Why are listener lists Lists?

Question

Why are listener lists (e.g. in Java those that use addXxxListener() and removeXxxListener() to register and unregister listeners) called lists, and usually implemented as Lists? Wouldn't a Set be a better fit, since in the case of listeners there's

• No matter in which order they get called (although there may well be such needs, but they're special cases; ordinary listener mechanisms make no such guarantees), and
• No need to register the same listener more than once (whether doing that should result in calling the same listener 1 times or N times, or be an error, is another question)

Is it just a matter of tradition? Sets are some kind of lists under the hood anyway. Are there performance differences? Is iterating through a List faster or slower than iterating through a Set? Does either take more or less memory? The differences are certainly almost negligible.

Answer 1

What kind of set? Should all listeners implement equals and hashCode so that a hash set is used, or would an identity hash set do? Is the use case of adding a listener to a list twice worth the complexity? Is there as simple a mechanism for making the set safe against adding or removing listeners during calls to their handlers?

There might be some performance differences, but there certainly are more complicated design, and it forces the multiple add-multiple remove decision into the library rather than leaving it to the application.