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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   * 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
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   *
   * 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;
  
  /**
   * This class represents an observable object, or "data"
   * in the model-view paradigm. It can be subclassed to represent an
   * object that the application wants to have observed.
   * <p>
   * An observable object can have one or more observers. An observer
   * may be any object that implements interface <tt>Observer</tt>. After an
   * observable instance changes, an application calling the
   * <code>Observable</code>'s <code>notifyObservers</code> method
   * causes all of its observers to be notified of the change by a call
   * to their <code>update</code> method.
   * <p>
   * The order in which notifications will be delivered is unspecified.
   * The default implementation provided in the Observable class will
   * notify Observers in the order in which they registered interest, but
   * subclasses may change this order, use no guaranteed order, deliver
   * notifications on separate threads, or may guarantee that their
   * subclass follows this order, as they choose.
   * <p>
   * Note that this notification mechanism has nothing to do with threads
   * and is completely separate from the <tt>wait</tt> and <tt>notify</tt>
   * mechanism of class <tt>Object</tt>.
   * <p>
   * When an observable object is newly created, its set of observers is
   * empty. Two observers are considered the same if and only if the
   * <tt>equals</tt> method returns true for them.
   *
   * @author  Chris Warth
   * @see     Observable#notifyObservers()
   * @see     Observable#notifyObservers(Object)
   * @see     Observer
   * @see     Observer#update(Observable, Object)
   * @since   JDK1.0
   */
  public class Observable {
      private boolean changed = false;
      private Vector<Observer> obs;
  
      /** Construct an Observable with zero Observers. */
  
      public Observable() {
          obs = new Vector<>();
      }
  
      /**
       * Adds an observer to the set of observers for this object, provided
       * that it is not the same as some observer already in the set.
       * The order in which notifications will be delivered to multiple
       * observers is not specified. See the class comment.
       *
       * @param   o   an observer to be added.
       * @throws NullPointerException   if the parameter o is null.
       */
      public synchronized void addObserver(Observer o) {
          if (o == null)
              throw new NullPointerException();
          if (!obs.contains(o)) {
              obs.addElement(o);
          }
      }
  
      /**
       * Deletes an observer from the set of observers of this object.
       * Passing <CODE>null</CODE> to this method will have no effect.
       * @param   o   the observer to be deleted.
       */
      public synchronized void deleteObserver(Observer o) {
          obs.removeElement(o);
      }
  
      /**
       * If this object has changed, as indicated by the
       * <code>hasChanged</code> method, then notify all of its observers
       * and then call the <code>clearChanged</code> method to
       * indicate that this object has no longer changed.
       * <p>
       * Each observer has its <code>update</code> method called with two
       * arguments: this observable object and <code>null</code>. In other
       * words, this method is equivalent to:
       * <blockquote><tt>
       * notifyObservers(null)</tt></blockquote>
       *
       * @see     Observable#clearChanged()
       * @see     Observable#hasChanged()
       * @see     Observer#update(Observable, Object)
       */
      public void notifyObservers() {
          notifyObservers(null);
      }
  
      /**
       * If this object has changed, as indicated by the
       * <code>hasChanged</code> method, then notify all of its observers
       * and then call the <code>clearChanged</code> method to indicate
       * that this object has no longer changed.
       * <p>
       * Each observer has its <code>update</code> method called with two
       * arguments: this observable object and the <code>arg</code> argument.
       *
       * @param   arg   any object.
       * @see     Observable#clearChanged()
       * @see     Observable#hasChanged()
       * @see     Observer#update(Observable, Object)
       */
      public void notifyObservers(Object arg) {
          /*
           * a temporary array buffer, used as a snapshot of the state of
           * current Observers.
           */
          Object[] arrLocal;
  
          synchronized (this) {
              /* We don't want the Observer doing callbacks into
               * arbitrary code while holding its own Monitor.
               * The code where we extract each Observable from
               * the Vector and store the state of the Observer
               * needs synchronization, but notifying observers
               * does not (should not).  The worst result of any
               * potential race-condition here is that:
               * 1) a newly-added Observer will miss a
               *   notification in progress
               * 2) a recently unregistered Observer will be
               *   wrongly notified when it doesn't care
               */
              // Android-changed: Call out to hasChanged() to figure out if something changes.
              // Upstream code avoids calling the nonfinal hasChanged() from the synchronized block,
              // but that would break compatibility for apps that override that method.
              // if (!changed)
              if (!hasChanged())
                  return;
              arrLocal = obs.toArray();
              clearChanged();
          }
  
          for (int i = arrLocal.length-1; i>=0; i--)
              ((Observer)arrLocal[i]).update(this, arg);
      }
  
      /**
       * Clears the observer list so that this object no longer has any observers.
       */
      public synchronized void deleteObservers() {
          obs.removeAllElements();
      }
  
      /**
       * Marks this <tt>Observable</tt> object as having been changed; the
       * <tt>hasChanged</tt> method will now return <tt>true</tt>.
       */
      protected synchronized void setChanged() {
          changed = true;
      }
  
      /**
       * Indicates that this object has no longer changed, or that it has
       * already notified all of its observers of its most recent change,
       * so that the <tt>hasChanged</tt> method will now return <tt>false</tt>.
       * This method is called automatically by the
       * <code>notifyObservers</code> methods.
       *
       * @see     Observable#notifyObservers()
       * @see     Observable#notifyObservers(Object)
       */
      protected synchronized void clearChanged() {
          changed = false;
      }
  
      /**
       * Tests if this object has changed.
       *
       * @return  <code>true</code> if and only if the <code>setChanged</code>
       *          method has been called more recently than the
       *          <code>clearChanged</code> method on this object;
       *          <code>false</code> otherwise.
       * @see     Observable#clearChanged()
       * @see     Observable#setChanged()
       */
      public synchronized boolean hasChanged() {
          return changed;
      }
  
      /**
       * Returns the number of observers of this <tt>Observable</tt> object.
       *
       * @return  the number of observers of this object.
       */
      public synchronized int countObservers() {
          return obs.size();
      }
  }
  