Data Structures Using Java1 Chapter 7 Queues. Data Structures Using Java2 Chapter Objectives Learn about queues Examine various queue operations Learn.

Data Structures Using Java 1

Chapter 7

Queues


Chapter Objectives

• Learn about queues• Examine various queue operations• Learn how to implement a queue as an array• Learn how to implement a queue as a linked list• Discover priority queues• Discover queue applications


Queues

• Definition: data structure in which the elements are added at one end, called the rear, and deleted from the other end, called the front or first

• First In First Out (LIFO) data structure


Basic Operations on a Queue

• initializeQueue: Initializes the queue to an empty state

• isEmptyQueue: Determines whether the queue is empty. If the queue is empty, it returns the value true; otherwise, it returns the value false


Basic Operations on a queue

• isFullQueue: Determines whether the queue is full. If the queue is full, it returns the value true; otherwise, it returns the value false

• front: Returns the front (first) element of the queue; the queue must exist

• back: Returns the front (first) element of the queue; the queue must exist


Basic Operations on a queue

• addQueue: Adds a new element to the rear of the queue; the queue must exist and must not be full

• deleteQueue: Removes the front element of the queue; the queue must exist and must not be empty


Queue Exception Class

• Adding an element to a full queue, and removing an element from an empty queue, generates errors and exceptions called queue overflow and queue underflow exception

• Exception classes designed to handle these exceptions


Implementation of Queues as Arrays

• Initially queue is empty; queueFront and queueRear point directly to first and last elements of queue

• To implement a queue as an array we need:– An array

– The variables queueFront and queueRear to keep track of the first and last elements of the queue

– The variable maxQueueSize to specify the maximum size of the queue






Circular Queue

• Possible problem: If a sequence of operations eventually sets index queueRear to point to last array position, it gives the impression that the queue is full.

• However, the queue has only two or three elements and front of the array is empty (see Figure 7-4).


Circular Queue


Circular Queue


Circular Queue



Case 1: Suppose that after certain operations, the array containing the queue is as shown below



deleteQueue operation results in an empty queue



Case 2: Let us now consider the queue shown below



Resulting array in Figure 7-11 represents a full queue


Full Queue vs. Empty Queue

• Problem: distinguishing between an empty and a full queue

• Arrays in Figures 7-9 and 7-11 have identical values for queueFront and queueRear

• Solutions:– Keep a count

– Let queueFront indicate index of array position preceding first element of queue, rather than index of actual first element itself (see Figure 7-12)


UML Diagram of theclass QueueClass


Initialize Queue

public void initializeQueue(){ for(int i = queueFront; i < queueRear;

i = (i + 1) % maxQueueSize)

list[i] = null; queueFront = 0; queueRear = maxQueueSize - 1; count = 0;}


Empty Queue and Full Queue

public boolean isEmptyQueue(){ return (count == 0);}

public boolean isFullQueue(){ return (count == maxQueueSize);}


front

public DataElement front() throws QueueUnderflowException

{

if(isEmptyQueue())

throw new QueueUnderflowException();

DataElement temp = list[queueFront].getCopy();

return temp;

}


back

public DataElement back() throws QueueUnderflowException

{ if(isEmptyQueue()) throw new QueueUnderflowException(); DataElement temp = list[queueRear].getCopy(); return temp; }


Add Queue

public void addQueue(DataElement queueElement) throws QueueOverflowException{ if(isFullQueue()) throw new QueueOverflowException(); queueRear = (queueRear + 1) % maxQueueSize; //use the

mod //operator to advance

queueRear //because the array is

circular count++; list[queueRear] = queueElement.getCopy();}


Delete Queue

public void deleteQueue() throws QueueUnderflowException{ if(isEmptyQueue()) throw new QueueUnderflowException(); count--; list[queueFront] = null; queueFront = (queueFront + 1) % maxQueueSize; //use the mod //operator to advance queueFront //because the array is circular }


Constructor

• Constructor – creates an array of the size specified by the user– Default value is 100– Initializes queueFront queueRear to indicate

that the queue is empty


Linked Queue as an ADT


Empty and Full Queue

• Queue is empty if queueFront is NULL

• Queue is full only if we run out of memory


addQueue

• Adds a new element to the end of the queue

• Access the reference variable queueRear to implement addQueue


Front, Back, and Delete Queue

• If queue is nonempty:– operation front returns the first element of the queue

– operation back returns the last element of the queue

– operation deleteQueue removes the first element of the queue

• If queue is empty:– method front terminates the program

– method back terminates the program


Priority Queue

• FIFO rules of a queue are relaxed

• Customers or jobs with higher priority are pushed to front of queue

• To implement:– use an ordinary linked list, which keeps the

items in order from the highest to lowest priority

– use a treelike structure


Application of Queues

• Simulation: technique in which one system models the behavior of another system; used when it is too expensive or dangerous to experiment with real systems

• Simulation examples:– wind tunnels used to experiment with design of car

bodies– flight simulators used to train airline pilots

• Computer simulations: objects being usually represented as data


Theater Problem• The manager of a local movie theater is hearing

complaints from customers about the time they have to wait in line to buy tickets. The theater currently has only one cashier.

• Another theater is preparing to open in the neighborhood and the manager is afraid of losing customers. The manager wants to hire enough cashiers so that a customer does not have to wait too long to buy a ticket, but does not want to hire extra cashiers on a trial basis and potentially waste time and money.

• One thing that the manager would like to know is the average time a customer has to wait for service.

• The manager wants someone to write a program to simulate the behavior of the theater.


Queuing System

• Server: object that provides the service• Customer: object receiving the service• transaction time: service time; time it takes

to serve a customer • time-driven simulation: clock is

implemented as a counter and the passage of time (e.g. 1 minute) can be implemented by incrementing the counter (by 1)








waitingCustomerQueueclass WaitingCustomerQueue extends QueueClass{ //default constructor public WaitingCustomerQueue() { super(); } //constructor with a parameter public WaitingCustomerQueue(int size) { super(size); } //copy constructor public WaitingCustomerQueue(WaitingCustomerQueue otherQ) { super(otherQ); } //Method to increment the waiting time of each //customer in the queue by one time unit. //Postcondition: The waiting time of each customer in // the queue is incremented by one time unit. public void updateWaitingQueue() { //Definition as given below. }}


Poisson Distribution

Need to know the number of customers arriving at a given time unit and how long it takes to serve each customer.

Use Poisson distribution from statistics, which says probability of y events occurring at a given time is given by:

where is the expected value that y events occur at that time.


Chapter Summary

• Queue Data Structure– Restricted Version of arrays and linked

list

– Basic operations

• First In First Out (FIFO)

• Queues Implemented as Arrays


Chapter Summary

• Queues Implemented as Linked Lists

• Priority Queues

• Application of Queues

Data Structures Using Java1 Chapter 7 Queues. Data Structures Using Java2 Chapter Objectives Learn about queues Examine various queue operations Learn.

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