Countdown.java

public class CountDown implements Runnable
{
    private JavaPlugin plugin;
    private int countdown = 0;
    private int threadId  = 0;

    private final Map<Integer, List<CountdownCallback>> callbacks = new HashMap<>();

    public CountDown(JavaPlugin plugin, int countdown)
    {
        this.plugin = plugin;
        this.countdown = countdown;
    }

    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object's
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p/>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see Thread#run()
     */
    @Override
    public void run()
    {
        if (this.threadId == 0 || this.threadId == -1)
        {
            return;
        }

        if (this.callbacks.containsKey(countdown))
        {
            for (CountdownCallback countdownCallback : this.callbacks.get(this.countdown))
            {
                countdownCallback.onTick(this.countdown);
            }

        }

        this.countdown--;

        if (this.countdown == -1)
        {
            Bukkit.getScheduler().cancelTask(this.threadId);
        }
    }

    public void start() throws Exception
    {
        this.threadId = Bukkit.getScheduler().scheduleSyncRepeatingTask(this.plugin, this, 0, 20);

        if (this.threadId == -1)
        {
            Exception exception = new Exception("Can't start thread");
            StackTraceElement stackTraceElement = new StackTraceElement("CountDown.class", "start", "CountDown.java", 61);
            StackTraceElement[] stackTraceElements = new StackTraceElement[1];
            stackTraceElements[0] = stackTraceElement;
            exception.setStackTrace(stackTraceElements);
            throw exception;
        }
    }

    public void addCallback(int seconds, CountdownCallback countdownCallback)
    {
        if (!this.callbacks.containsKey(seconds))
        {
            this.callbacks.put(seconds, new ArrayList<CountdownCallback>());
        }

        this.callbacks.get(seconds).add(countdownCallback);
    }
}

CountdownCallback.java

public interface CountdownCallback
{
    void onTick(int secondsRemaining);
}