Chapter 18: java.util Part 2: More Utility Classes 521
More than one object can be an observer. For example, the following program implements
two observing classes and adds an object of each class to theBeingWatchedobserver list. The
second observer waits until the count reaches zero and then rings the bell.
/ An object may be observed by two or more
observers.
/
import java.util.*;
// This is the first observing class.
class Watcher1 implements Observer {
public void update(Observable obj, Object arg) {
System.out.println("update() called, count is " +
((Integer)arg).intValue());
}
}
// This is the second observing class.
class Watcher2 implements Observer {
public void update(Observable obj, Object arg) {
// Ring bell when done
if(((Integer)arg).intValue() == 0)
System.out.println("Done" + '\7');
}
}
// This is the class being observed.
class BeingWatched extends Observable {
void counter(int period) {
for( ; period >=0; period--) {
setChanged();
notifyObservers(new Integer(period));
try {
Thread.sleep(100);
} catch(InterruptedException e) {
System.out.println("Sleep interrupted");
}
}
}
}
class TwoObservers {
public static void main(String args[]) {
BeingWatched observed = new BeingWatched();
Watcher1 observing1 = new Watcher1();
Watcher2 observing2 = new Watcher2();
// add both observers
observed.addObserver(observing1);
observed.addObserver(observing2);observed.counter(10);
}
}