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Observable

public class Observable
extends Object

java.lang.Object
   ↳ java.util.Observable


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.

Summary

Public constructors

Observable()

Construct an Observable with zero Observers.

Public methods

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.

int countObservers()

Returns the number of observers of this Observable object.

void deleteObserver(Observer o)

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(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 methods

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.

void setChanged()

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

Inherited methods

Public constructors

Observable

added in API level 1
public Observable ()

Construct an Observable with zero Observers.

Public methods

addObserver

added in API level 1
public 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 Observer: an observer to be added.

Throws
NullPointerException if the parameter o is null.

countObservers

added in API level 1
public int countObservers ()

Returns the number of observers of this Observable object.

Returns
int the number of observers of this object.

deleteObserver

added in API level 1
public 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 Observer: the observer to be deleted.

deleteObservers

added in API level 1
public void deleteObservers ()

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

hasChanged

added in API level 1
public boolean hasChanged ()

Tests if this object has changed.

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

notifyObservers

added in API level 1
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 Object: any object.

notifyObservers

added in API level 1
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)

Protected methods

clearChanged

added in API level 1
protected 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.

setChanged

added in API level 1
protected void setChanged ()

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