 <p>If all the elements to be added were already in the Collection (prior to the call to addAll), and the Collection doesn't allow duplicates, it will return false, since all the individual <code>add</code> method calls would return false. This is true for Collections such as <code>Set</code>.</p> <p>For other Collections, <code>add</code> always returns true, and therefore <code>addAll</code> would return true, unless you call it with an empty list of elements to be added.</p>