Copyque.add(d);
if (que.size() > YOUR_LIMIT)
     que.poll();

or did I missunderstand your question?

edit: forgot to mention that for this to work you probably have to invert your comparTo function since it will throw away the one with highest priority each cycle. (if a is "better" b compare (a, b) should return a positvie number.

example to keep the biggest numbers use something like this:

Copypublic int compare(Double first, Double second) {
            // keep the biggest values
            return first > second ? 1 : -1;
        }

Two thoughts:

1. If there is other code and you need all the features of a TreeSet great, otherwise delegate to a TreeSet member variable.

2. Your code if (capacity <= 0) is somewhat superfluous because of the next test if (size() < capacity)

How can you say Lucene's doesn't support a custom comparator?

Its abstract and you must implement the abstract method lessThan(T a, T b)

As the oracle documentation here states, http://docs.oracle.com/javase/7/docs/api/java/util/PriorityQueue.html:

A priority queue is unbounded, but has an internal capacity governing the size of an array used to store the elements on the queue. It is always at least as large as the queue size. As elements are added to a priority queue, its capacity grows automatically. The details of the growth policy are not specified.

A possible workaround may be that you can check the size before performing any operation:

if (q.size() <= QUEUE_LIMIT)
     //your code