QueuesQueues

Chapter 6Chapter 6

Chapter ObjectivesChapter Objectives

To learn how to represent a waiting line (queue) and how to use the methods in the Queue interface for insertion (offer and add), removal (remove and poll), and for accessing the element at the front (peek and element)To understand how to implement the Queue interface using a single-linked list, a circular array, and a double-linked listTo understand how to simulate the operation of a physical system that has one or more waiting lines using Queues and random number generators

To learn how to represent a waiting line (queue) and how to use the methods in the Queue interface for insertion (offer and add), removal (remove and poll), and for accessing the element at the front (peek and element)To understand how to implement the Queue interface using a single-linked list, a circular array, and a double-linked listTo understand how to simulate the operation of a physical system that has one or more waiting lines using Queues and random number generators

Queue Abstract Data TypeQueue Abstract Data Type

Can visualize a queue as a line of customers waiting for serviceThe next person to be served is the one who has waited the longestNew elements are placed at the end of the line

Can visualize a queue as a line of customers waiting for serviceThe next person to be served is the one who has waited the longestNew elements are placed at the end of the line

Queue Abstract Data Type (continued)

Queue Abstract Data Type (continued)

A Print QueueA Print Queue

Operating systems use queues to Track of tasks waiting for a scarce resourceTo ensure that the tasks are carried out in the order that they were generated

Print queue: printing is much slower than the process of selecting pages to print and so a queue is used

Operating systems use queues to Track of tasks waiting for a scarce resourceTo ensure that the tasks are carried out in the order that they were generated

Print queue: printing is much slower than the process of selecting pages to print and so a queue is used

A Print Queue (continued)A Print Queue (continued)

The Unsuitability of a Print StackThe Unsuitability of a Print Stack

Stacks are last-in, first-out (LIFO)The most recently selected document would be the next to printUnless the printer queue is empty, your print job may never get executed if others are issuing print jobs

Stacks are last-in, first-out (LIFO)The most recently selected document would be the next to printUnless the printer queue is empty, your print job may never get executed if others are issuing print jobs

Using a Queue for Traversing a Multi-Branch Data Structure

Using a Queue for Traversing a Multi-Branch Data Structure

A graph models a network of nodes, with many links connecting each node to other nodes in the networkA node in a graph may have several successorsProgrammers often use a queue to ensure that nodes closer to the starting point are visited before nodes that are farther away

A graph models a network of nodes, with many links connecting each node to other nodes in the networkA node in a graph may have several successorsProgrammers often use a queue to ensure that nodes closer to the starting point are visited before nodes that are farther away

Specification for a Queue Interface

Specification for a Queue Interface

Class LinkedList Implements the Queue Interface

Class LinkedList Implements the Queue Interface

LinkedList class provides methods for inserting and removing elements at either end of a double-linked listThe Java 5.0 LinkedList class implements the Queue interfaceQueue<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.

LinkedList class provides methods for inserting and removing elements at either end of a double-linked listThe Java 5.0 LinkedList class implements the Queue interfaceQueue<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.

Maintaining a Queue of CustomersMaintaining a Queue of Customers

Queue is good for storing a list of customers as they should be serviced in the order in which they arrivedAlgorithm for processCustomers

While the user is not finishedDisplay the menu and get the operation selectedPerform the operation selected

Queue is good for storing a list of customers as they should be serviced in the order in which they arrivedAlgorithm for processCustomers

While the user is not finishedDisplay the menu and get the operation selectedPerform the operation selected

Maintaining a Queue of Customers (continued)

Maintaining a Queue of Customers (continued)

Using a Double-Linked List to Implement the Queue Interface

Using a Double-Linked List 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 queueJava designers decided to make the head of the linked list the front of the queue and the tail the rear of the queueLimitation: LinkedList object is used as a queue, it may be possible to apply other LinkedList methods in addition to the ones required by 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 queueJava designers decided to make the head of the linked list the front of the queue and the tail the rear of the queueLimitation: LinkedList object is used as a queue, it may be possible to apply other LinkedList methods in addition to the ones required by the Queue interface

Using a Single-Linked List to Implement a Queue

Using a Single-Linked List to Implement a Queue

Can implement a queue using a single-linked listClass ListQueue contains a collection of Node<E> objectsInsertions are at the rear of a queue and removals are from the frontNeed a reference to the last list nodeNumber of elements in the queue is changed by methods insert and remove

Can implement a queue using a single-linked listClass ListQueue contains a collection of Node<E> objectsInsertions are at the rear of a queue and removals are from the frontNeed a reference to the last list nodeNumber of elements in the queue is changed by methods insert and remove

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

Implement a Queue (continued)

Implementing a Queue Using a Circular Array

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 inefficienciesStorage space is increased when using a linked list due to references stored at each list nodeArray Implementation

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

Time efficiency of using a single- or double-linked list to implement a queue is acceptable

However there are some space inefficienciesStorage space is increased when using a linked list due to references stored at each list nodeArray Implementation

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

Implementing a Queue Using a Circular Array (continued)

Implementing a Queue Using a Circular Array (continued)

Implementing a Queue Using a Circular Array (continued)

Implementing a Queue Using a Circular Array (continued)

Implementing a Queue Using a Circular Array (continued)

Implementing a Queue Using a Circular Array (continued)

Implementing a Queue Using a Circular Array (continued)

Implementing a Queue Using a Circular Array (continued)

Implementing Class ArrayQueue<E>.IterImplementing Class

ArrayQueue<E>.IterJust as for class ListQueue<E>, we must implement the missing Queue methods and an inner class Iter to fully implement the Queue interfaceData field index stores the subscript of the next element to accessThe constructor initializes index to front when a new Iter object is createdData field count keeps track of the number of items accessed so farMethod Iter.remove throws an Unsupported-OperationException because it would violate the contract for a queue to remove an item other than the first one

Just as for class ListQueue<E>, we must implement the missing Queue methods and an inner class Iter to fully implement the Queue interfaceData field index stores the subscript of the next element to accessThe constructor initializes index to front when a new Iter object is createdData field count keeps track of the number of items accessed so farMethod Iter.remove throws an Unsupported-OperationException because it would violate the contract for a queue to remove an item other than the first one

Comparing the Three Implementations

Comparing the Three Implementations

All three implementations are comparable in terms of computation timeLinked-list implementations require more storage because of the extra space required for the links

Each 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

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

All three implementations are comparable in terms of computation timeLinked-list implementations require more storage because of the extra space required for the links

Each 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

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

Simulating Waiting Lines Using Queues

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 systemSimulation allows designers of a new system to estimate the expected performance before building itSimulation can lead to changes in the design that will improve the expected performance of the new systemUseful when the real system would be too expensive to build or too dangerous to experiment with after its construction

Simulation is used to study the performance of a physical system by using a physical, mathematical, or computer model of the systemSimulation allows designers of a new system to estimate the expected performance before building itSimulation can lead to changes in the design that will improve the expected performance of the new systemUseful when the real system would be too expensive to build or too dangerous to experiment with after its construction

Simulating Waiting Lines Using Queues (continued)

Simulating Waiting Lines Using Queues (continued)

System designers often use computer models to simulate physical systems

Airline check-in counter for exampleA special branch of mathematics called queuing theory has been developed to study such problems

System designers often use computer models to simulate physical systems

Airline check-in counter for exampleA special branch of mathematics called queuing theory has been developed to study such problems

Simulate a Strategy for Serving Airline Passengers

Simulate a Strategy for Serving Airline Passengers

Simulate a Strategy for Serving Airline Passengers (continued)

Simulate a Strategy for Serving Airline Passengers (continued)

Simulate a Strategy for Serving Airline Passengers (continued)

Simulate a Strategy for Serving Airline Passengers (continued)

Simulating Waiting Lines Using Queues (continued)

Simulating Waiting Lines Using Queues (continued)

Simulating Waiting Lines Using Queues (continued)

Simulating Waiting Lines Using Queues (continued)

Simulating Waiting Lines Using Queues (continued)

Simulating Waiting Lines Using Queues (continued)

Simulating Waiting Lines Using Queues (continued)

Simulating Waiting Lines Using Queues (continued)

Simulating Waiting Lines Using Queues (continued)

Simulating Waiting Lines Using Queues (continued)

Simulating Waiting Lines Using Queues (continued)

Simulating Waiting Lines Using Queues (continued)

Simulating Waiting Lines Using Queues (continued)

Simulating Waiting Lines Using Queues (continued)

Chapter ReviewChapter Review

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.Three ways to implement the Queue interface: double-linked list, single-linked list, and circular arrayTo 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

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.Three ways to implement the Queue interface: double-linked list, single-linked list, and circular arrayTo 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