java.util: public class: Observable

java.util
public class: Observable [javadoc | source]

java.lang.Object
   java.util.Observable

Direct Known Subclasses:

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.

An observable object can have one or more observers. An observer may be any object that implements interface Observer. After an observable instance changes, an application calling the Observable's notifyObservers method causes all of its observers to be notified of the change by a call to their update method.

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.

Note that this notification mechanism is has nothing to do with threads and is completely separate from the wait and notify mechanism of class Object.

When an observable object is newly created, its set of observers is empty. Two observers are considered the same if and only if the equals method returns true for them.

Also see:

java.util.Observable#notifyObservers()

java.util.Observable#notifyObservers(java.lang.Object)

java.util.Observer

java.util.Observer#update(java.util.Observable, java.lang.Object)

author: Chris - Warth

since: JDK1.0 -

Constructor:
public Observable() { obs = new Vector(); } Construct an Observable with zero Observers.

Constructor:

 public Observable() {
           
        obs = new Vector();
}

Construct an Observable with zero Observers.

Method from java.util.Observable Summary:
addObserver, clearChanged, countObservers, deleteObserver, deleteObservers, hasChanged, notifyObservers, notifyObservers, setChanged

Methods from java.lang.Object:
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method from java.util.Observable Detail:
public synchronized void addObserver(Observer o) { if (o == null) throw new NullPointerException(); if (!obs.contains(o)) { obs.addElement(o); } } 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.
protected synchronized void clearChanged() { changed = false; } 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 `hasChanged` method will now return `false`. This method is called automatically by the `notifyObservers` methods.
public synchronized int countObservers() { return obs.size(); } Returns the number of observers of this `Observable` object.
public synchronized void deleteObserver(Observer o) { obs.removeElement(o); } Deletes an observer from the set of observers of this object. Passing `null` to this method will have no effect.
public synchronized void deleteObservers() { obs.removeAllElements(); } Clears the observer list so that this object no longer has any observers.
public synchronized boolean hasChanged() { return changed; } Tests if this object has changed.
public void notifyObservers() { notifyObservers(null); } If this object has changed, as indicated by the `hasChanged` method, then notify all of its observers and then call the `clearChanged` method to indicate that this object has no longer changed. Each observer has its `update` method called with two arguments: this observable object and `null`. In other words, this method is equivalent to: `notifyObservers(null)`
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); } If this object has changed, as indicated by the `hasChanged` method, then notify all of its observers and then call the `clearChanged` method to indicate that this object has no longer changed. Each observer has its `update` method called with two arguments: this observable object and the `arg` argument.
protected synchronized void setChanged() { changed = true; } Marks this `Observable` object as having been changed; the `hasChanged` method will now return `true`.

Method from java.util.Observable Detail:

 public synchronized  void addObserver(Observer o) {
        if (o == null)
            throw new NullPointerException();
        if (!obs.contains(o)) {
            obs.addElement(o);
        }
}

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.

 protected synchronized  void clearChanged() {
        changed = false;
}

hasChanged

false

notifyObservers

 public synchronized int countObservers() {
        return obs.size();
}

Observable

 public synchronized  void deleteObserver(Observer o) {
        obs.removeElement(o);
}

null

 public synchronized  void deleteObservers() {
        obs.removeAllElements();
}

Clears the observer list so that this object no longer has any observers.

 public synchronized boolean hasChanged() {
        return changed;
}

Tests if this object has changed.

 public  void notifyObservers() {
        notifyObservers(null);
}

hasChanged

clearChanged

Each observer has its update method called with two arguments: this observable object and null. In other words, this method is equivalent to:

notifyObservers(null)

 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);
}

hasChanged

clearChanged

Each observer has its update method called with two arguments: this observable object and the arg argument.

 protected synchronized  void setChanged() {
        changed = true;
}

Observable

hasChanged

true