An unbounded
blocking queue that uses
the same ordering rules as class
PriorityQueue and supplies
blocking retrieval operations. While this queue is logically
unbounded, attempted additions may fail due to resource exhaustion
(causing
OutOfMemoryError). This class does not permit
null elements. A priority queue relying on
natural ordering also does not permit insertion of
non-comparable objects (doing so results in
ClassCastException).
This class and its iterator implement all of the
optional methods of the
Collection and
Iterator interfaces. The Iterator provided in method
iterator() is
not guaranteed to traverse the elements of
the PriorityBlockingQueue in any particular order. If you need
ordered traversal, consider using
Arrays.sort(pq.toArray()). Also, method
drainTo
can be used to
remove some or all elements in priority
order and place them in another collection.
Operations on this class make no guarantees about the ordering
of elements with equal priority. If you need to enforce an
ordering, you can define custom classes or comparators that use a
secondary key to break ties in primary priority values. For
example, here is a class that applies first-in-first-out
tie-breaking to comparable elements. To use it, you would insert a
new FIFOEntry(anEntry) instead of a plain entry object.
class FIFOEntry<E extends Comparable<? super E>>
implements Comparable<FIFOEntry<E>> {
final static AtomicLong seq = new AtomicLong();
final long seqNum;
final E entry;
public FIFOEntry(E entry) {
seqNum = seq.getAndIncrement();
this.entry = entry;
}
public E getEntry() { return entry; }
public int compareTo(FIFOEntry<E> other) {
int res = entry.compareTo(other.entry);
if (res == 0 && other.entry != this.entry)
res = (seqNum < other.seqNum ? -1 : 1);
return res;
}
}
This class is a member of the
../../../../technotes/guides/collections/index.html">
Java Collections Framework.
T[] toArray
public T[] toArray(T[] a)
Returns an array containing all of the elements in this queue; the
runtime type of the returned array is that of the specified array.
The returned array elements are in no particular order.
If the queue fits in the specified array, it is returned therein.
Otherwise, a new array is allocated with the runtime type of the
specified array and the size of this queue.
If this queue fits in the specified array with room to spare
(i.e., the array has more elements than this queue), the element in
the array immediately following the end of the queue is set to
null.
Like the
toArray() method, this method acts as bridge between
array-based and collection-based APIs. Further, this method allows
precise control over the runtime type of the output array, and may,
under certain circumstances, be used to save allocation costs.
Suppose
x is a queue known to contain only strings.
The following code can be used to dump the queue into a newly
allocated array of
String:
String[] y = x.toArray(new String[0]);
Note that
toArray(new Object[0]) is identical in function to
toArray().
- T[] toArray in interface Collection<E>
- T[] toArray in interface AbstractCollection<E>
a - the array into which the elements of the queue are to
be stored, if it is big enough; otherwise, a new array of the
same runtime type is allocated for this purpose
- an array containing all of the elements in this queue
iterator
public Iterator iterator()
Returns an iterator over the elements in this queue. The
iterator does not return the elements in any particular order.
The returned
Iterator is a "weakly consistent"
iterator that will never throw
ConcurrentModificationException, and guarantees to traverse
elements as they existed upon construction of the iterator, and
may (but is not guaranteed to) reflect any modifications
subsequent to construction.
- iterator in interface Collection<E>
- iterator in interface Iterable<E>
- iterator in interface AbstractCollection<E>
- an iterator over the elements in this queue
offer
public boolean offer(E e,
long timeout,
TimeUnit unit) Inserts the specified element into this priority queue. As the queue is
unbounded this method will never block.
- offer in interface BlockingQueue<E>
e - the element to addtimeout - This parameter is ignored as the method never blocksunit - This parameter is ignored as the method never blocks
ClassCastException - if the specified element cannot be compared
with elements currently in the priority queue according to the
priority queue's orderingNullPointerException - if the specified element is null
peek
public E peek()
Retrieves, but does not remove, the head of this queue,
or returns null if this queue is empty.
- peek in interface Queue<E>
- the head of this queue, or null if this queue is empty
poll
public E poll()
Retrieves and removes the head of this queue,
or returns null if this queue is empty.
- poll in interface Queue<E>
- the head of this queue, or null if this queue is empty
put
public void put(E e)
Inserts the specified element into this priority queue. As the queue is
unbounded this method will never block.
- put in interface BlockingQueue<E>
ClassCastException - if the specified element cannot be compared
with elements currently in the priority queue according to the
priority queue's orderingNullPointerException - if the specified element is null
remove
public boolean remove(Object o)
Removes a single instance of the specified element from this queue,
if it is present. More formally, removes an element e such
that o.equals(e), if this queue contains one or more such
elements. Returns true if and only if this queue contained
the specified element (or equivalently, if this queue changed as a
result of the call).
- remove in interface BlockingQueue<E>
- remove in interface Collection<E>
- remove in interface AbstractCollection<E>
o - element to be removed from this queue, if present
- true if this queue changed as a result of the call
super E> comparator
public Comparatorsuper E> comparator()
Returns the comparator used to order the elements in this queue,
or
null if this queue uses the
natural ordering of its elements.
- the comparator used to order the elements in this queue,
or null if this queue uses the natural
ordering of its elements
toArray
public Object[] toArray()
Returns an array containing all of the elements in this queue.
The returned array elements are in no particular order.
The returned array will be "safe" in that no references to it are
maintained by this queue. (In other words, this method must allocate
a new array). The caller is thus free to modify the returned array.
This method acts as bridge between array-based and collection-based
APIs.
- toArray in interface Collection<E>
- toArray in interface AbstractCollection<E>
- an array containing all of the elements in this queue
toString
public String toString()
Convert this Object to a human-readable String.
There are no limits placed on how long this String
should be or what it should contain. We suggest you
make it as intuitive as possible to be able to place
it into
System.out.println()
and such.
It is typical, but not required, to ensure that this method
never completes abruptly with a
RuntimeException.
This method will be called when performing string
concatenation with this object. If the result is
null, string concatenation will instead
use
"null".
The default implementation returns
getClass().getName() + "@" +
Integer.toHexString(hashCode()).
- toString in interface AbstractCollection<E>
- the String representing this Object, which may be null