insertWithOverflow() is the same as insert() except its return value: it returns the object (if any) that was dropped off the heap because it was full. This can be the given parameter (in case it is smaller than the full heap's minimum, and couldn't be added), or another object that was previously the smallest value in the heap and now has been replaced by a larger one, or null if the queue wasn't yet full with maxSize elements.
PriorityQueue Class | Lucene.Net.Util Namespace