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  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

  *

  * This code is free software; you can redistribute it and/or modify it

  * under the terms of the GNU General Public License version 2 only, as

  * published by the Free Software Foundation.  Oracle designates this

  * particular file as subject to the "Classpath" exception as provided

  * by Oracle in the LICENSE file that accompanied this code.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 package java.util;

 import java.util.Date;

 import java.util.concurrent.atomic.AtomicInteger;

 import org.apidesign.bck2brwsr.core.JavaScriptBody;

 /**

  * A facility for threads to schedule tasks for future execution in a

  * background thread.  Tasks may be scheduled for one-time execution, or for

  * repeated execution at regular intervals.

  *

  * <p>Corresponding to each <tt>Timer</tt> object is a single background

  * thread that is used to execute all of the timer's tasks, sequentially.

  * Timer tasks should complete quickly.  If a timer task takes excessive time

  * to complete, it "hogs" the timer's task execution thread.  This can, in

  * turn, delay the execution of subsequent tasks, which may "bunch up" and

  * execute in rapid succession when (and if) the offending task finally

  * completes.

  *

  * <p>After the last live reference to a <tt>Timer</tt> object goes away

  * <i>and</i> all outstanding tasks have completed execution, the timer's task

  * execution thread terminates gracefully (and becomes subject to garbage

  * collection).  However, this can take arbitrarily long to occur.  By

  * default, the task execution thread does not run as a <i>daemon thread</i>,

  * so it is capable of keeping an application from terminating.  If a caller

  * wants to terminate a timer's task execution thread rapidly, the caller

  * should invoke the timer's <tt>cancel</tt> method.

  *

  * <p>If the timer's task execution thread terminates unexpectedly, for

  * example, because its <tt>stop</tt> method is invoked, any further

  * attempt to schedule a task on the timer will result in an

  * <tt>IllegalStateException</tt>, as if the timer's <tt>cancel</tt>

  * method had been invoked.

  *

  * <p>This class is thread-safe: multiple threads can share a single

  * <tt>Timer</tt> object without the need for external synchronization.

  *

  * <p>This class does <i>not</i> offer real-time guarantees: it schedules

  * tasks using the <tt>Object.wait(long)</tt> method.

  *

  * <p>Java 5.0 introduced the {@code java.util.concurrent} package and

  * one of the concurrency utilities therein is the {@link

  * java.util.concurrent.ScheduledThreadPoolExecutor

  * ScheduledThreadPoolExecutor} which is a thread pool for repeatedly

  * executing tasks at a given rate or delay.  It is effectively a more

  * versatile replacement for the {@code Timer}/{@code TimerTask}

  * combination, as it allows multiple service threads, accepts various

  * time units, and doesn't require subclassing {@code TimerTask} (just

  * implement {@code Runnable}).  Configuring {@code

  * ScheduledThreadPoolExecutor} with one thread makes it equivalent to

  * {@code Timer}.

  *

  * <p>Implementation note: This class scales to large numbers of concurrently

  * scheduled tasks (thousands should present no problem).  Internally,

  * it uses a binary heap to represent its task queue, so the cost to schedule

  * a task is O(log n), where n is the number of concurrently scheduled tasks.

  *

  * <p>Implementation note: All constructors start a timer thread.

  *

  * @author  Josh Bloch

  * @see     TimerTask

  * @see     Object#wait(long)

  * @since   1.3

  */

 public class Timer {

     /**

      * The timer task queue.  This data structure is shared with the timer

      * thread.  The timer produces tasks, via its various schedule calls,

      * and the timer thread consumes, executing timer tasks as appropriate,

      * and removing them from the queue when they're obsolete.

      */

     private final TaskQueue queue = new TaskQueue();

     /**

      * The timer thread.

      */

     private final TimerThread thread = new TimerThread(queue);

     /**

      * This object causes the timer's task execution thread to exit

      * gracefully when there are no live references to the Timer object and no

      * tasks in the timer queue.  It is used in preference to a finalizer on

      * Timer as such a finalizer would be susceptible to a subclass's

      * finalizer forgetting to call it.

      */

     private final Object threadReaper = new Object() {

         protected void finalize() throws Throwable {

             synchronized(queue) {

                 thread.newTasksMayBeScheduled = false;

                 thread.notifyQueue(1); // In case queue is empty.

             }

         }

     };

     /**

      * This ID is used to generate thread names.

      */

     private final static AtomicInteger nextSerialNumber = new AtomicInteger(0);

     private static int serialNumber() {

         return nextSerialNumber.getAndIncrement();

     }

     /**

      * Creates a new timer.  The associated thread does <i>not</i>

      * {@linkplain Thread#setDaemon run as a daemon}.

      */

     public Timer() {

         this("Timer-" + serialNumber());

     }

     /**

      * Creates a new timer whose associated thread may be specified to

      * {@linkplain Thread#setDaemon run as a daemon}.

      * A daemon thread is called for if the timer will be used to

      * schedule repeating "maintenance activities", which must be

      * performed as long as the application is running, but should not

      * prolong the lifetime of the application.

      *

      * @param isDaemon true if the associated thread should run as a daemon.

      */

     public Timer(boolean isDaemon) {

         this("Timer-" + serialNumber(), isDaemon);

     }

     /**

      * Creates a new timer whose associated thread has the specified name.

      * The associated thread does <i>not</i>

      * {@linkplain Thread#setDaemon run as a daemon}.

      *

      * @param name the name of the associated thread

      * @throws NullPointerException if {@code name} is null

      * @since 1.5

      */

     public Timer(String name) {

     }

     /**

      * Creates a new timer whose associated thread has the specified name,

      * and may be specified to

      * {@linkplain Thread#setDaemon run as a daemon}.

      *

      * @param name the name of the associated thread

      * @param isDaemon true if the associated thread should run as a daemon

      * @throws NullPointerException if {@code name} is null

      * @since 1.5

      */

     public Timer(String name, boolean isDaemon) {

     }

     /**

      * Schedules the specified task for execution after the specified delay.

      *

      * @param task  task to be scheduled.

      * @param delay delay in milliseconds before task is to be executed.

      * @throws IllegalArgumentException if <tt>delay</tt> is negative, or

      *         <tt>delay + System.currentTimeMillis()</tt> is negative.

      * @throws IllegalStateException if task was already scheduled or

      *         cancelled, timer was cancelled, or timer thread terminated.

      * @throws NullPointerException if {@code task} is null

      */

     public void schedule(TimerTask task, long delay) {

         if (delay < 0)

             throw new IllegalArgumentException("Negative delay.");

         sched(task, System.currentTimeMillis()+delay, 0);

     }

     /**

      * Schedules the specified task for execution at the specified time.  If

      * the time is in the past, the task is scheduled for immediate execution.

      *

      * @param task task to be scheduled.

      * @param time time at which task is to be executed.

      * @throws IllegalArgumentException if <tt>time.getTime()</tt> is negative.

      * @throws IllegalStateException if task was already scheduled or

      *         cancelled, timer was cancelled, or timer thread terminated.

      * @throws NullPointerException if {@code task} or {@code time} is null

      */

     public void schedule(TimerTask task, Date time) {

         sched(task, time.getTime(), 0);

     }

     /**

      * Schedules the specified task for repeated <i>fixed-delay execution</i>,

      * beginning after the specified delay.  Subsequent executions take place

      * at approximately regular intervals separated by the specified period.

      *

      * <p>In fixed-delay execution, each execution is scheduled relative to

      * the actual execution time of the previous execution.  If an execution

      * is delayed for any reason (such as garbage collection or other

      * background activity), subsequent executions will be delayed as well.

      * In the long run, the frequency of execution will generally be slightly

      * lower than the reciprocal of the specified period (assuming the system

      * clock underlying <tt>Object.wait(long)</tt> is accurate).

      *

      * <p>Fixed-delay execution is appropriate for recurring activities

      * that require "smoothness."  In other words, it is appropriate for

      * activities where it is more important to keep the frequency accurate

      * in the short run than in the long run.  This includes most animation

      * tasks, such as blinking a cursor at regular intervals.  It also includes

      * tasks wherein regular activity is performed in response to human

      * input, such as automatically repeating a character as long as a key

      * is held down.

      *

      * @param task   task to be scheduled.

      * @param delay  delay in milliseconds before task is to be executed.

      * @param period time in milliseconds between successive task executions.

      * @throws IllegalArgumentException if {@code delay < 0}, or

      *         {@code delay + System.currentTimeMillis() < 0}, or

      *         {@code period <= 0}

      * @throws IllegalStateException if task was already scheduled or

      *         cancelled, timer was cancelled, or timer thread terminated.

      * @throws NullPointerException if {@code task} is null

      */

     public void schedule(TimerTask task, long delay, long period) {

         if (delay < 0)

             throw new IllegalArgumentException("Negative delay.");

         if (period <= 0)

             throw new IllegalArgumentException("Non-positive period.");

         sched(task, System.currentTimeMillis()+delay, -period);

     }

     /**

      * Schedules the specified task for repeated <i>fixed-delay execution</i>,

      * beginning at the specified time. Subsequent executions take place at

      * approximately regular intervals, separated by the specified period.

      *

      * <p>In fixed-delay execution, each execution is scheduled relative to

      * the actual execution time of the previous execution.  If an execution

      * is delayed for any reason (such as garbage collection or other

      * background activity), subsequent executions will be delayed as well.

      * In the long run, the frequency of execution will generally be slightly

      * lower than the reciprocal of the specified period (assuming the system

      * clock underlying <tt>Object.wait(long)</tt> is accurate).  As a

      * consequence of the above, if the scheduled first time is in the past,

      * it is scheduled for immediate execution.

      *

      * <p>Fixed-delay execution is appropriate for recurring activities

      * that require "smoothness."  In other words, it is appropriate for

      * activities where it is more important to keep the frequency accurate

      * in the short run than in the long run.  This includes most animation

      * tasks, such as blinking a cursor at regular intervals.  It also includes

      * tasks wherein regular activity is performed in response to human

      * input, such as automatically repeating a character as long as a key

      * is held down.

      *

      * @param task   task to be scheduled.

      * @param firstTime First time at which task is to be executed.

      * @param period time in milliseconds between successive task executions.

      * @throws IllegalArgumentException if {@code firstTime.getTime() < 0}, or

      *         {@code period <= 0}

      * @throws IllegalStateException if task was already scheduled or

      *         cancelled, timer was cancelled, or timer thread terminated.

      * @throws NullPointerException if {@code task} or {@code firstTime} is null

      */

     public void schedule(TimerTask task, Date firstTime, long period) {

         if (period <= 0)

             throw new IllegalArgumentException("Non-positive period.");

         sched(task, firstTime.getTime(), -period);

     }

     /**

      * Schedules the specified task for repeated <i>fixed-rate execution</i>,

      * beginning after the specified delay.  Subsequent executions take place

      * at approximately regular intervals, separated by the specified period.

      *

      * <p>In fixed-rate execution, each execution is scheduled relative to the

      * scheduled execution time of the initial execution.  If an execution is

      * delayed for any reason (such as garbage collection or other background

      * activity), two or more executions will occur in rapid succession to

      * "catch up."  In the long run, the frequency of execution will be

      * exactly the reciprocal of the specified period (assuming the system

      * clock underlying <tt>Object.wait(long)</tt> is accurate).

      *

      * <p>Fixed-rate execution is appropriate for recurring activities that

      * are sensitive to <i>absolute</i> time, such as ringing a chime every

      * hour on the hour, or running scheduled maintenance every day at a

      * particular time.  It is also appropriate for recurring activities

      * where the total time to perform a fixed number of executions is

      * important, such as a countdown timer that ticks once every second for

      * ten seconds.  Finally, fixed-rate execution is appropriate for

      * scheduling multiple repeating timer tasks that must remain synchronized

      * with respect to one another.

      *

      * @param task   task to be scheduled.

      * @param delay  delay in milliseconds before task is to be executed.

      * @param period time in milliseconds between successive task executions.

      * @throws IllegalArgumentException if {@code delay < 0}, or

      *         {@code delay + System.currentTimeMillis() < 0}, or

      *         {@code period <= 0}

      * @throws IllegalStateException if task was already scheduled or

      *         cancelled, timer was cancelled, or timer thread terminated.

      * @throws NullPointerException if {@code task} is null

      */

     public void scheduleAtFixedRate(TimerTask task, long delay, long period) {

         if (delay < 0)

             throw new IllegalArgumentException("Negative delay.");

         if (period <= 0)

             throw new IllegalArgumentException("Non-positive period.");

         sched(task, System.currentTimeMillis()+delay, period);

     }

     /**

      * Schedules the specified task for repeated <i>fixed-rate execution</i>,

      * beginning at the specified time. Subsequent executions take place at

      * approximately regular intervals, separated by the specified period.

      *

      * <p>In fixed-rate execution, each execution is scheduled relative to the

      * scheduled execution time of the initial execution.  If an execution is

      * delayed for any reason (such as garbage collection or other background

      * activity), two or more executions will occur in rapid succession to

      * "catch up."  In the long run, the frequency of execution will be

      * exactly the reciprocal of the specified period (assuming the system

      * clock underlying <tt>Object.wait(long)</tt> is accurate).  As a

      * consequence of the above, if the scheduled first time is in the past,

      * then any "missed" executions will be scheduled for immediate "catch up"

      * execution.

      *

      * <p>Fixed-rate execution is appropriate for recurring activities that

      * are sensitive to <i>absolute</i> time, such as ringing a chime every

      * hour on the hour, or running scheduled maintenance every day at a

      * particular time.  It is also appropriate for recurring activities

      * where the total time to perform a fixed number of executions is

      * important, such as a countdown timer that ticks once every second for

      * ten seconds.  Finally, fixed-rate execution is appropriate for

      * scheduling multiple repeating timer tasks that must remain synchronized

      * with respect to one another.

      *

      * @param task   task to be scheduled.

      * @param firstTime First time at which task is to be executed.

      * @param period time in milliseconds between successive task executions.

      * @throws IllegalArgumentException if {@code firstTime.getTime() < 0} or

      *         {@code period <= 0}

      * @throws IllegalStateException if task was already scheduled or

      *         cancelled, timer was cancelled, or timer thread terminated.

      * @throws NullPointerException if {@code task} or {@code firstTime} is null

      */

     public void scheduleAtFixedRate(TimerTask task, Date firstTime,

                                     long period) {

         if (period <= 0)

             throw new IllegalArgumentException("Non-positive period.");

         sched(task, firstTime.getTime(), period);

     }

     /**

      * Schedule the specified timer task for execution at the specified

      * time with the specified period, in milliseconds.  If period is

      * positive, the task is scheduled for repeated execution; if period is

      * zero, the task is scheduled for one-time execution. Time is specified

      * in Date.getTime() format.  This method checks timer state, task state,

      * and initial execution time, but not period.

      *

      * @throws IllegalArgumentException if <tt>time</tt> is negative.

      * @throws IllegalStateException if task was already scheduled or

      *         cancelled, timer was cancelled, or timer thread terminated.

      * @throws NullPointerException if {@code task} is null

      */

     private void sched(TimerTask task, long time, long period) {

         if (time < 0)

             throw new IllegalArgumentException("Illegal execution time.");

         // Constrain value of period sufficiently to prevent numeric

         // overflow while still being effectively infinitely large.

         if (Math.abs(period) > (Long.MAX_VALUE >> 1))

             period >>= 1;

         synchronized(queue) {

             if (!thread.newTasksMayBeScheduled)

                 throw new IllegalStateException("Timer already cancelled.");

             synchronized(task.lock) {

                 if (task.state != TimerTask.VIRGIN)

                     throw new IllegalStateException(

                         "Task already scheduled or cancelled");

                 task.nextExecutionTime = time;

                 task.period = period;

                 task.state = TimerTask.SCHEDULED;

             }

             queue.add(task);

             if (queue.getMin() == task)

                 thread.notifyQueue(1);

         }

     }

     /**

      * Terminates this timer, discarding any currently scheduled tasks.

      * Does not interfere with a currently executing task (if it exists).

      * Once a timer has been terminated, its execution thread terminates

      * gracefully, and no more tasks may be scheduled on it.

      *

      * <p>Note that calling this method from within the run method of a

      * timer task that was invoked by this timer absolutely guarantees that

      * the ongoing task execution is the last task execution that will ever

      * be performed by this timer.

      *

      * <p>This method may be called repeatedly; the second and subsequent

      * calls have no effect.

      */

     public void cancel() {

         synchronized(queue) {

             thread.newTasksMayBeScheduled = false;

             queue.clear();

             thread.notifyQueue(1);  // In case queue was already empty.

         }

     }

     /**

      * Removes all cancelled tasks from this timer's task queue.  <i>Calling

      * this method has no effect on the behavior of the timer</i>, but

      * eliminates the references to the cancelled tasks from the queue.

      * If there are no external references to these tasks, they become

      * eligible for garbage collection.

      *

      * <p>Most programs will have no need to call this method.

      * It is designed for use by the rare application that cancels a large

      * number of tasks.  Calling this method trades time for space: the

      * runtime of the method may be proportional to n + c log n, where n

      * is the number of tasks in the queue and c is the number of cancelled

      * tasks.

      *

      * <p>Note that it is permissible to call this method from within a

      * a task scheduled on this timer.

      *

      * @return the number of tasks removed from the queue.

      * @since 1.5

      */

      public int purge() {

          int result = 0;

          synchronized(queue) {

              for (int i = queue.size(); i > 0; i--) {

                  if (queue.get(i).state == TimerTask.CANCELLED) {

                      queue.quickRemove(i);

                      result++;

                  }

              }

              if (result != 0)

                  queue.heapify();

          }

          return result;

      }

 }

 /**

  * This "helper class" implements the timer's task execution thread, which

  * waits for tasks on the timer queue, executions them when they fire,

  * reschedules repeating tasks, and removes cancelled tasks and spent

  * non-repeating tasks from the queue.

  */

 class TimerThread implements Runnable {

     /**

      * This flag is set to false by the reaper to inform us that there

      * are no more live references to our Timer object.  Once this flag

      * is true and there are no more tasks in our queue, there is no

      * work left for us to do, so we terminate gracefully.  Note that

      * this field is protected by queue's monitor!

      */

     boolean newTasksMayBeScheduled = true;

     /**

      * Our Timer's queue.  We store this reference in preference to

      * a reference to the Timer so the reference graph remains acyclic.

      * Otherwise, the Timer would never be garbage-collected and this

      * thread would never go away.

      */

     private TaskQueue queue;

     TimerThread(TaskQueue queue) {

         this.queue = queue;

     }

     void notifyQueue(int delay) {

         if (delay < 1) {

             delay = 1;

         }

         setTimeout(delay, this);

     }

     @JavaScriptBody(args = { "delay", "r" }, body = "window.setTimeout(function() { r.run__V(); }, delay);")

     private static native void setTimeout(int delay, Runnable r);

     public void run() {

         mainLoop(1);

 //        try {

 //            mainLoop(0);

 //        } finally {

 //            // Someone killed this Thread, behave as if Timer cancelled

 //            synchronized(queue) {

 //                newTasksMayBeScheduled = false;

 //                queue.clear();  // Eliminate obsolete references

 //            }

 //        }

     }

     /**

      * The main timer loop.  (See class comment.)

      */

     private void mainLoop(int inc) {

         for (int i = 0; i < 1; i += inc) {

             try {

                 TimerTask task;

                 boolean taskFired;

                 synchronized(queue) {

                     // Wait for queue to become non-empty

                     while (queue.isEmpty() && newTasksMayBeScheduled)

                         break;

                     if (queue.isEmpty())

                         break; // Queue is empty and will forever remain; die

                     // Queue nonempty; look at first evt and do the right thing

                     long currentTime, executionTime;

                     task = queue.getMin();

                     synchronized(task.lock) {

                         if (task.state == TimerTask.CANCELLED) {

                             queue.removeMin();

                             continue;  // No action required, poll queue again

                         }

                         currentTime = System.currentTimeMillis();

                         executionTime = task.nextExecutionTime;

                         if (taskFired = (executionTime<=currentTime)) {

                             if (task.period == 0) { // Non-repeating, remove

                                 queue.removeMin();

                                 task.state = TimerTask.EXECUTED;

                             } else { // Repeating task, reschedule

                                 queue.rescheduleMin(

                                   task.period<0 ? currentTime   - task.period

                                                 : executionTime + task.period);

                             }

                         }

                     }

                     if (!taskFired) {

                         // Task hasn't yet fired; wait

                         notifyQueue((int)(executionTime - currentTime));

                         return;

                     }

                 }

                 if (taskFired)  // Task fired; run it, holding no locks

                     task.run();

             } catch(Exception e) {

                 e.printStackTrace();

             }

         }

     }

 }

 /**

  * This class represents a timer task queue: a priority queue of TimerTasks,

  * ordered on nextExecutionTime.  Each Timer object has one of these, which it

  * shares with its TimerThread.  Internally this class uses a heap, which

  * offers log(n) performance for the add, removeMin and rescheduleMin

  * operations, and constant time performance for the getMin operation.

  */

 class TaskQueue {

     /**

      * Priority queue represented as a balanced binary heap: the two children

      * of queue[n] are queue[2*n] and queue[2*n+1].  The priority queue is

      * ordered on the nextExecutionTime field: The TimerTask with the lowest

      * nextExecutionTime is in queue[1] (assuming the queue is nonempty).  For

      * each node n in the heap, and each descendant of n, d,

      * n.nextExecutionTime <= d.nextExecutionTime.

      */

     private TimerTask[] queue = new TimerTask[128];

     /**

      * The number of tasks in the priority queue.  (The tasks are stored in

      * queue[1] up to queue[size]).

      */

     private int size = 0;

     /**

      * Returns the number of tasks currently on the queue.

      */

     int size() {

         return size;

     }

     /**

      * Adds a new task to the priority queue.

      */

     void add(TimerTask task) {

         // Grow backing store if necessary

         if (size + 1 == queue.length)

             queue = Arrays.copyOf(queue, 2*queue.length);

         queue[++size] = task;

         fixUp(size);

     }

     /**

      * Return the "head task" of the priority queue.  (The head task is an

      * task with the lowest nextExecutionTime.)

      */

     TimerTask getMin() {

         return queue[1];

     }

     /**

      * Return the ith task in the priority queue, where i ranges from 1 (the

      * head task, which is returned by getMin) to the number of tasks on the

      * queue, inclusive.

      */

     TimerTask get(int i) {

         return queue[i];

     }

     /**

      * Remove the head task from the priority queue.

      */

     void removeMin() {

         queue[1] = queue[size];

         queue[size--] = null;  // Drop extra reference to prevent memory leak

         fixDown(1);

     }

     /**

      * Removes the ith element from queue without regard for maintaining

      * the heap invariant.  Recall that queue is one-based, so

      * 1 <= i <= size.

      */

     void quickRemove(int i) {

         assert i <= size;

         queue[i] = queue[size];

         queue[size--] = null;  // Drop extra ref to prevent memory leak

     }

     /**

      * Sets the nextExecutionTime associated with the head task to the

      * specified value, and adjusts priority queue accordingly.

      */

     void rescheduleMin(long newTime) {

         queue[1].nextExecutionTime = newTime;

         fixDown(1);

     }

     /**

      * Returns true if the priority queue contains no elements.

      */

     boolean isEmpty() {

         return size==0;

     }

     /**

      * Removes all elements from the priority queue.

      */

     void clear() {

         // Null out task references to prevent memory leak

         for (int i=1; i<=size; i++)

             queue[i] = null;

         size = 0;

     }

     /**

      * Establishes the heap invariant (described above) assuming the heap

      * satisfies the invariant except possibly for the leaf-node indexed by k

      * (which may have a nextExecutionTime less than its parent's).

      *

      * This method functions by "promoting" queue[k] up the hierarchy

      * (by swapping it with its parent) repeatedly until queue[k]'s

      * nextExecutionTime is greater than or equal to that of its parent.

      */

     private void fixUp(int k) {

         while (k > 1) {

             int j = k >> 1;

             if (queue[j].nextExecutionTime <= queue[k].nextExecutionTime)

                 break;

             TimerTask tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;

             k = j;

         }

     }

     /**

      * Establishes the heap invariant (described above) in the subtree

      * rooted at k, which is assumed to satisfy the heap invariant except

      * possibly for node k itself (which may have a nextExecutionTime greater

      * than its children's).

      *

      * This method functions by "demoting" queue[k] down the hierarchy

      * (by swapping it with its smaller child) repeatedly until queue[k]'s

      * nextExecutionTime is less than or equal to those of its children.

      */

     private void fixDown(int k) {

         int j;

         while ((j = k << 1) <= size && j > 0) {

             if (j < size &&

                 queue[j].nextExecutionTime > queue[j+1].nextExecutionTime)

                 j++; // j indexes smallest kid

             if (queue[k].nextExecutionTime <= queue[j].nextExecutionTime)

                 break;

             TimerTask tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;

             k = j;

         }

     }

     /**

      * Establishes the heap invariant (described above) in the entire tree,

      * assuming nothing about the order of the elements prior to the call.

      */

     void heapify() {

         for (int i = size/2; i >= 1; i--)

             fixDown(i);

     }

 }