As of JDK 7 b37, when you call the EventClient method addEventClientListener(), it doesn't add an entry in the Collection returned by the method getListeners().
Is it an implementation defect ?
getListeners() is documented that way:

Returns the set of listeners that have been added through this EventClient and not subsequently removed.

I'm not sure it's clear enough listeners added with addEventClientListener() aren't added "through this EventClient".
That said, if getListeners()'s implementation is right to not return listener(s) added with addEventClientListener() then how can we list them ? Need for an extra method ?