Class Observable
- java.lang.Object
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- java.util.Observable
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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'snotifyObserversmethod causes all of its observers to be notified of the change by a call to theirupdatemethod.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.
- Since:
- JDK1.0
- See Also:
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notifyObservers(),notifyObservers(java.lang.Object),Observer,Observer.update(java.util.Observable, java.lang.Object)
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Constructor Summary
Constructors Constructor and Description Observable()Construct an Observable with zero Observers.
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method and Description voidaddObserver(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 voidclearChanged()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.intcountObservers()Returns the number of observers of this Observable object.voiddeleteObserver(Observer o)Deletes an observer from the set of observers of this object.voiddeleteObservers()Clears the observer list so that this object no longer has any observers.booleanhasChanged()Tests if this object has changed.voidnotifyObservers()If this object has changed, as indicated by thehasChangedmethod, then notify all of its observers and then call theclearChangedmethod to indicate that this object has no longer changed.voidnotifyObservers(Object arg)If this object has changed, as indicated by thehasChangedmethod, then notify all of its observers and then call theclearChangedmethod to indicate that this object has no longer changed.protected voidsetChanged()Marks this Observable object as having been changed; the hasChanged method will now return true.
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Method Detail
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.- Parameters:
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o- an observer to be added. - Throws:
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NullPointerException- if the parameter o is null.
deleteObserver
public void deleteObserver(Observer o)
Deletes an observer from the set of observers of this object. Passingnullto this method will have no effect.- Parameters:
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o- the observer to be deleted.
notifyObservers
public void notifyObservers()
If this object has changed, as indicated by thehasChangedmethod, then notify all of its observers and then call theclearChangedmethod to indicate that this object has no longer changed.Each observer has its
updatemethod called with two arguments: this observable object andnull. In other words, this method is equivalent to:notifyObservers(null)
notifyObservers
public void notifyObservers(Object arg)
If this object has changed, as indicated by thehasChangedmethod, then notify all of its observers and then call theclearChangedmethod to indicate that this object has no longer changed.Each observer has its
updatemethod called with two arguments: this observable object and theargargument.- Parameters:
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arg- any object. - See Also:
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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 thenotifyObserversmethods.
hasChanged
public boolean hasChanged()
Tests if this object has changed.- Returns:
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trueif and only if thesetChangedmethod has been called more recently than theclearChangedmethod on this object;falseotherwise. - See Also:
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clearChanged(),setChanged()
countObservers
public int countObservers()
Returns the number of observers of this Observable object.- Returns:
- the number of observers of this object.
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Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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