Observable

public class Observable extends 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.

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 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.

See Also

• notifyObservers()
• notifyObservers(java.lang.Object)
• Observer
• Observer.update(java.util.Observable, java.lang.Object)

Public Constructor Summary

Observable() Construct an Observable with zero Observers.

Public Method Summary

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

Protected Method Summary

synchronized void	clearChanged() 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.
synchronized void	setChanged() Marks this Observable object as having been changed; the hasChanged method will now return true.

Inherited Method Summary

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

Public Constructors

public Observable ()

Construct an Observable with zero Observers.

Public Methods

public synchronized void addObserver (Observer 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.

Parameters

o	an observer to be added.

Throws

NullPointerException	if the parameter o is null.

public synchronized int countObservers ()

Returns the number of observers of this Observable object.

Returns

• the number of observers of this object.

public synchronized void deleteObserver (Observer o)

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

Parameters

o	the observer to be deleted.

public synchronized void deleteObservers ()

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

public synchronized 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.

See Also

• clearChanged()
• setChanged()

public void notifyObservers (Object 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.

Parameters

arg	any object.

See Also

• clearChanged()
• hasChanged()
• Observer.update(java.util.Observable, java.lang.Object)

public 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)

See Also

• clearChanged()
• hasChanged()
• Observer.update(java.util.Observable, java.lang.Object)

Protected Methods

protected synchronized void clearChanged ()

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.

See Also

• notifyObservers()
• notifyObservers(java.lang.Object)

protected synchronized void setChanged ()

Marks this Observable object as having been changed; the hasChanged method will now return true.