queues chapter 6. chapter 6: queues chapter objectives to learn how to represent a waiting line...
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Queues
Chapter 6
Chapter 6: Queues
Chapter Objectives
• To learn how to represent a waiting line (queue) and how to use the five methods in the Queue interface: insert, retrieve, peek, getSize, and isEmpty
• To understand how to implement the Queue interface using a single-linked list, a circular array, and a double-linked list
• To understand how to simulate the operation of a physical system that has one or more waiting lines using Queues and random number generators
Chapter 6: Queues
Queue Abstract Data Type
• 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 are placed at the end of the line
Chapter 6: Queues
Queue Abstract Data Type (continued)
Chapter 6: Queues
A Print Queue
• Operating systems use queues to • Track of tasks waiting for a scarce resource• To 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
Chapter 6: Queues
A Print Queue (continued)
Chapter 6: Queues
The Unsuitability of a Print Stack
• 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
Chapter 6: Queues
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 network
• A node in a graph may have several successors• Programmers often use a queue to ensure that nodes
closer to the starting point are visited before nodes that are farther away
Chapter 6: Queues
Specification for a Queue Interface
Chapter 6: Queues
Maintaining a Queue of Customers
• Queue is good for storing a list of customers as they should be serviced in the order in which they arrived
• Algorithm for processCustomers• While the user is not finished
• Display the menu and get the operation selected• Perform the operation selected
Chapter 6: Queues
Maintaining a Queue of Customers (continued)
Chapter 6: Queues
Maintaining a Queue of Customers (continued)
Chapter 6: Queues
Using a Single-Linked List to Implement a Queue
• Can implement a queue using a single-linked list• 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
methods insert and remove
Chapter 6: Queues
Using a Single-Linked List to Implement a Queue (continued)
Chapter 6: Queues
Implementing a Queue Using Java’s LinkedList
• Can be implemented as an adapter of any class that implements the List interface• ArrayList• Vector• LinkedList
• Removal is O(n) with a linked list• O(n) when using ArrayList or Vector
Chapter 6: Queues
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
• Array Implementation• Insertion at rear of array is constant time• Removal from the front is linear time• Removal from rear of array is constant time• Insertion at the front is linear time
Chapter 6: Queues
Implementing a Queue Using a Circular Array (continued)
Chapter 6: Queues
Implementing a Queue Using a Circular Array (continued)
Chapter 6: Queues
Implementing a Queue Using a Circular Array (continued)
Chapter 6: Queues
Implementing a Queue Using a Circular Array (continued)
Chapter 6: Queues
Comparing the Three Implementations
• All three implementations are comparable in terms of computation time
• Linked-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 references• Each 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
Chapter 6: 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 system
• Simulation allows designers of a new system to estimate the expected performance before building it
• 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
Chapter 6: Queues
Simulating Waiting Lines Using Queues (continued)
• System designers often use computer models to simulate physical systems• Airline check-in counter for example• A special branch of mathematics called queuing
theory has been developed to study such problems
Chapter 6: Queues
Simulate a Strategy for Serving Airline Passengers
Chapter 6: Queues
Simulate a Strategy for Serving Airline Passengers (continued)
Chapter 6: Queues
Simulate a Strategy for Serving Airline Passengers (continued)
Chapter 6: Queues
Simulating Waiting Lines Using Queues (continued)
Chapter 6: Queues
Simulating Waiting Lines Using Queues (continued)
Chapter 6: Queues
Simulating Waiting Lines Using Queues (continued)
Chapter 6: Queues
Simulating Waiting Lines Using Queues (continued)
Chapter 6: Queues
Chapter Review
• Queue is an abstract data type with a first-in, first-out structure (FIFO)
• The Queue interface declares methods insert, remove, peek, size, and isEmpty
• Three ways to implement the Queue interface: single-linked list, circular array, and as an adapter of the LinkedList class
• 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