A Queue that ensure uniqueness of the elements?

Question

I\'m looking for a implementation of java.util.Queue or something in the Google collection who behave like a Queue, but also ensure that each element of the queue is unique.

Answer 1

How about a LinkedHashSet? Its iterator preserves insertion order, but because it's a Set, its elements are unique.

As its documentation says,

Note that insertion order is not affected if an element is re-inserted into the set.

In order to efficiently remove elements from the head of this "queue", go through its iterator:

Iterator<?> i = queue.iterator();
...
Object next = i.next();
i.remove();

Answer 2

Just to complete Adamski's answer:

/**
 * A queue that keeps each element only once. 
 * If you try to add an element that already exists - nothing will happen.
 * 
 * @author Adamski http://stackoverflow.com/a/2319156/827927
 * @NotThreadSafe
 */
public class UniqueQueue<T> implements Queue<T> {

private final Queue<T> queue = new LinkedList<T>();
private final Set<T> set = new HashSet<T>();

@Override public boolean add(T t) {
    // Only add element to queue if the set does not contain the specified element.
    if (set.add(t))
        queue.add(t);
    return true; // Must always return true as per API def.
}

@Override public boolean addAll(Collection<? extends T> arg0) {
    boolean ret = false;
    for (T t: arg0)
        if (set.add(t)) {
            queue.add(t);
            ret = true;
        }
    return ret;
}

@Override public T remove() throws NoSuchElementException {
    T ret = queue.remove();
    set.remove(ret);
    return ret;
}

@Override public boolean remove(Object arg0) {
    boolean ret = queue.remove(arg0);
    set.remove(arg0);
    return ret;
}

@Override public boolean removeAll(Collection<?> arg0) {
    boolean ret = queue.removeAll(arg0);
    set.removeAll(arg0);
    return ret;
}

@Override public void clear() {
    set.clear();
    queue.clear();
}

@Override public boolean contains(Object arg0) {
    return set.contains(arg0);
}

@Override public boolean containsAll(Collection<?> arg0) {
    return set.containsAll(arg0);
}

@Override public boolean isEmpty() {
    return set.isEmpty();
}

@Override public Iterator<T> iterator() {
    return queue.iterator();
}

@Override public boolean retainAll(Collection<?> arg0) {
    throw new UnsupportedOperationException();
}

@Override public int size() {
    return queue.size();
}

@Override public Object[] toArray() {
    return queue.toArray();
}

@Override public <T> T[] toArray(T[] arg0) {
    return queue.toArray(arg0);
}

@Override public T element() {
    return queue.element();
}

@Override public boolean offer(T e) {
    return queue.offer(e);
}

@Override public T peek() {
    return queue.peek();
}

@Override public T poll() {
    return queue.poll();
}
}