Collections - Beginning Java 8 Language Features - page 545

person, who showed up after you, is served before you, based on a priority? Java also has this kind of queue and it
is called a priority queue . In a priority queue, you define the priority using a Comparator object or implement the
Comparable interface in an element's class, and the next element in the queue to come out will be decided based on
the priority of the elements in the queue.

■ typically, a null element does not make sense in a Queue . after all, the purpose of having a queue is to apply
some processing logic on its elements or use the elements to perform some logic. in either case, a null value does not
make sense. it is up to the implementation of the Queue interface to allow or disallow null values. the use of null
elements in a queue is not recommended. if you use null elements in a queue, you will not be able to distinguish
between the null value returned from its method to indicate a special situation and the null value of the element.

Tip

A queue lets you perform three basic operations:

• Add an element to its tail

• Remove an element from its head

•
The Queue interface defines two methods for each of the three operations. One method throws an exception if the
operation is not possible; the other method returns a value ( false or null ) to indicate the failure. The method you use
to perform the specific operation depends on your requirements. The Queue interface adds six methods to provide the
functionality of a FIFO queue. They are listed in Table 12-1 .

Peek the element at its head

Table 12-1. Additional Methods Declared by the Queue Interface

Category Method Description

boolean add(E e)

Adding an element to the queue Adds an element to the queue if it is possible.

Otherwise, it throws an exception.

boolean offer(E e) Adds an element to the queue without throwing an

exception if the element cannot not be added. It
returns false on failure and true on success. It is
the preferred way to add an element in a bounded
queue.

E remove() Removing an element from the

queue

Retrieves and removes the head of the queue.
It throws an exception if the queue is empty.

E poll() Performs the same job as the remove() method.

However, it returns null if the queue is empty
instead of throwing an exception.

E element() Peeking at the head of the queue Retrieves the head of the queue without removing it

from the queue. It throws an exception if the queue
is empty.

E peek() Performs the same job as the element() method.

However, it returns null if the queue is empty
instead of throwing an exception.