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COMP 121

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 interface Double-linked list Single-linked list Circular array. Queue.

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COMP 121

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  1. COMP 121 Week 14: Queues

  2. Objectives • Learn how to represent a queue • Learn how to use the methods in the Queue interface • Understand how to implement the Queue interface • Double-linked list • Single-linked list • Circular array

  3. 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.

  4. 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.

  5. 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.

  6. 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.

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

  8. 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

  9. 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.

  10. 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.

  11. 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.

  12. Queue Example • Queue<String> names = new LinkedList<String>(); • Creates a new Queue reference, names, that stores references to String objects • The 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.

  13. 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.

  14. 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.

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

  16. Answer: Adapter Pattern • A better approach would be to create a new class that wrappers a LinkedList and implements the Queue interface • Only Queue methods would be available • Queue methods would be implemented by calling methods on the LinkedList • Encapsulation would hide linked list implementation from clients

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

  18. 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.

  19. 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.

  20. Implementing a Queue Using a Circular Array (continued) • 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 • Circular Array Implementation • Both 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.

  21. 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.

  22. 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.

  23. 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.

  24. 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.

  25. Comparing the Implementations • All three implementations are comparable in terms of computation time • All operations are O(1) • 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 Koffman, E.B. & Wolfgang, P.A.T. (2003). Objects, Abstraction, Data Structures, and Design Using Java Version 5.0. New York: John Wiley & Sons.

  26. 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.

  27. 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.

  28. 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.

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

  30. 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.

  31. Any Questions?

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