package Observer.example03;

import java.util.Vector;

public class Observable<T> {

    private boolean changed = false;
    private Vector<Observer<T>> 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<T> 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<T> 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(T arg) {
        /*
         * a temporary array buffer, used as a snapshot of the state of
         * current Observers.
         */
        Vector<Observer<T>> 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 = (Vector<Observer<T>>) obs.clone();
            clearChanged();
        }

        for (Observer<T> observer: arrLocal) {
            observer.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}.
     */
    public 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();
    }

    public interface Observer<T> {
        /**
         * This method is called whenever the observed object is changed. An
         * application calls an {@code Observable} object's
         * {@code notifyObservers} method to have all the object's
         * observers notified of the change.
         *
         * @param   o     the observable object.
         * @param   arg   an argument passed to the {@code notifyObservers}
         *                 method.
         */
        void update(Observable<T> o, T arg);
    }

}
