Chapter 17
The java.util Package

Observable

Name

Observable

Synopsis

Class Name:

java.util.Observable

Superclass:

java.lang.Object

Immediate Subclasses:

None

Interfaces Implemented:

None

Availability:

JDK 1.0 or later

Subclasses of the Observable class are used to implement the model portion of the model-view paradigm. The idea is that an Observable object, the model, represents some data that is being manipulated through a user interface, while Observer objects provide the user with a view of the data. When the Observable object is modified, it tells the Observer objects that the model has been modified by calling notifyObservers(). An Observer object registers with an Observable object to receive notifications when the Observable is modified. The Observer object is then notified of changes via the update() method.

Class Summary

public class java.util.Observable extends java.lang.Object {
  // Constructors
  public Observable();                          
  // Instance Methods
  public synchronized void addObserver(Observer o);
  public synchronized int countObservers();
  public synchronized void deleteObserver(Observer o);
  public synchronized void deleteObservers();
  public synchronized boolean hasChanged();
  public void notifyObservers();
  public void notifyObservers(Object arg);
  // Protected Instance Methods
  protected synchronized void clearChanged();
  protected synchronized void setChanged();
}

Constructors

Observable

public Observable()

Description: This constructor creates an Observable object with no registered Observer objects.

InstanceMethods

addObserver

public synchronized void addObserver(Observer o)

Parameters

o: The Observer to be added.

Description

This method registers the given Observer with this Observable object. The given Observer is then notified when notifyObservers() is called.

countObservers

public synchronized int countObservers()

Returns

The number of registered Observer objects for this Observable object.

Description

This method returns the number of Observer objects that are registered with this Observable object.

deleteObserver

public synchronized void deleteObserver(Observer o)

Parameters

o: The Observer to be removed.

Description

This method unregisters the given Observer with this Observable object. The given Observer is no longer notified when notifyObservers() is called.

deleteObservers

public synchronized void deleteObservers()

Description: This method unregisters all of the Observer objects of this Observable object. Thus, no objects are notified if notifyObservers() is called.

hasChanged

public synchronized boolean hasChanged()

Returns

true if this object has been flagged as changed; false otherwise.

Description

This method returns the value of an internal "dirty" flag. The flag can be modified using the protected methods setChanged() and clearChanged().

notifyObservers

public void notifyObservers()

Description: This method calls the update() method of all registered Observer objects. The value passed as the second argument to each of the update() method calls is null.

public void notifyObservers(Object arg)

Parameters

arg: A "hint" object that describes a change.

Description

This method calls the update() method of all registered Observer objects. The value passed as the second argument to each of the update() method calls is the given object arg.

This "hint" object can be used to efficiently update the views of a model. For example, an Observable object could represent satellite image data. A set of Observer objects would provide different graphical views of the data. If the model data changes, the arg object describes the part of the data that changed, and the Observer views could use this "hint" to update only parts of their displays.

Description: This method sets an internal "dirty" flag to false. After this method is called, this object's hasChanged() method returns false.

setChanged

protected synchronized void setChanged()

Description: This method sets an internal "dirty" flag to true. After this method is called, this object's hasChanged() method returns true.

Inherited Methods

Method	Inherited From	Method	Inherited From
`clone()`	`Object`	`equals(Object)`	`Object`
`finalize()`	`Object`	`getClass()`	`Object`
`hashCode()`	`Object`	`notify()`	`Object`
`notifyAll()`	`Object`	`toString()`	`Object`
`wait()`	`Object`	`wait(long)`	`Object`
`wait(long, int)`	`Object`