gcc/libjava/java/util/Observable.java

/* Observable.java -- an object to be observed
   Copyright (C) 1999, 2000, 2001, 2002 Free Software Foundation, Inc.

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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any later version.

GNU Classpath 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 for more details.

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along with GNU Classpath; see the file COPYING.  If not, write to the
Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
02111-1307 USA.

Linking this library statically or dynamically with other modules is
making a combined work based on this library.  Thus, the terms and
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As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
executable, regardless of the license terms of these independent
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terms of your choice, provided that you also meet, for each linked
independent module, the terms and conditions of the license of that
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this exception to your version of the library, but you are not
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exception statement from your version. */


package java.util;

/**
 * This class represents an object which is observable.  Other objects may
 * register their intent to be notified when this object changes; and when
 * this object does change, it will trigger the <code>update</code> method
 * of each observer.
 *
 * Note that the <code>notifyObservers()</code> method of this class is
 * unrelated to the <code>notify()</code> of Object.
 *
 * @author Warren Levy <warrenl@cygnus.com>
 * @author Eric Blake <ebb9@email.byu.edu>
 * @see Observer
 * @status updated to 1.4
 */
public class Observable
{
  /** Tracks whether this object has changed. */
  private boolean changed;

  /* List of the Observers registered as interested in this Observable. */
  private LinkedHashSet observers;

  /**
   * Constructs an Observable with zero Observers.
   */
  public Observable()
  {
    observers = new LinkedHashSet();
  }

  /**
   * Adds an Observer. If the observer was already added this method does
   * nothing.
   *
   * @param observer Observer to add
   * @throws NullPointerException if observer is null
   */
  public synchronized void addObserver(Observer observer)
  {
    observers.add(observer);
  }

  /**
   * Reset this Observable's state to unchanged. This is called automatically
   * by <code>notifyObservers</code> once all observers have been notified.
   *
   * @see #notifyObservers()
   */
  protected synchronized void clearChanged()
  {
    changed = false;
  }

  /**
   * Returns the number of observers for this object.
   *
   * @return number of Observers for this
   */
  public synchronized int countObservers()
  {
    return observers.size();
  }

  /**
   * Deletes an Observer of this Observable.
   *
   * @param victim Observer to delete
   */
  public synchronized void deleteObserver(Observer victim)
  {
    observers.remove(victim);
  }

  /**
   * Deletes all Observers of this Observable.
   */
  public synchronized void deleteObservers()
  {
    observers.clear();
  }

  /**
   * True if <code>setChanged</code> has been called more recently than
   * <code>clearChanged</code>.
   *
   * @return whether or not this Observable has changed
   */
  public synchronized boolean hasChanged()
  {
    return changed;
  }

  /**
   * If the Observable has actually changed then tell all Observers about it,
   * then reset state to unchanged.
   *
   * @see #notifyObservers(Object)
   * @see Observer#update(Observable, Object)
   */
  public void notifyObservers()
  {
    notifyObservers(null);
  }

  /**
   * If the Observable has actually changed then tell all Observers about it,
   * then reset state to unchanged. Note that though the order of
   * notification is unspecified in subclasses, in Observable it is in the
   * order of registration.
   *
   * @param obj argument to Observer's update method
   * @see Observer#update(Observable, Object)
   */
  public void notifyObservers(Object obj)
  {
    if (! hasChanged())
      return;
    // Create clone inside monitor, as that is relatively fast and still
    // important to keep threadsafe, but update observers outside of the
    // lock since update() can call arbitrary code.
    Set s;
    synchronized (this)
      {
        s = (Set) observers.clone();
      }
    int i = s.size();
    Iterator iter = s.iterator();
    while (--i >= 0)
      ((Observer) iter.next()).update(this, obj);
    clearChanged();
  }

  /**
   * Marks this Observable as having changed.
   */
  protected synchronized void setChanged()
  {
    changed = true;
  }
}