Queues, Deques, and Priority Queues

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Queues, Deques, and Priority Queues

• Chapter 22

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Chapter Contents

• Specifications for the ADT Queue
• Simulating a Waiting Line
• The Classes WaitLine and Customer
• Computing capital gain in a Sale of Stock
• The Classes StockLedger and StockPurchase
• Specifications of the ADT Deque
• Computing Capital Gain in a Sale of Stock
• Specifications of the ADT Priority Queue
• Computing Capital Gain in a Sale of Stock

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Specifications for the ADT Queue

• Queue organizes entries according to order of
  entry
• Exhibits first-in, first-out behavior (FIFO)
• Contrast to stack...last-in, first-out (LIFO)
• All additions are at the back of the queue
• Front of queue has oldest items (those added
  first)

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Specifications for the ADT Queue
Fig. 22-1 Some everyday queues.
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Specifications for the ADT Queue

• Interface for a queue of objects

public interface QueueInterface / Task Add
item to back / public void enqueue(Object
newEntry) / Task Removes and returns the
front / public Object dequeue() / Task
Retrieves front of queue without removing
/ public Object getFront() / Task
Determines whether the queue is empty./ public
boolean isEmpty() / Task Removes all entries
from the queue. / public void clear()
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Specifications for the ADT Queue
Fig. 22-2 Queue of strings after (a) enqueue adds
Jim (b) Jess (c) Jill (d) Jane (e) Joe (f)
dequeue retrieves, removes Jim (g) enqueue adds
Jerry (h) dequeue retrieves, removes Jess.
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Using a Queue to Simulate a Waiting Line
Fig. 22-3 A line, or queue of people.
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Classes WaitLine and Customer
Fig. 22-4 A CRC card for the class WaitLine.
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Classes WaitLine and Customer
Fig. 22-5 A diagram of the classes WaitLine and
Customer.
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Classes WaitLine and Customer
Fig. 22-6 A simulated waiting line continued ?
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Classes WaitLine and Customer
Fig. 22-6 (ctd) A simulated waiting line.
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Using a Queue to Compute Capital Gain in a Sale
of Stock

• Must sell stocks in order of purchase
• Must use the difference in selling price and
  purchase price to calculate capital gain
• We use a queue to
• Record investment transactions chronologically
• Compute capital gain of the stock

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Classes StockLedger and StockPurchase
Fig. 22-7 A CRC card for the class StockLedger
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Classes StockLedger and StockPurchase
Fig. 22-8 A diagram of the class StockLedger and
StockPurchase.
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Classes StockLedger and StockPurchase
Fig. 22-9 A queue of (a) individual shares of
stock (b) grouped shares.
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Specifications of the ADT Deque

• A Double ended queue
• Has operations that
• Add, remove, or retrieve entries
• At both its front and back
• Combines and expands the operations of queue and
  stack

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Specifications of the ADT Deque
Fig. 22-10 An instance of a deque.
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Specifications of the ADT Deque

• A Java interface

public interface DequeInterface public void
addToFront(Object newEntry) public void
addToBack(Object newEntry) public Object
removeFront() public Object removeBack() publi
c Object getFront() public Object
getBack() public boolean isEmpty() public
void clear() // end DequeInterface
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Specifications of the ADT Deque
Fig. 22-11 A comparison of the operations that
add, remove, and retrieve the entries of a stack
s, queue q, and a deque d (a) add (b) remove
(c) retrieve.
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Using a Deque to Compute Capital Gain in Sale of
Stock

• Revise the class StockPurchase
• Represents purchase of n shares at d dollars per
  share
• Constructor is changed
• Accessor methods for new fields added
• Revise class StockLedger
• Method ledger now an instance of a deque
• Method buy now creates instance of StockPurchase,
  places it at back of deque
• Method sell also altered

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Specifications of the ADT Priority Queue

• Organizes objects according to priorities
• Contrast to regular queue in order of arrival
• Priority queue example a hospital ER
• Priority can be specified by an integer
• Must define whether high number is high priority
  or
• Low number (say 0) is high priority
• Other objects can specify priority
• Object must have a compareTo method

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Specifications of the ADT Priority Queue
public interface PriorityQueueInterface public
void add(Comparable newEntry) public Comparable
remove() public Comparable get() public
boolean isEmpty() public int getSize() public
void clear() // end PriorityQueueInterface
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Using Priority Queue to Compute Capital Gain in
Sale of Stock

• Revise class StockPurchase
• Include data field date
• Method compareTo is made available
• This field is the priority earliest date is
  highest priority