Swing Hacks

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Chapter 12, Miscellany

#94 Fire Events and Stay Bug Free

HACK

The obvious question here is: why didn’t D get called? Well, think about it:

you iterate over the listeners by index, from 0 to 4. On index 2, the code

calls

C

’s

handleEvent( )

, which removes itself from the

ArrayList

. As a result,

D

, which was at index 3, is now at index 2. But having serviced index 2

(which was

C

), the

for

loop moves on to index 3, which is now listener

E

.

Thus,

D

never gets called.

That was a lot of fun the first time I got to debug it.

Hacking a Solution

Consider an alternative approach. Counting up gets you in trouble because a

listener that removes itself shifts the indices of all subsequent listeners. But if

you counted down—from the last listener to the first—then a listener could

remove itself safely.

In these examples, this would mean counting from index 4 down to 0. On

index 2, listener

C removes itself, but that doesn’t change the indices of the

listeners that haven’t been called yet, which are at indices 0 and 1.

All you have to do is change the

fireEvent( ) method in the

BackwardsForLoopEventSource class:

public void fireEvent (EventObject o) {

for (int i=listeners.size( )-1; i>=0; i--) {

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

l.handleEvent (o);

}

}

Run this modified code and you get the output shown here:

[tonberry] cadamson% java BackwardsForLoopEventSource

E called

D called

C called

B called