimeSinceLastTick += deltaTime;
        if (elapsedTimeSinceLastTick >= ONE_SECOND) {
            elapsedTimeSinceLastTick = 0;
            notifyCountDown();
        }

        if (isFinished()) {
            notifyCountDown();
            notifyFinish();
        }
    }

    private boolean isFinished() {
        return totalElapsedTime >= countDownMillis;
    }

    private void notifyCountDown() {
        for(WeakReference<CountDownObserver> ref : this.observers) {
            CountDownObserver observer = ref.get();
            if (observer != null) {
                observer.onCountDown(
                        totalElapsedTime,
                        Math.max(0, (countDownMillis - totalElapsedTime)));
            } else {
                System.out.println("!!! DELETED");
            }
        }
    }

    private void notifyStart() {
        for(WeakReference<CountDownObserver> ref : this.observers) {
            CountDownObserver observer = ref.get();
            if (observer != null) {
                observer.onStart();
            }
        }
    }

    private void notifyFinish() {
        for(WeakReference<CountDownObserver> ref : this.observers) {
            CountDownObserver observer = ref.get();
            if (observer != null) {
                observer.onFinish();
            }
        }
    }

    public void registerObserver(CountDownObserver observer) {
        WeakReference<CountDownObserver> weakReference = findWeakReference(observer);
        if (weakReference == null) {
            weakReference = new WeakReference<>(observer);
            this.observers.add(weakReference);
        }
    }

    public void unregisterObserver(CountDownObserver observer) {
        WeakReference<CountDownObserver> weakReference = findWeakReference(observer);
        if (weakReference != null) {
            this.observers.remove(weakReference);
        }
    }

    private WeakReference<CountDownObserver> findWeakReference(CountDownObserver observer) {
        WeakReference<CountDownObserver> weakReference = null;
        for(WeakReference<CountDownObserver> ref : this.observers) {
            if (ref.get() == observer) {
                weakReference = ref;
            }
        }
        return weakReference;
    }
}