Collections - The Java Programming Language - page 866

Attempts to insert the given element into this queue. If the
attempt is successful, true is returned, otherwise false . For
queues that have genuine reason to reject a requestsuch as a
queue with finite capacitythis method is preferable to the Col-
lectionadd method which can only indicate failure by throwing
an exception.

Queues in general should not accept null elements because null is used
as a sentinel value for poll and peek to indicate an empty queue.

The LinkedList class provides the simplest implementation of Queue . For
historical reasons the LinkedList class accepts null elements, but you
should avoid inserting null elements when using a LinkedList instance
as a queue.

21.7.1. PriorityQueue

The other Queue implementation is PriorityQueue , an unbounded queue,
based on a priority heap. The head of the queue is the smallest element
in it, where smallest is determined either by the elements' natural order
or by a supplied comparator. A PriorityQueue is not a sorted queue in the
general senseyou can't pass it to a method expecting a sorted collection-
because the iterator returned by iterator is not guaranteed to traverse
the elements in priority order; rather it guarantees that removing ele-
ments from the queue occurs in a given order. The iterator can traverse
the elements in any order. If you need ordered traversal you could ex-
tract the elements to an array and then sort the array (see " The Arrays
Utility Class " on page 607 ) .

Whether the smallest element represents the element with the highest
or lowest "priority" depends on how the natural order or the comparator
is defined. For example, if queuing Thread objects according to their ex-
ecution priority, then the smallest element represents a Thread with the
lowest execution priority.

The performance characteristics of a PriorityQueue are unspecified. A
good implementation based on a priority heap would provide