src/java.base/share/classes/java/util/Observable.java

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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 {@code Observer}. After an
 * observable instance changes, an application calling the
 * {@code Observable}'s {@code notifyObservers} method
 * causes all of its observers to be notified of the change by a call
 * to their {@code update} 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 {@code wait} and {@code notify}
 * mechanism of class {@code Object}.
 * <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
 * {@code equals} method returns true for them.
 *
 * @author  Chris Warth
 * @see     java.util.Observable#notifyObservers()
 * @see     java.util.Observable#notifyObservers(java.lang.Object)
 * @see     java.util.Observer
 * @see     java.util.Observer#update(java.util.Observable, java.lang.Object)
 * @since   1.0
 *
 * @deprecated
 * This class and the {@link Observer} interface have been deprecated.
 * The event model supported by {@code Observer} and {@code Observable}
 * is quite limited, the order of notifications delivered by
 * {@code Observable} is unspecified, and state changes are not in
 * one-for-one correspondence with notifications.
 * For a richer event model, consider using the
 * {@link java.desktop/java.beans} package.  For reliable and ordered
 * messaging among threads, consider using one of the concurrent data
 * structures in the {@link java.util.concurrent} package.
 * For reactive streams style programming, see the
 * {@link java.util.concurrent.Flow} API.
 */
@Deprecated(since="9")
@SuppressWarnings("doclint:reference") // cross-module links
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} 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} method, then notify all of its observers
     * and then call the {@code clearChanged} method to
     * indicate that this object has no longer changed.
     * <p>
     * Each observer has its {@code update} method called with two
     * arguments: this observable object and {@code null}. In other
     * words, this method is equivalent to:
     * <blockquote>{@code
     * notifyObservers(null)}</blockquote>
     *
     * @see     java.util.Observable#clearChanged()
     * @see     java.util.Observable#hasChanged()
     * @see     java.util.Observer#update(java.util.Observable, java.lang.Object)
     */
    public void notifyObservers() {
        notifyObservers(null);
    }

    /**
     * If this object has changed, as indicated by the
     * {@code hasChanged} method, then notify all of its observers
     * and then call the {@code clearChanged} method to indicate
     * that this object has no longer changed.
     * <p>
     * Each observer has its {@code update} method called with two
     * arguments: this observable object and the {@code arg} argument.
     *
     * @param   arg   any object.
     * @see     java.util.Observable#clearChanged()
     * @see     java.util.Observable#hasChanged()
     * @see     java.util.Observer#update(java.util.Observable, java.lang.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
             */
            if (!changed)
                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 {@code Observable} object as having been changed; the
     * {@code hasChanged} method will now return {@code true}.
     */
    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 {@code hasChanged} method will now return {@code false}.
     * This method is called automatically by the
     * {@code notifyObservers} methods.
     *
     * @see     java.util.Observable#notifyObservers()
     * @see     java.util.Observable#notifyObservers(java.lang.Object)
     */
    protected synchronized void clearChanged() {
        changed = false;
    }

    /**
     * Tests if this object has changed.
     *
     * @return  {@code true} if and only if the {@code setChanged}
     *          method has been called more recently than the
     *          {@code clearChanged} method on this object;
     *          {@code false} otherwise.
     * @see     java.util.Observable#clearChanged()
     * @see     java.util.Observable#setChanged()
     */
    public synchronized boolean hasChanged() {
        return changed;
    }

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