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25 
26 package java.util;
27 import java.util.Date;
28 import java.util.concurrent.atomic.AtomicInteger;
29 
30 /**
31  * A facility for threads to schedule tasks for future execution in a
32  * background thread.  Tasks may be scheduled for one-time execution, or for
33  * repeated execution at regular intervals.
34  *
35  * <p>Corresponding to each <tt>Timer</tt> object is a single background
36  * thread that is used to execute all of the timer's tasks, sequentially.
37  * Timer tasks should complete quickly.  If a timer task takes excessive time
38  * to complete, it "hogs" the timer's task execution thread.  This can, in
39  * turn, delay the execution of subsequent tasks, which may "bunch up" and
40  * execute in rapid succession when (and if) the offending task finally
41  * completes.
42  *
43  * <p>After the last live reference to a <tt>Timer</tt> object goes away
44  * <i>and</i> all outstanding tasks have completed execution, the timer's task
45  * execution thread terminates gracefully (and becomes subject to garbage
46  * collection).  However, this can take arbitrarily long to occur.  By
47  * default, the task execution thread does not run as a <i>daemon thread</i>,
48  * so it is capable of keeping an application from terminating.  If a caller
49  * wants to terminate a timer's task execution thread rapidly, the caller
50  * should invoke the timer's <tt>cancel</tt> method.
51  *
52  * <p>If the timer's task execution thread terminates unexpectedly, for
53  * example, because its <tt>stop</tt> method is invoked, any further
54  * attempt to schedule a task on the timer will result in an
55  * <tt>IllegalStateException</tt>, as if the timer's <tt>cancel</tt>
56  * method had been invoked.
57  *
58  * <p>This class is thread-safe: multiple threads can share a single
59  * <tt>Timer</tt> object without the need for external synchronization.
60  *
61  * <p>This class does <i>not</i> offer real-time guarantees: it schedules
62  * tasks using the <tt>Object.wait(long)</tt> method.
63  *
64  * <p>Java 5.0 introduced the {@code java.util.concurrent} package and
65  * one of the concurrency utilities therein is the {@link
66  * java.util.concurrent.ScheduledThreadPoolExecutor
67  * ScheduledThreadPoolExecutor} which is a thread pool for repeatedly
68  * executing tasks at a given rate or delay.  It is effectively a more
69  * versatile replacement for the {@code Timer}/{@code TimerTask}
70  * combination, as it allows multiple service threads, accepts various
71  * time units, and doesn't require subclassing {@code TimerTask} (just
72  * implement {@code Runnable}).  Configuring {@code
73  * ScheduledThreadPoolExecutor} with one thread makes it equivalent to
74  * {@code Timer}.
75  *
76  * <p>Implementation note: This class scales to large numbers of concurrently
77  * scheduled tasks (thousands should present no problem).  Internally,
78  * it uses a binary heap to represent its task queue, so the cost to schedule
79  * a task is O(log n), where n is the number of concurrently scheduled tasks.
80  *
81  * <p>Implementation note: All constructors start a timer thread.
82  *
83  * @author  Josh Bloch
84  * @see     TimerTask
85  * @see     Object#wait(long)
86  * @since   1.3
87  */
88 
89 public class Timer {
90     /**
91      * The timer task queue.  This data structure is shared with the timer
92      * thread.  The timer produces tasks, via its various schedule calls,
93      * and the timer thread consumes, executing timer tasks as appropriate,
94      * and removing them from the queue when they're obsolete.
95      */
96     private final TaskQueue queue = new TaskQueue();
97 
98     /**
99      * The timer thread.
100      */
101     private final TimerThread thread = new TimerThread(queue);
102 
103     /**
104      * This object causes the timer's task execution thread to exit
105      * gracefully when there are no live references to the Timer object and no
106      * tasks in the timer queue.  It is used in preference to a finalizer on
107      * Timer as such a finalizer would be susceptible to a subclass's
108      * finalizer forgetting to call it.
109      */
110     private final Object threadReaper = new Object() {
111         protected void finalize() throws Throwable {
112             synchronized(queue) {
113                 thread.newTasksMayBeScheduled = false;
114                 queue.notify(); // In case queue is empty.
115             }
116         }
117     };
118 
119     /**
120      * This ID is used to generate thread names.
121      */
122     private final static AtomicInteger nextSerialNumber = new AtomicInteger(0);
serialNumber()123     private static int serialNumber() {
124         return nextSerialNumber.getAndIncrement();
125     }
126 
127     /**
128      * Creates a new timer.  The associated thread does <i>not</i>
129      * {@linkplain Thread#setDaemon run as a daemon}.
130      */
Timer()131     public Timer() {
132         this("Timer-" + serialNumber());
133     }
134 
135     /**
136      * Creates a new timer whose associated thread may be specified to
137      * {@linkplain Thread#setDaemon run as a daemon}.
138      * A daemon thread is called for if the timer will be used to
139      * schedule repeating "maintenance activities", which must be
140      * performed as long as the application is running, but should not
141      * prolong the lifetime of the application.
142      *
143      * @param isDaemon true if the associated thread should run as a daemon.
144      */
Timer(boolean isDaemon)145     public Timer(boolean isDaemon) {
146         this("Timer-" + serialNumber(), isDaemon);
147     }
148 
149     /**
150      * Creates a new timer whose associated thread has the specified name.
151      * The associated thread does <i>not</i>
152      * {@linkplain Thread#setDaemon run as a daemon}.
153      *
154      * @param name the name of the associated thread
155      * @throws NullPointerException if {@code name} is null
156      * @since 1.5
157      */
Timer(String name)158     public Timer(String name) {
159         thread.setName(name);
160         thread.start();
161     }
162 
163     /**
164      * Creates a new timer whose associated thread has the specified name,
165      * and may be specified to
166      * {@linkplain Thread#setDaemon run as a daemon}.
167      *
168      * @param name the name of the associated thread
169      * @param isDaemon true if the associated thread should run as a daemon
170      * @throws NullPointerException if {@code name} is null
171      * @since 1.5
172      */
Timer(String name, boolean isDaemon)173     public Timer(String name, boolean isDaemon) {
174         thread.setName(name);
175         thread.setDaemon(isDaemon);
176         thread.start();
177     }
178 
179     /**
180      * Schedules the specified task for execution after the specified delay.
181      *
182      * @param task  task to be scheduled.
183      * @param delay delay in milliseconds before task is to be executed.
184      * @throws IllegalArgumentException if <tt>delay</tt> is negative, or
185      *         <tt>delay + System.currentTimeMillis()</tt> is negative.
186      * @throws IllegalStateException if task was already scheduled or
187      *         cancelled, timer was cancelled, or timer thread terminated.
188      * @throws NullPointerException if {@code task} is null
189      */
schedule(TimerTask task, long delay)190     public void schedule(TimerTask task, long delay) {
191         if (delay < 0)
192             throw new IllegalArgumentException("Negative delay.");
193         sched(task, System.currentTimeMillis()+delay, 0);
194     }
195 
196     /**
197      * Schedules the specified task for execution at the specified time.  If
198      * the time is in the past, the task is scheduled for immediate execution.
199      *
200      * @param task task to be scheduled.
201      * @param time time at which task is to be executed.
202      * @throws IllegalArgumentException if <tt>time.getTime()</tt> is negative.
203      * @throws IllegalStateException if task was already scheduled or
204      *         cancelled, timer was cancelled, or timer thread terminated.
205      * @throws NullPointerException if {@code task} or {@code time} is null
206      */
schedule(TimerTask task, Date time)207     public void schedule(TimerTask task, Date time) {
208         sched(task, time.getTime(), 0);
209     }
210 
211     /**
212      * Schedules the specified task for repeated <i>fixed-delay execution</i>,
213      * beginning after the specified delay.  Subsequent executions take place
214      * at approximately regular intervals separated by the specified period.
215      *
216      * <p>In fixed-delay execution, each execution is scheduled relative to
217      * the actual execution time of the previous execution.  If an execution
218      * is delayed for any reason (such as garbage collection or other
219      * background activity), subsequent executions will be delayed as well.
220      * In the long run, the frequency of execution will generally be slightly
221      * lower than the reciprocal of the specified period (assuming the system
222      * clock underlying <tt>Object.wait(long)</tt> is accurate).
223      *
224      * <p>Fixed-delay execution is appropriate for recurring activities
225      * that require "smoothness."  In other words, it is appropriate for
226      * activities where it is more important to keep the frequency accurate
227      * in the short run than in the long run.  This includes most animation
228      * tasks, such as blinking a cursor at regular intervals.  It also includes
229      * tasks wherein regular activity is performed in response to human
230      * input, such as automatically repeating a character as long as a key
231      * is held down.
232      *
233      * @param task   task to be scheduled.
234      * @param delay  delay in milliseconds before task is to be executed.
235      * @param period time in milliseconds between successive task executions.
236      * @throws IllegalArgumentException if {@code delay < 0}, or
237      *         {@code delay + System.currentTimeMillis() < 0}, or
238      *         {@code period <= 0}
239      * @throws IllegalStateException if task was already scheduled or
240      *         cancelled, timer was cancelled, or timer thread terminated.
241      * @throws NullPointerException if {@code task} is null
242      */
schedule(TimerTask task, long delay, long period)243     public void schedule(TimerTask task, long delay, long period) {
244         if (delay < 0)
245             throw new IllegalArgumentException("Negative delay.");
246         if (period <= 0)
247             throw new IllegalArgumentException("Non-positive period.");
248         sched(task, System.currentTimeMillis()+delay, -period);
249     }
250 
251     /**
252      * Schedules the specified task for repeated <i>fixed-delay execution</i>,
253      * beginning at the specified time. Subsequent executions take place at
254      * approximately regular intervals, separated by the specified period.
255      *
256      * <p>In fixed-delay execution, each execution is scheduled relative to
257      * the actual execution time of the previous execution.  If an execution
258      * is delayed for any reason (such as garbage collection or other
259      * background activity), subsequent executions will be delayed as well.
260      * In the long run, the frequency of execution will generally be slightly
261      * lower than the reciprocal of the specified period (assuming the system
262      * clock underlying <tt>Object.wait(long)</tt> is accurate).  As a
263      * consequence of the above, if the scheduled first time is in the past,
264      * it is scheduled for immediate execution.
265      *
266      * <p>Fixed-delay execution is appropriate for recurring activities
267      * that require "smoothness."  In other words, it is appropriate for
268      * activities where it is more important to keep the frequency accurate
269      * in the short run than in the long run.  This includes most animation
270      * tasks, such as blinking a cursor at regular intervals.  It also includes
271      * tasks wherein regular activity is performed in response to human
272      * input, such as automatically repeating a character as long as a key
273      * is held down.
274      *
275      * @param task   task to be scheduled.
276      * @param firstTime First time at which task is to be executed.
277      * @param period time in milliseconds between successive task executions.
278      * @throws IllegalArgumentException if {@code firstTime.getTime() < 0}, or
279      *         {@code period <= 0}
280      * @throws IllegalStateException if task was already scheduled or
281      *         cancelled, timer was cancelled, or timer thread terminated.
282      * @throws NullPointerException if {@code task} or {@code firstTime} is null
283      */
schedule(TimerTask task, Date firstTime, long period)284     public void schedule(TimerTask task, Date firstTime, long period) {
285         if (period <= 0)
286             throw new IllegalArgumentException("Non-positive period.");
287         sched(task, firstTime.getTime(), -period);
288     }
289 
290     /**
291      * Schedules the specified task for repeated <i>fixed-rate execution</i>,
292      * beginning after the specified delay.  Subsequent executions take place
293      * at approximately regular intervals, separated by the specified period.
294      *
295      * <p>In fixed-rate execution, each execution is scheduled relative to the
296      * scheduled execution time of the initial execution.  If an execution is
297      * delayed for any reason (such as garbage collection or other background
298      * activity), two or more executions will occur in rapid succession to
299      * "catch up."  In the long run, the frequency of execution will be
300      * exactly the reciprocal of the specified period (assuming the system
301      * clock underlying <tt>Object.wait(long)</tt> is accurate).
302      *
303      * <p>Fixed-rate execution is appropriate for recurring activities that
304      * are sensitive to <i>absolute</i> time, such as ringing a chime every
305      * hour on the hour, or running scheduled maintenance every day at a
306      * particular time.  It is also appropriate for recurring activities
307      * where the total time to perform a fixed number of executions is
308      * important, such as a countdown timer that ticks once every second for
309      * ten seconds.  Finally, fixed-rate execution is appropriate for
310      * scheduling multiple repeating timer tasks that must remain synchronized
311      * with respect to one another.
312      *
313      * @param task   task to be scheduled.
314      * @param delay  delay in milliseconds before task is to be executed.
315      * @param period time in milliseconds between successive task executions.
316      * @throws IllegalArgumentException if {@code delay < 0}, or
317      *         {@code delay + System.currentTimeMillis() < 0}, or
318      *         {@code period <= 0}
319      * @throws IllegalStateException if task was already scheduled or
320      *         cancelled, timer was cancelled, or timer thread terminated.
321      * @throws NullPointerException if {@code task} is null
322      */
scheduleAtFixedRate(TimerTask task, long delay, long period)323     public void scheduleAtFixedRate(TimerTask task, long delay, long period) {
324         if (delay < 0)
325             throw new IllegalArgumentException("Negative delay.");
326         if (period <= 0)
327             throw new IllegalArgumentException("Non-positive period.");
328         sched(task, System.currentTimeMillis()+delay, period);
329     }
330 
331     /**
332      * Schedules the specified task for repeated <i>fixed-rate execution</i>,
333      * beginning at the specified time. Subsequent executions take place at
334      * approximately regular intervals, separated by the specified period.
335      *
336      * <p>In fixed-rate execution, each execution is scheduled relative to the
337      * scheduled execution time of the initial execution.  If an execution is
338      * delayed for any reason (such as garbage collection or other background
339      * activity), two or more executions will occur in rapid succession to
340      * "catch up."  In the long run, the frequency of execution will be
341      * exactly the reciprocal of the specified period (assuming the system
342      * clock underlying <tt>Object.wait(long)</tt> is accurate).  As a
343      * consequence of the above, if the scheduled first time is in the past,
344      * then any "missed" executions will be scheduled for immediate "catch up"
345      * execution.
346      *
347      * <p>Fixed-rate execution is appropriate for recurring activities that
348      * are sensitive to <i>absolute</i> time, such as ringing a chime every
349      * hour on the hour, or running scheduled maintenance every day at a
350      * particular time.  It is also appropriate for recurring activities
351      * where the total time to perform a fixed number of executions is
352      * important, such as a countdown timer that ticks once every second for
353      * ten seconds.  Finally, fixed-rate execution is appropriate for
354      * scheduling multiple repeating timer tasks that must remain synchronized
355      * with respect to one another.
356      *
357      * @param task   task to be scheduled.
358      * @param firstTime First time at which task is to be executed.
359      * @param period time in milliseconds between successive task executions.
360      * @throws IllegalArgumentException if {@code firstTime.getTime() < 0} or
361      *         {@code period <= 0}
362      * @throws IllegalStateException if task was already scheduled or
363      *         cancelled, timer was cancelled, or timer thread terminated.
364      * @throws NullPointerException if {@code task} or {@code firstTime} is null
365      */
scheduleAtFixedRate(TimerTask task, Date firstTime, long period)366     public void scheduleAtFixedRate(TimerTask task, Date firstTime,
367                                     long period) {
368         if (period <= 0)
369             throw new IllegalArgumentException("Non-positive period.");
370         sched(task, firstTime.getTime(), period);
371     }
372 
373     /**
374      * Schedule the specified timer task for execution at the specified
375      * time with the specified period, in milliseconds.  If period is
376      * positive, the task is scheduled for repeated execution; if period is
377      * zero, the task is scheduled for one-time execution. Time is specified
378      * in Date.getTime() format.  This method checks timer state, task state,
379      * and initial execution time, but not period.
380      *
381      * @throws IllegalArgumentException if <tt>time</tt> is negative.
382      * @throws IllegalStateException if task was already scheduled or
383      *         cancelled, timer was cancelled, or timer thread terminated.
384      * @throws NullPointerException if {@code task} is null
385      */
sched(TimerTask task, long time, long period)386     private void sched(TimerTask task, long time, long period) {
387         if (time < 0)
388             throw new IllegalArgumentException("Illegal execution time.");
389 
390         // Constrain value of period sufficiently to prevent numeric
391         // overflow while still being effectively infinitely large.
392         if (Math.abs(period) > (Long.MAX_VALUE >> 1))
393             period >>= 1;
394 
395         synchronized(queue) {
396             if (!thread.newTasksMayBeScheduled)
397                 throw new IllegalStateException("Timer already cancelled.");
398 
399             synchronized(task.lock) {
400                 if (task.state != TimerTask.VIRGIN)
401                     throw new IllegalStateException(
402                         "Task already scheduled or cancelled");
403                 task.nextExecutionTime = time;
404                 task.period = period;
405                 task.state = TimerTask.SCHEDULED;
406             }
407 
408             queue.add(task);
409             if (queue.getMin() == task)
410                 queue.notify();
411         }
412     }
413 
414     /**
415      * Terminates this timer, discarding any currently scheduled tasks.
416      * Does not interfere with a currently executing task (if it exists).
417      * Once a timer has been terminated, its execution thread terminates
418      * gracefully, and no more tasks may be scheduled on it.
419      *
420      * <p>Note that calling this method from within the run method of a
421      * timer task that was invoked by this timer absolutely guarantees that
422      * the ongoing task execution is the last task execution that will ever
423      * be performed by this timer.
424      *
425      * <p>This method may be called repeatedly; the second and subsequent
426      * calls have no effect.
427      */
cancel()428     public void cancel() {
429         synchronized(queue) {
430             thread.newTasksMayBeScheduled = false;
431             queue.clear();
432             queue.notify();  // In case queue was already empty.
433         }
434     }
435 
436     /**
437      * Removes all cancelled tasks from this timer's task queue.  <i>Calling
438      * this method has no effect on the behavior of the timer</i>, but
439      * eliminates the references to the cancelled tasks from the queue.
440      * If there are no external references to these tasks, they become
441      * eligible for garbage collection.
442      *
443      * <p>Most programs will have no need to call this method.
444      * It is designed for use by the rare application that cancels a large
445      * number of tasks.  Calling this method trades time for space: the
446      * runtime of the method may be proportional to n + c log n, where n
447      * is the number of tasks in the queue and c is the number of cancelled
448      * tasks.
449      *
450      * <p>Note that it is permissible to call this method from within a
451      * a task scheduled on this timer.
452      *
453      * @return the number of tasks removed from the queue.
454      * @since 1.5
455      */
purge()456      public int purge() {
457          int result = 0;
458 
459          synchronized(queue) {
460              for (int i = queue.size(); i > 0; i--) {
461                  if (queue.get(i).state == TimerTask.CANCELLED) {
462                      queue.quickRemove(i);
463                      result++;
464                  }
465              }
466 
467              if (result != 0)
468                  queue.heapify();
469          }
470 
471          return result;
472      }
473 }
474 
475 /**
476  * This "helper class" implements the timer's task execution thread, which
477  * waits for tasks on the timer queue, executions them when they fire,
478  * reschedules repeating tasks, and removes cancelled tasks and spent
479  * non-repeating tasks from the queue.
480  */
481 class TimerThread extends Thread {
482     /**
483      * This flag is set to false by the reaper to inform us that there
484      * are no more live references to our Timer object.  Once this flag
485      * is true and there are no more tasks in our queue, there is no
486      * work left for us to do, so we terminate gracefully.  Note that
487      * this field is protected by queue's monitor!
488      */
489     boolean newTasksMayBeScheduled = true;
490 
491     /**
492      * Our Timer's queue.  We store this reference in preference to
493      * a reference to the Timer so the reference graph remains acyclic.
494      * Otherwise, the Timer would never be garbage-collected and this
495      * thread would never go away.
496      */
497     private TaskQueue queue;
498 
TimerThread(TaskQueue queue)499     TimerThread(TaskQueue queue) {
500         this.queue = queue;
501     }
502 
run()503     public void run() {
504         try {
505             mainLoop();
506         } finally {
507             // Someone killed this Thread, behave as if Timer cancelled
508             synchronized(queue) {
509                 newTasksMayBeScheduled = false;
510                 queue.clear();  // Eliminate obsolete references
511             }
512         }
513     }
514 
515     /**
516      * The main timer loop.  (See class comment.)
517      */
mainLoop()518     private void mainLoop() {
519         while (true) {
520             try {
521                 TimerTask task;
522                 boolean taskFired;
523                 synchronized(queue) {
524                     // Wait for queue to become non-empty
525                     while (queue.isEmpty() && newTasksMayBeScheduled)
526                         queue.wait();
527                     if (queue.isEmpty())
528                         break; // Queue is empty and will forever remain; die
529 
530                     // Queue nonempty; look at first evt and do the right thing
531                     long currentTime, executionTime;
532                     task = queue.getMin();
533                     synchronized(task.lock) {
534                         if (task.state == TimerTask.CANCELLED) {
535                             queue.removeMin();
536                             continue;  // No action required, poll queue again
537                         }
538                         currentTime = System.currentTimeMillis();
539                         executionTime = task.nextExecutionTime;
540                         if (taskFired = (executionTime<=currentTime)) {
541                             if (task.period == 0) { // Non-repeating, remove
542                                 queue.removeMin();
543                                 task.state = TimerTask.EXECUTED;
544                             } else { // Repeating task, reschedule
545                                 queue.rescheduleMin(
546                                   task.period<0 ? currentTime   - task.period
547                                                 : executionTime + task.period);
548                             }
549                         }
550                     }
551                     if (!taskFired) // Task hasn't yet fired; wait
552                         queue.wait(executionTime - currentTime);
553                 }
554                 if (taskFired)  // Task fired; run it, holding no locks
555                     task.run();
556             } catch(InterruptedException e) {
557             }
558         }
559     }
560 }
561 
562 /**
563  * This class represents a timer task queue: a priority queue of TimerTasks,
564  * ordered on nextExecutionTime.  Each Timer object has one of these, which it
565  * shares with its TimerThread.  Internally this class uses a heap, which
566  * offers log(n) performance for the add, removeMin and rescheduleMin
567  * operations, and constant time performance for the getMin operation.
568  */
569 class TaskQueue {
570     /**
571      * Priority queue represented as a balanced binary heap: the two children
572      * of queue[n] are queue[2*n] and queue[2*n+1].  The priority queue is
573      * ordered on the nextExecutionTime field: The TimerTask with the lowest
574      * nextExecutionTime is in queue[1] (assuming the queue is nonempty).  For
575      * each node n in the heap, and each descendant of n, d,
576      * n.nextExecutionTime <= d.nextExecutionTime.
577      */
578     private TimerTask[] queue = new TimerTask[128];
579 
580     /**
581      * The number of tasks in the priority queue.  (The tasks are stored in
582      * queue[1] up to queue[size]).
583      */
584     private int size = 0;
585 
586     /**
587      * Returns the number of tasks currently on the queue.
588      */
589     int size() {
590         return size;
591     }
592 
593     /**
594      * Adds a new task to the priority queue.
595      */
596     void add(TimerTask task) {
597         // Grow backing store if necessary
598         if (size + 1 == queue.length)
599             queue = Arrays.copyOf(queue, 2*queue.length);
600 
601         queue[++size] = task;
602         fixUp(size);
603     }
604 
605     /**
606      * Return the "head task" of the priority queue.  (The head task is an
607      * task with the lowest nextExecutionTime.)
608      */
609     TimerTask getMin() {
610         return queue[1];
611     }
612 
613     /**
614      * Return the ith task in the priority queue, where i ranges from 1 (the
615      * head task, which is returned by getMin) to the number of tasks on the
616      * queue, inclusive.
617      */
618     TimerTask get(int i) {
619         return queue[i];
620     }
621 
622     /**
623      * Remove the head task from the priority queue.
624      */
625     void removeMin() {
626         queue[1] = queue[size];
627         queue[size--] = null;  // Drop extra reference to prevent memory leak
628         fixDown(1);
629     }
630 
631     /**
632      * Removes the ith element from queue without regard for maintaining
633      * the heap invariant.  Recall that queue is one-based, so
634      * 1 <= i <= size.
635      */
636     void quickRemove(int i) {
637         assert i <= size;
638 
639         queue[i] = queue[size];
640         queue[size--] = null;  // Drop extra ref to prevent memory leak
641     }
642 
643     /**
644      * Sets the nextExecutionTime associated with the head task to the
645      * specified value, and adjusts priority queue accordingly.
646      */
647     void rescheduleMin(long newTime) {
648         queue[1].nextExecutionTime = newTime;
649         fixDown(1);
650     }
651 
652     /**
653      * Returns true if the priority queue contains no elements.
654      */
655     boolean isEmpty() {
656         return size==0;
657     }
658 
659     /**
660      * Removes all elements from the priority queue.
661      */
662     void clear() {
663         // Null out task references to prevent memory leak
664         for (int i=1; i<=size; i++)
665             queue[i] = null;
666 
667         size = 0;
668     }
669 
670     /**
671      * Establishes the heap invariant (described above) assuming the heap
672      * satisfies the invariant except possibly for the leaf-node indexed by k
673      * (which may have a nextExecutionTime less than its parent's).
674      *
675      * This method functions by "promoting" queue[k] up the hierarchy
676      * (by swapping it with its parent) repeatedly until queue[k]'s
677      * nextExecutionTime is greater than or equal to that of its parent.
678      */
679     private void fixUp(int k) {
680         while (k > 1) {
681             int j = k >> 1;
682             if (queue[j].nextExecutionTime <= queue[k].nextExecutionTime)
683                 break;
684             TimerTask tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;
685             k = j;
686         }
687     }
688 
689     /**
690      * Establishes the heap invariant (described above) in the subtree
691      * rooted at k, which is assumed to satisfy the heap invariant except
692      * possibly for node k itself (which may have a nextExecutionTime greater
693      * than its children's).
694      *
695      * This method functions by "demoting" queue[k] down the hierarchy
696      * (by swapping it with its smaller child) repeatedly until queue[k]'s
697      * nextExecutionTime is less than or equal to those of its children.
698      */
699     private void fixDown(int k) {
700         int j;
701         while ((j = k << 1) <= size && j > 0) {
702             if (j < size &&
703                 queue[j].nextExecutionTime > queue[j+1].nextExecutionTime)
704                 j++; // j indexes smallest kid
705             if (queue[k].nextExecutionTime <= queue[j].nextExecutionTime)
706                 break;
707             TimerTask tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;
708             k = j;
709         }
710     }
711 
712     /**
713      * Establishes the heap invariant (described above) in the entire tree,
714      * assuming nothing about the order of the elements prior to the call.
715      */
716     void heapify() {
717         for (int i = size/2; i >= 1; i--)
718             fixDown(i);
719     }
720 }
721