/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/licenses/publicdomain
 */

package java.util.concurrent;
import java.util.concurrent.atomic.*;
import java.util.*;

/**
 * A {@link ThreadPoolExecutor} that can additionally schedule
 * commands to run after a given delay, or to execute
 * periodically. This class is preferable to {@link java.util.Timer}
 * when multiple worker threads are needed, or when the additional
 * flexibility or capabilities of {@link ThreadPoolExecutor} (which
 * this class extends) are required.
 *
 * <p> Delayed tasks execute no sooner than they are enabled, but
 * without any real-time guarantees about when, after they are
 * enabled, they will commence. Tasks scheduled for exactly the same
 * execution time are enabled in first-in-first-out (FIFO) order of
 * submission.
 *
 * <p>While this class inherits from {@link ThreadPoolExecutor}, a few
 * of the inherited tuning methods are not useful for it. In
 * particular, because it acts as a fixed-sized pool using
 * <tt>corePoolSize</tt> threads and an unbounded queue, adjustments
 * to <tt>maximumPoolSize</tt> have no useful effect. Additionally, it
 * is almost never a good idea to set <tt>corePoolSize</tt> to zero or
 * use <tt>allowCoreThreadTimeOut</tt> because this may leave the pool
 * without threads to handle tasks once they become eligible to run.
 *
 * <p><b>Extension notes:</b> This class overrides {@link
 * AbstractExecutorService} <tt>submit</tt> methods to generate
 * internal objects to control per-task delays and scheduling. To
 * preserve functionality, any further overrides of these methods in
 * subclasses must invoke superclass versions, which effectively
 * disables additional task customization. However, this class
 * provides alternative protected extension method
 * <tt>decorateTask</tt> (one version each for <tt>Runnable</tt> and
 * <tt>Callable</tt>) that can be used to customize the concrete task
 * types used to execute commands entered via <tt>execute</tt>,
 * <tt>submit</tt>, <tt>schedule</tt>, <tt>scheduleAtFixedRate</tt>,
 * and <tt>scheduleWithFixedDelay</tt>.  By default, a
 * <tt>ScheduledThreadPoolExecutor</tt> uses a task type extending
 * {@link FutureTask}. However, this may be modified or replaced using
 * subclasses of the form:
 *
 * <pre>
 * public class CustomScheduledExecutor extends ScheduledThreadPoolExecutor {
 *
 *   static class CustomTask&lt;V&gt; implements RunnableScheduledFuture&lt;V&gt; { ... }
 *
 *   protected &lt;V&gt; RunnableScheduledFuture&lt;V&gt; decorateTask(
 *                Runnable r, RunnableScheduledFuture&lt;V&gt; task) {
 *       return new CustomTask&lt;V&gt;(r, task);
 *   }
 *
 *   protected &lt;V&gt; RunnableScheduledFuture&lt;V&gt; decorateTask(
 *                Callable&lt;V&gt; c, RunnableScheduledFuture&lt;V&gt; task) {
 *       return new CustomTask&lt;V&gt;(c, task);
 *   }
 *   // ... add constructors, etc.
 * }
 * </pre>
 * @since 1.5
 * @author Doug Lea
 */
public class ScheduledThreadPoolExecutor
        extends ThreadPoolExecutor
        implements ScheduledExecutorService {

    /*
     * This class specializes ThreadPoolExecutor implementation by
     *
     * 1. Using a custom task type, ScheduledFutureTask for
     *    tasks, even those that don't require scheduling (i.e.,
     *    those submitted using ExecutorService execute, not
     *    ScheduledExecutorService methods) which are treated as
     *    delayed tasks with a delay of zero.
     *
     * 2. Using a custom queue (DelayedWorkQueue) based on an
     *    unbounded DelayQueue. The lack of capacity constraint and
     *    the fact that corePoolSize and maximumPoolSize are
     *    effectively identical simplifies some execution mechanics
     *    (see delayedExecute) compared to ThreadPoolExecutor
     *    version.
     *
     *    The DelayedWorkQueue class is defined below for the sake of
     *    ensuring that all elements are instances of
     *    RunnableScheduledFuture.  Since DelayQueue otherwise
     *    requires type be Delayed, but not necessarily Runnable, and
     *    the workQueue requires the opposite, we need to explicitly
     *    define a class that requires both to ensure that users don't
     *    add objects that aren't RunnableScheduledFutures via
     *    getQueue().add() etc.
     *
     * 3. Supporting optional run-after-shutdown parameters, which
     *    leads to overrides of shutdown methods to remove and cancel
     *    tasks that should NOT be run after shutdown, as well as
     *    different recheck logic when task (re)submission overlaps
     *    with a shutdown.
     *
     * 4. Task decoration methods to allow interception and
     *    instrumentation, which are needed because subclasses cannot
     *    otherwise override submit methods to get this effect. These
     *    don't have any impact on pool control logic though.
     */

    /**
     * False if should cancel/suppress periodic tasks on shutdown.
     */
    private volatile boolean continueExistingPeriodicTasksAfterShutdown;

    /**
     * False if should cancel non-periodic tasks on shutdown.
     */
    private volatile boolean executeExistingDelayedTasksAfterShutdown = true;

    /**
     * Sequence number to break scheduling ties, and in turn to
     * guarantee FIFO order among tied entries.
     */
    private static final AtomicLong sequencer = new AtomicLong(0);

    /** Base of nanosecond timings, to avoid wrapping */
    private static final long NANO_ORIGIN = System.nanoTime();

    /**
     * Returns nanosecond time offset by origin
     */
    final long now() {
        return System.nanoTime() - NANO_ORIGIN;
    }

    private class ScheduledFutureTask<V>
            extends FutureTask<V> implements RunnableScheduledFuture<V> {

        /** Sequence number to break ties FIFO */
        private final long sequenceNumber;
        /** The time the task is enabled to execute in nanoTime units */
        private long time;
        /**
         * Period in nanoseconds for repeating tasks.  A positive
         * value indicates fixed-rate execution.  A negative value
         * indicates fixed-delay execution.  A value of 0 indicates a
         * non-repeating task.
         */
        private final long period;

        /**
         * Creates a one-shot action with given nanoTime-based trigger time.
         */
        ScheduledFutureTask(Runnable r, V result, long ns) {
            super(r, result);
            this.time = ns;
            this.period = 0;
            this.sequenceNumber = sequencer.getAndIncrement();
        }

        /**
         * Creates a periodic action with given nano time and period.
         */
        ScheduledFutureTask(Runnable r, V result, long ns, long period) {
            super(r, result);
            this.time = ns;
            this.period = period;
            this.sequenceNumber = sequencer.getAndIncrement();
        }

        /**
         * Creates a one-shot action with given nanoTime-based trigger.
         */
        ScheduledFutureTask(Callable<V> callable, long ns) {
            super(callable);
            this.time = ns;
            this.period = 0;
            this.sequenceNumber = sequencer.getAndIncrement();
        }

        public long getDelay(TimeUnit unit) {
            long d = unit.convert(time - now(), TimeUnit.NANOSECONDS);
            return d;
        }

        public int compareTo(Delayed other) {
            if (other == this) // compare zero ONLY if same object
                return 0;
            if (other instanceof ScheduledFutureTask) {
                ScheduledFutureTask<?> x = (ScheduledFutureTask<?>)other;
                long diff = time - x.time;
                if (diff < 0)
                    return -1;
                else if (diff > 0)
                    return 1;
                else if (sequenceNumber < x.sequenceNumber)
                    return -1;
                else
                    return 1;
            }
            long d = (getDelay(TimeUnit.NANOSECONDS) -
                      other.getDelay(TimeUnit.NANOSECONDS));
            return (d == 0)? 0 : ((d < 0)? -1 : 1);
        }

        /**
         * Returns true if this is a periodic (not a one-shot) action.
         *
         * @return true if periodic
         */
        public boolean isPeriodic() {
            return period != 0;
        }

        /**
         * Sets the next time to run for a periodic task
         */
        private void setNextRunTime() {
            long p = period;
            if (p > 0)
                time += p;
            else
                time = now() - p;
        }

        /**
         * Overrides FutureTask version so as to reset/requeue if periodic.
         */
        public void run() {
            boolean periodic = isPeriodic();
            if (!canRunInCurrentRunState(periodic))
                cancel(false);
            else if (!periodic)
                ScheduledFutureTask.super.run();
            else if (ScheduledFutureTask.super.runAndReset()) {
                setNextRunTime();
                reExecutePeriodic(this);
            }
        }
    }

    /**
     * Returns true if can run a task given current run state
     * and run-after-shutdown parameters
     * @param periodic true if this task periodic, false if delayed
     */
    boolean canRunInCurrentRunState(boolean periodic) {
        return isRunningOrShutdown(periodic?
                                   continueExistingPeriodicTasksAfterShutdown :
                                   executeExistingDelayedTasksAfterShutdown);
    }

    /**
     * Main execution method for delayed or periodic tasks.  If pool
     * is shut down, rejects the task. Otherwise adds task to queue
     * and starts a thread, if necessary, to run it.  (We cannot
     * prestart the thread to run the task because the task (probably)
     * shouldn't be run yet,) If the pool is shut down while the task
     * is being added, cancel and remove it if required by state and
     * run-after-shutdown parameters
     * @param task the task
     */
    private void delayedExecute(RunnableScheduledFuture<?> task) {
        if (isShutdown())
            reject(task);
        else {
            super.getQueue().add(task);
            if (isShutdown() &&
                !canRunInCurrentRunState(task.isPeriodic()) &&
                remove(task))
                task.cancel(false);
            prestartCoreThread();
        }
    }

    /**
     * Requeues a periodic task unless current run state precludes
     * it. Same idea as delayedExecute except drops task rather than
     * rejecting.
     * @param task the task
     */
    void reExecutePeriodic(RunnableScheduledFuture<?> task) {
        if (canRunInCurrentRunState(true)) {
            super.getQueue().add(task);
            if (!canRunInCurrentRunState(true) && remove(task))
                task.cancel(false);
            prestartCoreThread();
        }
    }

    /**
     * Cancels and clears the queue of all tasks that should not be run
     * due to shutdown policy. Invoked within super.shutdown.
     */
    @Override void onShutdown() {
        BlockingQueue<Runnable> q = super.getQueue();
        boolean keepDelayed =
            getExecuteExistingDelayedTasksAfterShutdownPolicy();
        boolean keepPeriodic =
            getContinueExistingPeriodicTasksAfterShutdownPolicy();
        if (!keepDelayed && !keepPeriodic)
            q.clear();
        else {
            // Traverse snapshot to avoid iterator exceptions
            Object[] entries = q.toArray();
            for (int i = 0; i < entries.length; ++i) {
                Object e = entries[i];
                if (e instanceof RunnableScheduledFuture) {
                    RunnableScheduledFuture<?> t =
                        (RunnableScheduledFuture<?>)e;
                    if ((t.isPeriodic()? !keepPeriodic : !keepDelayed) ||
                        t.isCancelled()) { // also remove if already cancelled
                        if (q.remove(t))
                            t.cancel(false);
                    }
                }
            }
        }
    }

    /**
     * Modifies or replaces the task used to execute a runnable.
     * This method can be used to override the concrete
     * class used for managing internal tasks.
     * The default implementation simply returns the given task.
     *
     * @param runnable the submitted Runnable
     * @param task the task created to execute the runnable
     * @return a task that can execute the runnable
     * @since 1.6
     */
    protected <V> RunnableScheduledFuture<V> decorateTask(
        Runnable runnable, RunnableScheduledFuture<V> task) {
        return task;
    }

    /**
     * Modifies or replaces the task used to execute a callable.
     * This method can be used to override the concrete
     * class used for managing internal tasks.
     * The default implementation simply returns the given task.
     *
     * @param callable the submitted Callable
     * @param task the task created to execute the callable
     * @return a task that can execute the callable
     * @since 1.6
     */
    protected <V> RunnableScheduledFuture<V> decorateTask(
        Callable<V> callable, RunnableScheduledFuture<V> task) {
        return task;
    }

    /**
     * Creates a new ScheduledThreadPoolExecutor with the given core
     * pool size.
     *
     * @param corePoolSize the number of threads to keep in the pool,
     * even if they are idle, unless allowCoreThreadTimeOut is set
     * @throws IllegalArgumentException if <tt>corePoolSize &lt; 0</tt>
     */
    public ScheduledThreadPoolExecutor(int corePoolSize) {
        super(corePoolSize, Integer.MAX_VALUE, 0, TimeUnit.NANOSECONDS,
              new DelayedWorkQueue());
    }

    /**
     * Creates a new ScheduledThreadPoolExecutor with the given
     * initial parameters.
     *
     * @param corePoolSize the number of threads to keep in the pool,
     * even if they are idle, unless allowCoreThreadTimeOut is set
     * @param threadFactory the factory to use when the executor
     * creates a new thread
     * @throws IllegalArgumentException if <tt>corePoolSize &lt; 0</tt>
     * @throws NullPointerException if threadFactory is null
     */
    public ScheduledThreadPoolExecutor(int corePoolSize,
                             ThreadFactory threadFactory) {
        super(corePoolSize, Integer.MAX_VALUE, 0, TimeUnit.NANOSECONDS,
              new DelayedWorkQueue(), threadFactory);
    }

    /**
     * Creates a new ScheduledThreadPoolExecutor with the given
     * initial parameters.
     *
     * @param corePoolSize the number of threads to keep in the pool,
     * even if they are idle, unless allowCoreThreadTimeOut is set
     * @param handler the handler to use when execution is blocked
     * because the thread bounds and queue capacities are reached
     * @throws IllegalArgumentException if <tt>corePoolSize &lt; 0</tt>
     * @throws NullPointerException if handler is null
     */
    public ScheduledThreadPoolExecutor(int corePoolSize,
                              RejectedExecutionHandler handler) {
        super(corePoolSize, Integer.MAX_VALUE, 0, TimeUnit.NANOSECONDS,
              new DelayedWorkQueue(), handler);
    }

    /**
     * Creates a new ScheduledThreadPoolExecutor with the given
     * initial parameters.
     *
     * @param corePoolSize the number of threads to keep in the pool,
     * even if they are idle, unless allowCoreThreadTimeOut is set
     * @param threadFactory the factory to use when the executor
     * creates a new thread
     * @param handler the handler to use when execution is blocked
     * because the thread bounds and queue capacities are reached.
     * @throws IllegalArgumentException if <tt>corePoolSize &lt; 0</tt>
     * @throws NullPointerException if threadFactory or handler is null
     */
    public ScheduledThreadPoolExecutor(int corePoolSize,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler) {
        super(corePoolSize, Integer.MAX_VALUE, 0, TimeUnit.NANOSECONDS,
              new DelayedWorkQueue(), threadFactory, handler);
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public ScheduledFuture<?> schedule(Runnable command,
                                       long delay,
                                       TimeUnit unit) {
        if (command == null || unit == null)
            throw new NullPointerException();
        if (delay < 0) delay = 0;
        long triggerTime = now() + unit.toNanos(delay);
        RunnableScheduledFuture<?> t = decorateTask(command,
            new ScheduledFutureTask<Boolean>(command, null, triggerTime));
        delayedExecute(t);
        return t;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public <V> ScheduledFuture<V> schedule(Callable<V> callable,
                                           long delay,
                                           TimeUnit unit) {
        if (callable == null || unit == null)
            throw new NullPointerException();
        if (delay < 0) delay = 0;
        long triggerTime = now() + unit.toNanos(delay);
        RunnableScheduledFuture<V> t = decorateTask(callable,
            new ScheduledFutureTask<V>(callable, triggerTime));
        delayedExecute(t);
        return t;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     * @throws IllegalArgumentException   {@inheritDoc}
     */
    public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,
                                                  long initialDelay,
                                                  long period,
                                                  TimeUnit unit) {
        if (command == null || unit == null)
            throw new NullPointerException();
        if (period <= 0)
            throw new IllegalArgumentException();
        if (initialDelay < 0) initialDelay = 0;
        long triggerTime = now() + unit.toNanos(initialDelay);
        RunnableScheduledFuture<?> t = decorateTask(command,
            new ScheduledFutureTask<Object>(command,
                                            null,
                                            triggerTime,
                                            unit.toNanos(period)));
        delayedExecute(t);
        return t;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     * @throws IllegalArgumentException   {@inheritDoc}
     */
    public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command,
                                                     long initialDelay,
                                                     long delay,
                                                     TimeUnit unit) {
        if (command == null || unit == null)
            throw new NullPointerException();
        if (delay <= 0)
            throw new IllegalArgumentException();
        if (initialDelay < 0) initialDelay = 0;
        long triggerTime = now() + unit.toNanos(initialDelay);
        RunnableScheduledFuture<?> t = decorateTask(command,
            new ScheduledFutureTask<Boolean>(command,
                                             null,
                                             triggerTime,
                                             unit.toNanos(-delay)));
        delayedExecute(t);
        return t;
    }

    /**
     * Executes command with zero required delay. This has effect
     * equivalent to <tt>schedule(command, 0, anyUnit)</tt>.  Note
     * that inspections of the queue and of the list returned by
     * <tt>shutdownNow</tt> will access the zero-delayed
     * {@link ScheduledFuture}, not the <tt>command</tt> itself.
     *
     * @param command the task to execute
     * @throws RejectedExecutionException at discretion of
     * <tt>RejectedExecutionHandler</tt>, if task cannot be accepted
     * for execution because the executor has been shut down.
     * @throws NullPointerException if command is null
     */
    public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        schedule(command, 0, TimeUnit.NANOSECONDS);
    }

    // Override AbstractExecutorService methods

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public Future<?> submit(Runnable task) {
        return schedule(task, 0, TimeUnit.NANOSECONDS);
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public <T> Future<T> submit(Runnable task, T result) {
        return schedule(Executors.callable(task, result),
                        0, TimeUnit.NANOSECONDS);
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public <T> Future<T> submit(Callable<T> task) {
        return schedule(task, 0, TimeUnit.NANOSECONDS);
    }

    /**
     * Sets the policy on whether to continue executing existing
     * periodic tasks even when this executor has been
     * <tt>shutdown</tt>. In this case, these tasks will only
     * terminate upon <tt>shutdownNow</tt>, or after setting the
     * policy to <tt>false</tt> when already shutdown. This value is
     * by default false.
     *
     * @param value if true, continue after shutdown, else don't.
     * @see #getContinueExistingPeriodicTasksAfterShutdownPolicy
     */
    public void setContinueExistingPeriodicTasksAfterShutdownPolicy(boolean value) {
        continueExistingPeriodicTasksAfterShutdown = value;
        if (!value && isShutdown()) {
            onShutdown();
            tryTerminate();
        }
    }

    /**
     * Gets the policy on whether to continue executing existing
     * periodic tasks even when this executor has been
     * <tt>shutdown</tt>. In this case, these tasks will only
     * terminate upon <tt>shutdownNow</tt> or after setting the policy
     * to <tt>false</tt> when already shutdown. This value is by
     * default false.
     *
     * @return true if will continue after shutdown
     * @see #setContinueExistingPeriodicTasksAfterShutdownPolicy
     */
    public boolean getContinueExistingPeriodicTasksAfterShutdownPolicy() {
        return continueExistingPeriodicTasksAfterShutdown;
    }

    /**
     * Sets the policy on whether to execute existing delayed
     * tasks even when this executor has been <tt>shutdown</tt>. In
     * this case, these tasks will only terminate upon
     * <tt>shutdownNow</tt>, or after setting the policy to
     * <tt>false</tt> when already shutdown. This value is by default
     * true.
     *
     * @param value if true, execute after shutdown, else don't.
     * @see #getExecuteExistingDelayedTasksAfterShutdownPolicy
     */
    public void setExecuteExistingDelayedTasksAfterShutdownPolicy(boolean value) {
        executeExistingDelayedTasksAfterShutdown = value;
        if (!value && isShutdown()) {
            onShutdown();
            tryTerminate();
        }
    }

    /**
     * Gets the policy on whether to execute existing delayed
     * tasks even when this executor has been <tt>shutdown</tt>. In
     * this case, these tasks will only terminate upon
     * <tt>shutdownNow</tt>, or after setting the policy to
     * <tt>false</tt> when already shutdown. This value is by default
     * true.
     *
     * @return true if will execute after shutdown
     * @see #setExecuteExistingDelayedTasksAfterShutdownPolicy
     */
    public boolean getExecuteExistingDelayedTasksAfterShutdownPolicy() {
        return executeExistingDelayedTasksAfterShutdown;
    }

    /**
     * Initiates an orderly shutdown in which previously submitted
     * tasks are executed, but no new tasks will be accepted. If the
     * <tt>ExecuteExistingDelayedTasksAfterShutdownPolicy</tt> has
     * been set <tt>false</tt>, existing delayed tasks whose delays
     * have not yet elapsed are cancelled. And unless the
     * <tt>ContinueExistingPeriodicTasksAfterShutdownPolicy</tt> has
     * been set <tt>true</tt>, future executions of existing periodic
     * tasks will be cancelled.
     */
    public void shutdown() {
        super.shutdown();
    }

    /**
     * Attempts to stop all actively executing tasks, halts the
     * processing of waiting tasks, and returns a list of the tasks
     * that were awaiting execution.
     *
     * <p>There are no guarantees beyond best-effort attempts to stop
     * processing actively executing tasks.  This implementation
     * cancels tasks via {@link Thread#interrupt}, so any task that
     * fails to respond to interrupts may never terminate.
     *
     * @return list of tasks that never commenced execution.  Each
     * element of this list is a {@link ScheduledFuture},
     * including those tasks submitted using <tt>execute</tt>, which
     * are for scheduling purposes used as the basis of a zero-delay
     * <tt>ScheduledFuture</tt>.
     * @throws SecurityException {@inheritDoc}
     */
    public List<Runnable> shutdownNow() {
        return super.shutdownNow();
    }

    /**
     * Returns the task queue used by this executor.  Each element of
     * this queue is a {@link ScheduledFuture}, including those
     * tasks submitted using <tt>execute</tt> which are for scheduling
     * purposes used as the basis of a zero-delay
     * <tt>ScheduledFuture</tt>. Iteration over this queue is
     * <em>not</em> guaranteed to traverse tasks in the order in
     * which they will execute.
     *
     * @return the task queue
     */
    public BlockingQueue<Runnable> getQueue() {
        return super.getQueue();
    }

    /**
     * An annoying wrapper class to convince javac to use a
     * DelayQueue<RunnableScheduledFuture> as a BlockingQueue<Runnable>
     */
    private static class DelayedWorkQueue
        extends AbstractCollection<Runnable>
        implements BlockingQueue<Runnable> {

        private final DelayQueue<RunnableScheduledFuture> dq = new DelayQueue<RunnableScheduledFuture>();
        public Runnable poll() { return dq.poll(); }
        public Runnable peek() { return dq.peek(); }
        public Runnable take() throws InterruptedException { return dq.take(); }
        public Runnable poll(long timeout, TimeUnit unit) throws InterruptedException {
            return dq.poll(timeout, unit);
        }

        public boolean add(Runnable x) {
	    return dq.add((RunnableScheduledFuture)x);
	}
        public boolean offer(Runnable x) {
	    return dq.offer((RunnableScheduledFuture)x);
	}
        public void put(Runnable x) {
            dq.put((RunnableScheduledFuture)x);
        }
        public boolean offer(Runnable x, long timeout, TimeUnit unit) {
            return dq.offer((RunnableScheduledFuture)x, timeout, unit);
        }

        public Runnable remove() { return dq.remove(); }
        public Runnable element() { return dq.element(); }
        public void clear() { dq.clear(); }
        public int drainTo(Collection<? super Runnable> c) { return dq.drainTo(c); }
        public int drainTo(Collection<? super Runnable> c, int maxElements) {
            return dq.drainTo(c, maxElements);
        }

        public int remainingCapacity() { return dq.remainingCapacity(); }
        public boolean remove(Object x) { return dq.remove(x); }
        public boolean contains(Object x) { return dq.contains(x); }
        public int size() { return dq.size(); }
        public boolean isEmpty() { return dq.isEmpty(); }
        public Object[] toArray() { return dq.toArray(); }
        public <T> T[] toArray(T[] array) { return dq.toArray(array); }
        public Iterator<Runnable> iterator() {
            return new Iterator<Runnable>() {
                private Iterator<RunnableScheduledFuture> it = dq.iterator();
                public boolean hasNext() { return it.hasNext(); }
                public Runnable next() { return it.next(); }
                public void remove() { it.remove(); }
            };
        }
    }
}