edu.emory.mathcs.backport.java.util

Class AbstractQueue

Implemented Interfaces:
Collection, Queue
Known Direct Subclasses:
ArrayBlockingQueue, ConcurrentLinkedQueue, DelayQueue, LinkedBlockingDeque, LinkedBlockingQueue, PriorityBlockingQueue, PriorityQueue, SynchronousQueue

public abstract class AbstractQueue
extends AbstractCollection
implements Queue

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(Object) 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 ../../../../../../../guide/collections/index.html"> Java Collections Framework.

Author:
Doug Lea
Since:
1.5

Constructor Summary

AbstractQueue()
Constructor for use by subclasses.

Method Summary

boolean
add(Object 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(Collection c)
Adds all of the elements in the specified collection to this queue.
void
clear()
Removes all of the elements from this queue.
Object
element()
Retrieves, but does not remove, the head of this queue.
Object
remove()
Retrieves and removes the head of this queue.

Methods inherited from class edu.emory.mathcs.backport.java.util.AbstractCollection

toArray, toArray

Constructor Details

AbstractQueue

protected AbstractQueue()
Constructor for use by subclasses.

Method Details

add

public boolean add(Object 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
Parameters:
e - the element to add
Returns:
true (as specified by Collection.add)

addAll

public boolean addAll(Collection 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.

Parameters:
c - collection containing elements to be added to this queue
Returns:
true if this queue changed as a result of the call

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.


element

public Object 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
Returns:
the head of this queue

remove

public Object 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
Returns:
the head of this queue