Queues

From last time: Java's built-in java.util.Stack class

Consider the javadoc for Java’s built-in java.util.Stack class (http://download.oracle.com/javase/7/docs/api/java/util/Stack.html).

What are the design flaws in this class? Explain.

Hint: can it guarantee that the item you pop will always be the last item pushed?

Queues p. 2/19

OverviewWhat are queues useful for?The QueueADT interfaceLinkedQueue implementationTime complexity of LinkedQueue operationsArray implementationsApplication: ticket counterQueues vs. stacksComing attractions

Queues p. 3/19

What are queues useful for?Suppose we want to model:A.Customers waiting in line for an online helpdeskB.Print jobs waiting for the printerC.Customers waiting in line at a bank

Assuming no one cuts in line, what operations would the solutions to these problems have in common?

Queues p. 4/19

Now contrast the following:A. The undo operation in WordB. The Back button in a web browserC. A hundred names in alphabetical order, where

names are added and removed frequentlyD. The roads and intersections in a city

Would our queue operations help to solve these problems? Why or why not?

Queues p. 5/19

You try …Give three everyday examples of a situation that

could be modeled using a queue.

Queues p. 6/19

The QueueADT interfaceConsider the code p. 102Compare the operations given here with

those we listed.

Queues p. 7/19

The QueueADT interfaceIn the code on page 102, the capital letter “T”

stands for:A. a temporary value B. the type of the items in the Queue C. a class named T defined somewhere else in

the program D. the top of the Queue E. none of the above

Queues p. 8/19

A LinkedQueue implementationConsider the start of the implementation on the

top of page 114. What's the overall purpose? And what is each line there for?

Queues p. 9/19

A LinkedQueue implementationConsider the LinkedQueue code on pp. 114-116.

What happens when client code tries to enqueue an item?

Describe precisely with box and arrow diagrams and reference to particular lines of code.

Queues p. 10/19

Time Complexity of LinkedQueueConsider the LinkedQueue code on pp. 114-116. The Big-Oh time complexity of the enqueue

operation is: A. O(1) B. O(log2 n)

C. O(n) D. O(n2) E. none of the above

Queues p. 11/19

Time Complexity of LinkedQueueConsider the LinkedQueue code on pp. 114-116. The Big-Oh time complexity of the dequeue

operation is: A. O(1) B. O(log2 n)

C. O(n) D. O(n2) E. none of the above

Queues p. 12/19

Time Complexity of LinkedQueueConsider the LinkedQueue code on pp. 114-116. What is the Big-Oh time complexity of the other

Queue operations?

Queues p. 13/19

Array implementationsDraw a picture of an 8-element array. Insert the characters A, B, C, D, E into the

first five locations of the array.Suppose A is the front of a queue and E is the

rear.Dequeue 3 elements and then add 4.What happens?

Queues p. 14/19

Possible solution: “circular” arraysHow would these solve our problem?

Describe precisely using the example above.How do they compare with the linked list

implementation of queues? What are the advantages and limitations of each?

Queues p. 15/19

Application: ticket counter(In groups) Consider the ticket counter simulation problem and

the solution given in the text (pp. 107-112).

Hand trace the solution for 22 customers and 4 cashiers.

Queues p. 16/19

Queues vs. stacksMatch each stack operation with the related

queue operation:A. push 1. isEmptyB. pop 2. enqueueC. peek 3. dequeueD. isEmpty 4 toStringE. toString 5. front

Queues p. 17/19

Queues vs. stacksWhich operations work the same even though they

may have different names? Which work differently? (Answer with reference to specific lines of code. If no code is given in the book, explain how you would implement the operation).

A. push 1. isEmptyB. pop 2. enqueueC. peek 3. dequeueD. isEmpty 4 toStringE. toString 5. front

Queues p. 18/19

Coming attractionsNext time we'll look at our third Abstract

Datatype, ListADT, a new linear data structure that allows elements to be added at the beginning, the end, and the middle.

Homework: read Chapter 6 Lists (or the equivalent in the earlier edition).

Queues p. 19/19