>> The default implementation (for Observable) would look like this:
>> public default void ensureListener(InvalidationListener listener) {
>>    removeListener(listener);
>>    addListener(listener);
>> }
>> subclasses might do something more effective. The same would apply to
>> ObservableValue and ChangeListener and Observable[List|Set|Map] and
>> [List|Set|Map]ChangeListener.
> Well this would destroy the order! I expect listeners to be called in
> the correct order not?

That’s a good point :-(

> Why doing a remove and not simply check if the
> listener has already been added?

Because there is no way to check, except in the implementation. From the 
Observable interface level, there is no way to a) force all implementations of 
the interface to implement the method correctly (without breaking source 
compatibility anyway), or b) to provide a reasonable default implementation.

Maybe this is one of those things we can’t fix on the Observable interface and 
just have to provide implementations of on our concrete properties.


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