观察者模式

public class Observable {    private boolean changed = false;    private final ArrayList<Observer> observers;    /** Construct an Observable with zero Observers. */    public Observable() {        observers = new ArrayList<>();    }    /**     * 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 (!observers.contains(o)) {            observers.add(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) {        observers.remove(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     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</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     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.         */        Observer[] arrLocal;        synchronized (this) {            /* We don't want the Observer doing callbacks into             * arbitrary Observables while holding its own Monitor.             * The code where we extract each Observable from             * the ArrayList 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 (!hasChanged())                return;            arrLocal = observers.toArray(new Observer[observers.size()]);            clearChanged();        }        for (int i = arrLocal.length-1; i>=0; i--)            arrLocal[i].update(this, arg);    }    /**     * Clears the observer list so that this object no longer has any observers.     */    public synchronized void deleteObservers() {        observers.clear();    }    /**     * 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     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</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     java.util.Observable#clearChanged()     * @see     java.util.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 observers.size();    }}

public interface Observer {    /**     * This method is called whenever the observed object is changed. An     * application calls an <tt>Observable</tt> object's     * <code>notifyObservers</code> 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</code>     *                 method.     */    void update(Observable o, Object arg);}

public class WeatherStation extends Observable{public int temperature;//温度public int dampness;//湿度public void setTemperature(int temperature) {this.temperature = temperature;Weather weather=new Weather();weather.temperature=temperature;setChanged();notifyObservers(weather);}public void setDampness(int dampness) {this.dampness = dampness;}}

public class Weather {public int temperature;//温度public int dampness;//湿度}

public class LaoWang implements Observer{@Overridepublic void update(Observable o, Object arg) {Weather weather=(Weather)arg;if(weather.temperature>10){System.out.println("老王打开了空调");}}}

public class XiaoLi implements Observer{@Overridepublic void update(Observable o, Object arg) {WeatherStation station=(WeatherStation)o;if(station.temperature<0){System.out.println("小李穿上了棉袄");}}}

public static void main(String[] args) {WeatherStation station=new WeatherStation();XiaoLi xiaoli=new XiaoLi();LaoWang laowang=new LaoWang();station.addObserver(laowang);station.addObserver(xiaoli);station.setTemperature(12);}}