java.util

Class AbstractQueue<E>

Implemented Interfaces:
Collection<E>, Iterable<E>, Queue<E>
Known Direct Subclasses:
ArrayBlockingQueue<E>, ConcurrentLinkedQueue<E>, DelayQueue<E,extends,Delayed>, LinkedBlockingDeque<E>, LinkedBlockingQueue<E>, PriorityBlockingQueue<E>, PriorityQueue<E>, SynchronousQueue<E>

public abstract class AbstractQueue<E>
extends AbstractCollection<E>
implements Queue<E>

This class provides skeletal implementations of some Queue operations. The implementations in this class are appropriate when the base implementation does not allow null elements. Methods add, remove, and element are based on offer, poll, and peek, respectively but throw exceptions instead of indicating failure via false or null returns.

A Queue implementation that extends this class must minimally define a method Queue.offer(E) which does not permit insertion of null elements, along with methods Queue.peek(), Queue.poll(), Collection.size(), and a Collection.iterator() supporting Iterator.remove(). Typically, additional methods will be overridden as well. If these requirements cannot be met, consider instead subclassing AbstractCollection.

This class is a member of the ../../../technotes/guides/collections/index.html"> Java Collections Framework.

Since:
1.5

Constructor Summary

AbstractQueue()
Constructor for use by subclasses.

Method Summary

boolean
add(E e)
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning true upon success and throwing an IllegalStateException if no space is currently available.
boolean
addAll(E> c)
Adds all of the elements in the specified collection to this queue.
void
clear()
Removes all of the elements from this queue.
E
element()
Retrieves, but does not remove, the head of this queue.
E
remove()
Retrieves and removes the head of this queue.

Methods inherited from class java.util.AbstractCollection<E>

T[] toArray, add, addAll, clear, contains, containsAll, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toString

Methods inherited from class java.lang.Object

clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

AbstractQueue

protected AbstractQueue()
Constructor for use by subclasses.

Method Details

add

public boolean add(E e)
Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning true upon success and throwing an IllegalStateException if no space is currently available.

This implementation returns true if offer succeeds, else throws an IllegalStateException.

Specified by:
add in interface Queue<E>
add in interface Collection<E>
Overrides:
add in interface AbstractCollection<E>
Parameters:
e - the element to add
Returns:
true (as specified by Collection.add(E))
Throws:
IllegalStateException - if the element cannot be added at this time due to capacity restrictions
ClassCastException - if the class of the specified element prevents it from being added to this queue
NullPointerException - if the specified element is null and this queue does not permit null elements
IllegalArgumentException - if some property of this element prevents it from being added to this queue

addAll

public boolean addAll(E> c)
Adds all of the elements in the specified collection to this queue. Attempts to addAll of a queue to itself result in IllegalArgumentException. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress.

This implementation iterates over the specified collection, and adds each element returned by the iterator to this queue, in turn. A runtime exception encountered while trying to add an element (including, in particular, a null element) may result in only some of the elements having been successfully added when the associated exception is thrown.

Specified by:
addAll in interface Collection<E>
Overrides:
addAll in interface AbstractCollection<E>
Parameters:
c - collection containing elements to be added to this queue
Returns:
true if this queue changed as a result of the call
Throws:
ClassCastException - if the class of an element of the specified collection prevents it from being added to this queue
NullPointerException - if the specified collection contains a null element and this queue does not permit null elements, or if the specified collection is null
IllegalArgumentException - if some property of an element of the specified collection prevents it from being added to this queue, or if the specified collection is this queue
IllegalStateException - if not all the elements can be added at this time due to insertion restrictions
See Also:
add(Object)

clear

public void clear()
Removes all of the elements from this queue. The queue will be empty after this call returns.

This implementation repeatedly invokes poll until it returns null.

Specified by:
clear in interface Collection<E>
Overrides:
clear in interface AbstractCollection<E>

element

public E element()
Retrieves, but does not remove, the head of this queue. This method differs from peek only in that it throws an exception if this queue is empty.

This implementation returns the result of peek unless the queue is empty.

Specified by:
element in interface Queue<E>
Returns:
the head of this queue
Throws:
NoSuchElementException - if this queue is empty

remove

public E remove()
Retrieves and removes the head of this queue. This method differs from poll only in that it throws an exception if this queue is empty.

This implementation returns the result of poll unless the queue is empty.

Specified by:
remove in interface Queue<E>
Returns:
the head of this queue
Throws:
NoSuchElementException - if this queue is empty