COMP 121

Week 14: Queues

Objectives Learn how to represent a queue Learn how to use the methods in the

Queue interface Understand how to implement the Queue

interfaceDouble-linked listSingle-linked listCircular array

Queue

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Queue Abstract Data Type A First-In, First-Out (FIFO) data structure Can visualize a queue as a line of

customers waiting for service The next person to be served is the one

who has waited the longest New elements (people) are placed at the

end of the line

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

A Print Queue Operating systems use queues to:

Keep track of tasks waiting for a scarce resource Ensure that the tasks are carried out in the order that

they were generated

Print queue Printing is slower than the process of selecting pages

or documents to print Queue is used to store documents for printing Documents are printed in the order they were added

to the queue (unless a priority scheme is used)

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

A Print Queue (continued)

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Question: Why not use a Stack to hold documents for printing?

Answer: Stacks are last-in, first-out (LIFO) The most recently selected document

would be the next to print Unless the printer queue is empty, your

print job may never get executed if others are issuing print jobs

Specification of a Queue Interface

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Queue Methods add and offer

For a queue of unlimited size, methods are logically equivalent

For a bounded queue, add will throw an exception if the limit has been reached, but offer will return false

peek and element peek will return null if the queue is empty, but element

will throw an exception poll and remove

poll will return null if the queue is empty, but remove will throw an exception

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

LinkedList Class and Queue Interface LinkedList class provides methods for

inserting and removing elements at either end of a double-linked list

The Java 5.0 LinkedList class implements the Queue interface

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Queue Example Queue<String> names = new LinkedList<String>(); Creates a new Queue reference, names, that

stores references to String objectsThe actual object referenced by names is type

LinkedList<String> Because names is a type Queue<String>

reference, you can apply only the Queue methods to it

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Using a LinkedList to Implement the Queue Interface Insertion and removal from either end of a

double-linked list is O(1) so either end can be the front (or rear) of the queue

In the Java 5.0 implementation, designers decided to make the head of the linked list the front of the queue and the tail the rear of the queue

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Using a LinkedList to Implement the Queue Interface (continued) Because LinkedList implements Queue, it

is possible to apply other LinkedList methods to a Queue (in addition to the ones required by the Queue interface)

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Question: What design pattern could be used to create a better design for a Queue?

Answer: Adapter Pattern

A better approach would be to create a new class that wrappers a LinkedList and implements the Queue interfaceOnly Queue methods would be availableQueue methods would be implemented by

calling methods on the LinkedListEncapsulation would hide linked list

implementation from clients

Using a Single-Linked List to Implement a Queue Can implement a queue using a single-

linked list Class ListQueue contains a collection of

Node<E> objects Insertions are at the rear of a queue and

removals are from the front Need a reference to the last list node Number of elements in the queue is

changed by offer, poll, and removeKoffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using

Java Version 5.0. New York: John Wiley & Sons.

Using a Single-Linked List to Implement a Queue (continued)

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Implementing a Queue Using a Circular Array Time efficiency of using a single- or

double-linked list to implement a queue is acceptable However there are some space inefficiencies

Storage space is increased when using a linked list due to references stored at each list node

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Implementing a Queue Using a Circular Array (continued) Array Implementation

Insertion at rear of array is constant timeRemoval from the front is linear timeRemoval from rear of array is constant time Insertion at the front is linear time

Circular Array ImplementationBoth insertions and removals are constant

time

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Implementing a Queue Using a Circular Array (continued)

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Implementing a Queue Using a Circular Array (continued)

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Implementing a Queue Using a Circular Array (continued)

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Implementing a Queue Using a Circular Array (continued)

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Comparing the Implementations All three implementations are comparable in

terms of computation timeAll operations are O(1)

Linked-list implementations require more storage because of the extra space required for the linksEach node for a single-linked list would store a

total of two referencesEach node for a double-linked list would store a

total of three references

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Comparing the Implementations (continued) A circular array that is filled to capacity

would require half the storage of a single-linked list to store the same number of elements

A circular array that was just reallocated would be half empty and require the same amount of storage as a single-linked list

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Simulating Waiting Lines Using Queues Simulation is used to study the

performance of a physical system by using a physical, mathematical, or computer model of the system

Simulation allows designers of a new system to estimate the expected performance before building it

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Simulating Waiting Lines Using Queues (continued) Simulation can lead to changes in the

design that will improve the expected performance of the new system

Useful when the real system would be too expensive to build or too dangerous to experiment with after its construction

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Simulating Waiting Lines Using Queues (continued) System designers often use computer

models to simulate physical systemsAirline check-in counter for example

A special branch of mathematics called queuing theory has been developed to study such problems

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Summary Queue is an abstract data type with a first-in, first-

out structure (FIFO) The Queue interface declares methods offer,

remove, poll, peek, and element Multiple ways to implement the Queue interface

Double-linked list, single-linked list, and circular array Queues are often used in simulation

To avoid the cost of building a physical system or running an actual experiment, computer simulation can be used to evaluate the expected performance of a system or operation strategy

Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

Any Questions?