edu.emory.mathcs.backport.java.util
Class PriorityQueue
java.util.AbstractCollection
- Collection, java.io.Serializable, Queue
public class PriorityQueue
implements java.io.Serializable, Queue
An unbounded
queue that supports element retrieval
in the order of relative priority. The ordering can be defined via an
explicit comparator; otherwise, the natural ordering of elements is used.
Element at the head of the queue is always the
smallest one
according to the given ordering.
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 PriorityQueue in any particular order. If you need
ordered traversal, consider using
Arrays.sort(pq.toArray()).
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. See
PriorityBlockingQueue
for an example.
Implementation note: basic mutative methods (insert, offer,
remove, poll etc) have complexity O(log(n)). Parameterless inspection methods
(peek, element,isEmpty) have complexity O(1). Methods contains(Object) and
remove(Object) have complexity O(n).
PriorityQueue() - Creates a PriorityQueue with the default
initial capacity (11) that orders its elements according to
their natural ordering.
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PriorityQueue(Collection c) - Creates a PriorityQueue containing the elements
in the specified collection.
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PriorityQueue(Comparator comparator) - Creates a PriorityQueue with the specified initial
capacity that orders its elements according to the specified
comparator.
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PriorityQueue(SortedSet s) - Creates a PriorityQueue containing the elements
from the specified sorted set.
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PriorityQueue(PriorityQueue q) - Creates a PriorityQueue containing the elements from
the specified priority queue.
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PriorityQueue(int initialCapacity) - Creates a PriorityQueue with the specified
initial capacity that orders its elements according to their
natural ordering.
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PriorityQueue(int initialCapacity, Comparator comparator)
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boolean | add(Object o) - Inserts the specified element into this priority queue.
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void | clear() - Removes all of the elements from this queue.
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Comparator | comparator() - Returns the comparator used to order the elements in this queue,
or null if this queue uses the natural ordering of its elements.
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boolean | contains(Object o) - Returns true if this queue contains the specified element.
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Object | element() - Retrieves, but does not remove, the head of this queue.
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boolean | isEmpty() - Returns true if this queue contains no elements.
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Iterator | iterator() - Returns an iterator over the elements in this queue.
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boolean | offer(Object o) - Inserts the specified element into this priority queue.
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Object | peek() - Retrieves, but does not remove, the head of this queue, or returns
null if this queue is empty.
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Object | poll() - Retrieves and removes the head of this queue, or returns null
if this queue is empty.
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Object | remove() - Retrieves and removes the head of this queue.
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boolean | remove(Object o) - Removes a single instance of the specified element from this queue,
if it is present.
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int | size() - Returns the number of elements in this priority queue.
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Object[] | toArray() - Returns an array containing all of the elements in this queue.
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Object[] | toArray(Object[] 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.
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PriorityQueue
public PriorityQueue()
Creates a PriorityQueue with the default
initial capacity (11) that orders its elements according to
their natural ordering.
PriorityQueue
public PriorityQueue(Collection c)
Creates a
PriorityQueue containing the elements
in the specified collection. The priority queue has an initial
capacity of 110% of the size of the specified collection. If
the specified collection is a
java.util.SortedSet
or a
PriorityQueue
, this priority queue will be sorted according to
the same comparator, or according to the natural ordering of its
elements if the collection is sorted according to the natural
ordering of its elements. Otherwise, this priority queue is
ordered according to the natural ordering of its elements.
c
- the collection whose elements are to be placed
into this priority queue.
PriorityQueue
public PriorityQueue(Comparator comparator)
Creates a PriorityQueue with the specified initial
capacity that orders its elements according to the specified
comparator.
comparator
- the comparator used to order this priority queue.
If null then the order depends on the elements' natural
ordering.
PriorityQueue
public PriorityQueue(SortedSet s)
Creates a PriorityQueue containing the elements
from the specified sorted set. This priority queue has an initial
capacity of 110% of the size of the specified set, and it is
sorted according to the same comparator as the specified set,
or according to the natural ordering of its
elements if the specified set is sorted according to the natural
ordering of its elements.
s
- the set whose elements are to be placed
into this priority queue.
PriorityQueue
public PriorityQueue(PriorityQueue q)
Creates a PriorityQueue containing the elements from
the specified priority queue. This priority queue has an initial
capacity of 110% of the size of the specified queue, and it is
sorted according to the same comparator as the specified queue,
or according to the natural ordering of its
elements if the specified queue is sorted according to the natural
ordering of its elements.
q
- the queue whose elements are to be placed
into this priority queue.
PriorityQueue
public PriorityQueue(int initialCapacity)
Creates a PriorityQueue with the specified
initial capacity that orders its elements according to their
natural ordering.
initialCapacity
- the initial capacity for this priority queue
PriorityQueue
public PriorityQueue(int initialCapacity,
Comparator comparator)
add
public boolean add(Object o)
Inserts the specified element into this priority queue.
- add in interface Queue
- add in interface AbstractQueue
- true (as per the spec for
Collection.add
)
clear
public void clear()
Removes all of the elements from this queue.
The queue will be empty after this call returns.
- clear in interface AbstractQueue
comparator
public Comparator 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.
contains
public boolean contains(Object o)
Returns true if this queue contains the specified element.
More formally, returns true if and only if this queue contains
at least one element e such that o.equals(e).
o
- object to be checked for containment in this queue
- true if this queue contains the specified element
isEmpty
public boolean isEmpty()
Returns true if this queue contains no elements.
- true if this queue contains no elements
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 thread-safe "fast-fail" iterator
that will throw java.util.ConcurrentModificationException
upon
detected interference.
- an iterator over the elements in this queue
offer
public boolean offer(Object o)
Inserts the specified element into this priority queue.
- offer in interface Queue
peek
public Object peek()
Retrieves, but does not remove, the head of this queue, or returns
null if this queue is empty.
- peek in interface Queue
- the head of this queue, or null if this queue is
empty
poll
public Object poll()
Retrieves and removes the head of this queue, or returns null
if this queue is empty.
- poll in interface Queue
- the head of this queue, or null if this queue is
empty
remove
public boolean remove(Object o)
Removes a single instance of the specified element from this queue,
if it is present.
Returns true if this queue contained the specified element
(or equivalently, if this queue changed as a result of the call).
o
- element to be removed from this queue, if present
- true if this queue changed as a result of the call
size
public int size()
Returns the number of elements in this priority queue.
- the number of elements in this priority queue
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 AbstractCollection
- an array containing all of the elements in this queue
toArray
public Object[] toArray(Object[] 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().
- toArray in interface AbstractCollection
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