I look for a queue that stores up to N elements for a certain time (i.e. 10 sec) or should dispose the oldest value if full.
I found a similar queue in the Apache Collections (CircularFifoQueue JavaDoc) that misses the aspect of time to live. A full fletched message broker seems like an overhead for my small project.
Do you mind giving me a hint how I should implement this? Shall I filter out old values while I poll for elements?
I used to following queue implementation. The code is heavily based on Apaches CircularFifoQueue and is only weakly tested. Moreover the implementation is not thread-safe and not serializable.
Leave a comment if you spot a mistake.
import java.util.AbstractCollection;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Queue;
import java.util.concurrent.TimeUnit;
/**
* TimedQueue is a first-in first-out queue with a fixed size that
* replaces its oldest element if full.
* <p>
* The removal order of a {@link TimedQueue} is based on the
* insertion order; elements are removed in the same order in which they
* were added. The iteration order is the same as the removal order.
* <p>
* The {@link #add(Object)}, {@link #remove()}, {@link #peek()}, {@link #poll},
* {@link #offer(Object)} operations all perform in constant time.
* All other operations perform in linear time or worse.
* <p>
* This queue prevents null objects from being added and it is not thread-safe and not serializable.
*
* The majority of this source code was copied from Apaches {@link CircularFifoQueue}: http://commons.apache.org/proper/commons-collections/apidocs/org/apache/commons/collections4/queue/CircularFifoQueue.html
*
* @version 1.0
*/
public class TimedQueue<E> extends AbstractCollection<E>
implements Queue<E> {
/** Underlying storage array. */
private Item<E>[] elements;
/** Array index of first (oldest) queue element. */
private int start = 0;
/**
* Index mod maxElements of the array position following the last queue
* element. Queue elements start at elements[start] and "wrap around"
* elements[maxElements-1], ending at elements[decrement(end)].
* For example, elements = {c,a,b}, start=1, end=1 corresponds to
* the queue [a,b,c].
*/
private transient int end = 0;
/** Flag to indicate if the queue is currently full. */
private transient boolean full = false;
/** Capacity of the queue. */
private final int maxElements;
private TimeUnit unit;
private int delay;
/**
* Constructor that creates a queue with the default size of 32.
*/
public TimedQueue() {
this(32);
}
/**
* Constructor that creates a queue with the specified size.
*
* @param size the size of the queue (cannot be changed)
* @throws IllegalArgumentException if the size is < 1
*/
public TimedQueue(final int size) {
this(size, 3, TimeUnit.SECONDS);
}
@SuppressWarnings("unchecked")
public TimedQueue(final int size, int delay, TimeUnit unit) {
if (size <= 0) {
throw new IllegalArgumentException("The size must be greater than 0");
}
elements = new Item[size];
maxElements = elements.length;
this.unit = unit;
this.delay = delay;
}
/**
* Constructor that creates a queue from the specified collection.
* The collection size also sets the queue size.
*
* @param coll the collection to copy into the queue, may not be null
* @throws NullPointerException if the collection is null
*/
public TimedQueue(final Collection<? extends E> coll) {
this(coll.size());
addAll(coll);
}
/**
* Returns the number of elements stored in the queue.
*
* @return this queue's size
*/
@Override
public int size() {
int size = 0;
for(int i = 0; i < elements.length; i++) {
if(validElement(i) != null) {
size++;
}
}
return size;
}
/**
* Returns true if this queue is empty; false otherwise.
*
* @return true if this queue is empty
*/
@Override
public boolean isEmpty() {
return size() == 0;
}
private boolean isAtFullCapacity() {
return size() == maxElements;
}
/**
* Clears this queue.
*/
@Override
public void clear() {
full = false;
start = 0;
end = 0;
Arrays.fill(elements, null);
}
/**
* Adds the given element to this queue. If the queue is full, the least recently added
* element is discarded so that a new element can be inserted.
*
* @param element the element to add
* @return true, always
* @throws NullPointerException if the given element is null
*/
@Override
public boolean add(final E element) {
if (null == element) {
throw new NullPointerException("Attempted to add null object to queue");
}
if (isAtFullCapacity()) {
remove();
}
elements[end++] = new Item<E>(element);
if (end >= maxElements) {
end = 0;
}
if (end == start) {
full = true;
}
return true;
}
/**
* Returns the element at the specified position in this queue.
*
* @param index the position of the element in the queue
* @return the element at position {@code index}
* @throws NoSuchElementException if the requested position is outside the range [0, size)
*/
public E get(final int index) {
final int sz = size();
if (sz == 0) {
throw new NoSuchElementException(
String.format("The specified index (%1$d) is outside the available range because the queue is empty.", Integer.valueOf(index)));
}
if (index < 0 || index >= sz) {
throw new NoSuchElementException(
String.format("The specified index (%1$d) is outside the available range [0, %2$d]",
Integer.valueOf(index), Integer.valueOf(sz-1)));
}
final int idx = (start + index) % maxElements;
return validElement(idx);
}
private E validElement(int idx) {
if(elements[idx] == null){
return null;
}
long diff = System.currentTimeMillis() - elements[idx].getCreationTime();
if(diff < unit.toMillis(delay)) {
return (E) elements[idx].getValue();
} else {
elements[idx] = null;
return null;
}
}
//-----------------------------------------------------------------------
/**
* Adds the given element to this queue. If the queue is full, the least recently added
* element is discarded so that a new element can be inserted.
*
* @param element the element to add
* @return true, always
* @throws NullPointerException if the given element is null
*/
public boolean offer(E element) {
return add(element);
}
public E poll() {
if (isEmpty()) {
return null;
}
return remove();
}
public E element() {
if (isEmpty()) {
throw new NoSuchElementException("queue is empty");
}
return peek();
}
public E peek() {
if (isEmpty()) {
return null;
}
return (E) elements[start].getValue();
}
public E remove() {
if (isEmpty()) {
throw new NoSuchElementException("queue is empty");
}
final E element = validElement(start);
if (null != element) {
elements[start++] = null;
if (start >= maxElements) {
start = 0;
}
full = false;
}
return element;
}
/**
* Increments the internal index.
*
* @param index the index to increment
* @return the updated index
*/
private int increment(int index) {
index++;
if (index >= maxElements) {
index = 0;
}
return index;
}
/**
* Decrements the internal index.
*
* @param index the index to decrement
* @return the updated index
*/
private int decrement(int index) {
index--;
if (index < 0) {
index = maxElements - 1;
}
return index;
}
/**
* Returns an iterator over this queue's elements.
*
* @return an iterator over this queue's elements
*/
@Override
public Iterator<E> iterator() {
return new Iterator<E>() {
private int index = start;
private int lastReturnedIndex = -1;
private boolean isFirst = full;
public boolean hasNext() {
return (isFirst || index != end) && size() > 0;
}
public E next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
isFirst = false;
lastReturnedIndex = index;
index = increment(index);
if(validElement(lastReturnedIndex) == null) {
return next();
} else {
return validElement(lastReturnedIndex);
}
}
public void remove() {
if (lastReturnedIndex == -1) {
throw new IllegalStateException();
}
// First element can be removed quickly
if (lastReturnedIndex == start) {
TimedQueue.this.remove();
lastReturnedIndex = -1;
return;
}
int pos = lastReturnedIndex + 1;
if (start < lastReturnedIndex && pos < end) {
// shift in one part
System.arraycopy(elements, pos, elements, lastReturnedIndex, end - pos);
} else {
// Other elements require us to shift the subsequent elements
while (pos != end) {
if (pos >= maxElements) {
elements[pos - 1] = elements[0];
pos = 0;
} else {
elements[decrement(pos)] = elements[pos];
pos = increment(pos);
}
}
}
lastReturnedIndex = -1;
end = decrement(end);
elements[end] = null;
full = false;
index = decrement(index);
}
};
}
private static final class Item<E> {
private long creationTime;
private E in;
public Item(E in) {
this.in = in;
creationTime = System.currentTimeMillis();
}
public E getValue() {
return in;
}
public long getCreationTime() {
return creationTime;
}
}
}