Queues

1. Queues Chapter 6

2. 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

3. 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

4. Queue Abstract Data Type (continued) Chapter 6: Queues

5. 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

6. A Print Queue (continued) Chapter 6: Queues

7. 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

8. 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

9. Specification for a Queue Interface Chapter 6: Queues

10. 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

11. Maintaining a Queue of Customers (continued) Chapter 6: Queues

12. Maintaining a Queue of Customers (continued) Chapter 6: Queues

13. 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

14. Using a Single-Linked List to Implement a Queue (continued) Chapter 6: Queues

15. 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

16. 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

17. Implementing a Queue Using a Circular Array (continued) Chapter 6: Queues

18. Implementing a Queue Using a Circular Array (continued) Chapter 6: Queues

19. Implementing a Queue Using a Circular Array (continued) Chapter 6: Queues

20. Implementing a Queue Using a Circular Array (continued) Chapter 6: Queues

21. 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

22. 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

23. 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

24. Simulate a Strategy for Serving Airline Passengers Chapter 6: Queues

25. Simulate a Strategy for Serving Airline Passengers (continued) Chapter 6: Queues

26. Simulate a Strategy for Serving Airline Passengers (continued) Chapter 6: Queues

27. Simulating Waiting Lines Using Queues (continued) Chapter 6: Queues

28. Simulating Waiting Lines Using Queues (continued) Chapter 6: Queues

29. Simulating Waiting Lines Using Queues (continued) Chapter 6: Queues

30. Simulating Waiting Lines Using Queues (continued) Chapter 6: Queues

31. 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 Chapter 6: Queues