THE PACKAGE JAVA.UTIL

java.util.Observable

21.7

21.7 The Class

java.util.Observable

Each instance of class

Observable

 maintains a set of  observers  that are notified

whenever the

Observable

 object changes in some significant way. An observer

may be any object that implements interface

Observer

 ( 21.8).

Note that this notification mechanism is has nothing to do with threads

( 20.20) and is completely separate from the

wait

 and

notify

 mechanism of

class

Object

 ( 20.1).

public class Observable {

public void addObserver(Observer o);

public void deleteObserver(Observer o);

public void deleteObservers();

public int countObservers();

public void notifyObservers();

public void notifyObservers(Object arg);

protected void setChanged();

protected void clearChanged();

public boolean hasChanged();

}

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

( 20.1.3) returns

true

 for them.

21.7.1

public void addObserver(Observer o)

The observer

o

 is added to this

Observable

 object's set of observers, provided

that it is not the same as some observer already in the set.

21.7.2

public void deleteObserver(Observer o)

The observer

o

 is removed from this

Observable

 object's set of observers.

21.7.3

public void deleteObservers()

All observers are removed from this

Observable

 object's set of observers.

21.7.4

public int countObservers()

The number of observers in this

Observable

 object's set of observers is returned.

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