Fixed size priority queue

Question

I've tried implementing priority queue with fixed size in Java using TreeSet and overriding add() method:

public class FixedSizePriorityQueue<E> extends TreeSet<E> {
    private final int capacity;

    public FixedSizePriorityQueue(final int capacity) {
        this.capacity = capacity;
    }

    public FixedSizePriorityQueue(
            final int capacity,
            final Comparator<? super E> comparator) {

        super(comparator);
        this.capacity = capacity;
    }

    @Override
    public boolean add(final E e) {
        // initialized with 0 or less than zero capacity
        if (capacity <= 0) {
            return false;
        }

        // keep adding until we fill the queue
        if (size() < capacity) {
            return super.add(e);
        }

        if (comparator() != null
                && comparator().compare(this.last(), e) < 0) {
            pollLast();
            return super.add(e);
        }

        return false;
    }
}

I would really appreciate your thoughts, hints and comments.

Answer 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)

Answer 2

I rarely check the return of add methods. I would suggest throwing an exception to indicate the container is full. Add methods that return available space and full state.