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    4    *
    5    * This code is free software; you can redistribute it and/or modify it
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    9    * by Oracle in the LICENSE file that accompanied this code.
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   13    * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   14    * version 2 for more details (a copy is included in the LICENSE file that
   15    * accompanied this code).
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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);
  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        */
  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        */
  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        */
  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        */
  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        */
  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        */
  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        */
  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        */
  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        */
  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        */
  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        */
  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        */
  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        */
  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   
  499       TimerThread(TaskQueue queue) {
  500           this.queue = queue;
  501       }
  502   
  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        */
  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   }

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