Swing Hacks

Fire Events and Stay Bug Free #94

Chapter 12, Miscellany

|

475

HACK

So, what happened? The problem is obviously with the C listener, the one

that removes itself after being called. In fact, this removal is what causes the

disaster—the

ConcurrentModificationException

indicates that you’re trying

to change the

Collection

that underlies the

Iterator

, while iterating over it.

And It Gets Worse

At least Java 1.2 has a fail-fast exception for this. Back in Java 1.1, without

Collections, you might have used a

for loop instead of an Iterator to count

over the listeners. Example 12-14 shows what that might look like.

Good news: this doesn’t throw an exception. Bad news: this doesn’t throw

an exception, as seen in the console output:

[tonberry] cadamson% java PathologicalForLoopEventSource

A called

B called

C called

E called

Example 12-14. Using a for-loop to fire events

import java.util.*;

public class PathologicalForLoopEventSource

extends TestEventSource {

ArrayList listeners = new ArrayList( );

public void addListener (TestEventListener l) {

listeners.add (l);

}

public void removeListener (TestEventListener l) {

listeners.remove (l);

}

public void fireEvent (EventObject o) {

for (int i=0; i<listeners.size( ); i++) {

TestEventListener l = (TestEventListener) listeners.get(i);

l.handleEvent (o);

}

}

public static void main (String[] args) {

PathologicalForLoopEventSource pfles =

new PathologicalForLoopEventSource( );

pfles.test( );

}

}