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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;
  
  /**
   * A task that can be scheduled for one-time or repeated execution by a Timer.
   *
   * @author  Josh Bloch
   * @see     Timer
   * @since   1.3
   */
  
  public abstract class TimerTask implements Runnable {
      /**
       * This object is used to control access to the TimerTask internals.
       */
      final Object lock = new Object();
  
      /**
       * The state of this task, chosen from the constants below.
       */
      int state = VIRGIN;
  
      /**
       * This task has not yet been scheduled.
       */
      static final int VIRGIN = 0;
  
      /**
       * This task is scheduled for execution.  If it is a non-repeating task,
       * it has not yet been executed.
       */
      static final int SCHEDULED   = 1;
  
      /**
       * This non-repeating task has already executed (or is currently
       * executing) and has not been cancelled.
       */
      static final int EXECUTED    = 2;
  
      /**
       * This task has been cancelled (with a call to TimerTask.cancel).
       */
      static final int CANCELLED   = 3;
  
      /**
       * Next execution time for this task in the format returned by
       * System.currentTimeMillis, assuming this task is scheduled for execution.
       * For repeating tasks, this field is updated prior to each task execution.
       */
      long nextExecutionTime;
  
      /**
       * Period in milliseconds for repeating tasks.  A positive value indicates
       * fixed-rate execution.  A negative value indicates fixed-delay execution.
       * A value of 0 indicates a non-repeating task.
       */
      long period = 0;
  
      /**
       * Creates a new timer task.
       */
      protected TimerTask() {
      }
  
      /**
       * The action to be performed by this timer task.
       */
      public abstract void run();
  
      /**
       * Cancels this timer task.  If the task has been scheduled for one-time
       * execution and has not yet run, or has not yet been scheduled, it will
       * never run.  If the task has been scheduled for repeated execution, it
       * will never run again.  (If the task is running when this call occurs,
       * the task will run to completion, but will never run again.)
       *
       * <p>Note that calling this method from within the <tt>run</tt> method of
       * a repeating timer task absolutely guarantees that the timer task will
       * not run again.
       *
       * <p>This method may be called repeatedly; the second and subsequent
       * calls have no effect.
       *
       * @return true if this task is scheduled for one-time execution and has
       *         not yet run, or this task is scheduled for repeated execution.
       *         Returns false if the task was scheduled for one-time execution
       *         and has already run, or if the task was never scheduled, or if
       *         the task was already cancelled.  (Loosely speaking, this method
       *         returns <tt>true</tt> if it prevents one or more scheduled
       *         executions from taking place.)
       */
      public boolean cancel() {
          synchronized(lock) {
              boolean result = (state == SCHEDULED);
              state = CANCELLED;
              return result;
          }
      }
  
      /**
       * Returns the <i>scheduled</i> execution time of the most recent
       * <i>actual</i> execution of this task.  (If this method is invoked
       * while task execution is in progress, the return value is the scheduled
       * execution time of the ongoing task execution.)
       *
       * <p>This method is typically invoked from within a task's run method, to
       * determine whether the current execution of the task is sufficiently
       * timely to warrant performing the scheduled activity:
       * <pre>{@code
       *   public void run() {
       *       if (System.currentTimeMillis() - scheduledExecutionTime() >=
       *           MAX_TARDINESS)
       *               return;  // Too late; skip this execution.
       *       // Perform the task
       *   }
       * }</pre>
       * This method is typically <i>not</i> used in conjunction with
       * <i>fixed-delay execution</i> repeating tasks, as their scheduled
       * execution times are allowed to drift over time, and so are not terribly
       * significant.
       *
       * @return the time at which the most recent execution of this task was
       *         scheduled to occur, in the format returned by Date.getTime().
       *         The return value is undefined if the task has yet to commence
       *         its first execution.
       * @see Date#getTime()
       */
      public long scheduledExecutionTime() {
          synchronized(lock) {
              return (period < 0 ? nextExecutionTime + period
                                 : nextExecutionTime - period);
          }
      }
  }
  