Queues
From last time: Java's built-in java.util.Stack class Consider the javadoc for Java’s built-in java.util.Stack class ( til/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
Overview What are queues useful for? The QueueADT interface LinkedQueue implementation Time complexity of LinkedQueue operations Array implementations Application: ticket counter Queues vs. stacks Coming attractions Queues p. 3/19
What are queues useful for? Suppose we want to model: A. Customers waiting in line for an online helpdesk B. Print jobs waiting for the printer C. 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 Word B. The Back button in a web browser C. A hundred names in alphabetical order, where names are added and removed frequently D. 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 interface Consider the code p. 102 Compare the operations given here with those we listed. Queues p. 7/19
The QueueADT interface In 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 implementation Consider 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 implementation Consider the LinkedQueue code on pp 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 LinkedQueue Consider the LinkedQueue code on pp The Big-Oh time complexity of the enqueue operation is: A. O(1) B. O(log 2 n) C. O(n) D. O(n 2 ) E. none of the above Queues p. 11/19
Time Complexity of LinkedQueue Consider the LinkedQueue code on pp The Big-Oh time complexity of the dequeue operation is: A. O(1) B. O(log 2 n) C. O(n) D. O(n 2 ) E. none of the above Queues p. 12/19
Time Complexity of LinkedQueue Consider the LinkedQueue code on pp What is the Big-Oh time complexity of the other Queue operations? Queues p. 13/19
Array implementations Draw 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” arrays How 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 ). Hand trace the solution for 22 customers and 4 cashiers. Queues p. 16/19
Queues vs. stacks Match each stack operation with the related queue operation: A. push1. isEmpty B. pop2. enqueue C. peek3. dequeue D. isEmpty4 toString E. toString5. front Queues p. 17/19
Queues vs. stacks Which 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. push1. isEmpty B. pop2. enqueue C. peek3. dequeue D. isEmpty4 toString E. toString5. front Queues p. 18/19
Coming attractions Next 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