com.graemestevenson.observer
Class Observable<T>

java.lang.Object
  com.graemestevenson.observer.Observable<T>

Type Parameters:

T - the type of the argument passed to the notifyAll method.

public class Observable<T>
extends java.lang.Object

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.

This code (and documentation) is based on the Observable class provided with the Java SDK ( java.util.Observable), but extends it to use generics. The generic parameter associated with this class allows the programmer to state the type of the object passed using the notifyObervers method. This is coupled with the generic Observer interface which supports a typed updated method.

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.

Author:

Graeme Stevenson (graeme.stevenson@ucd.ie), based on Chris Warth's code and documentation from the Java SDK.

Constructor Summary

Observable()

Method Summary

void	addObserver(Observer<T> an_observer) 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.
int	countObservers() Returns the number of observers of this Observable object.
void	deleteObserver(Observer<T> an_observer) Deletes an observer from the set of observers of this object.
void	deleteObservers() Clears the observer list so that this object no longer has any observers.
boolean	hasChanged() Tests if this object has changed.
void	notifyObservers() 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.
void	notifyObservers(T an_argument) 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.

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Observable

public Observable()

Method Detail

addObserver

public final void addObserver(Observer<T> an_observer)
                       throws java.lang.IllegalArgumentException

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.

Parameters:

an_observer - an observer to be added.

Throws:

java.lang.IllegalArgumentException - if the parameter o is null.

deleteObserver

public final void deleteObserver(Observer<T> an_observer)

Deletes an observer from the set of observers of this object. Passing null to this method will have no effect.

Parameters:

an_observer - the observer to be deleted.

notifyObservers

public final void notifyObservers()

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)

notifyObservers

public final void notifyObservers(T an_argument)

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 an_argument argument.

Parameters:

an_argument - an object of the gerneric type associated with an instance of this class.

hasChanged

public final boolean hasChanged()

Tests if this object has changed.

Returns:

true if and only if the setChanged method has been called more recently than the clearChanged method on this object; false otherwise.

deleteObservers

public final void deleteObservers()

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

countObservers

public final int countObservers()

Returns the number of observers of this Observable object.

Returns:

the number of observers of this object.