程序包 java.util

类 Observable

java.lang.Object

java.util.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.

从以下版本开始:

JDK1.0

另请参阅:

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

构造器概要

构造器

构造器	说明
Observable()	Construct an Observable with zero Observers.

方法概要

限定符和类型	方法	说明
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.
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.
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()	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(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.
protected void	setChanged()	Marks this Observable object as having been changed; the hasChanged method will now return true.

从类继承的方法 java.lang.Object

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

构造器详细资料

Observable

public Observable()

Construct an Observable with zero Observers.

方法详细资料

addObserver

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.

参数:

o - an observer to be added.

抛出:

NullPointerException - if the parameter o is null.

deleteObserver

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.

参数:

o - the observer to be deleted.

notifyObservers

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)

另请参阅:

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

notifyObservers

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.

参数:

arg - any object.

另请参阅:

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

deleteObservers

public void deleteObservers()

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

setChanged

protected void setChanged()

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

clearChanged

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.

另请参阅:

notifyObservers(), notifyObservers(java.lang.Object)

hasChanged

public boolean hasChanged()

Tests if this object has changed.

返回:

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

另请参阅:

clearChanged(), setChanged()

countObservers

public int countObservers()

Returns the number of observers of this Observable object.

返回:

the number of observers of this object.