Queues, Deques, and Priority Queues

Chapter 22

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Chapter Contents

Specifications for the ADT QueueUsing a Queue to Simulate a Waiting Line• The Classes WaitLine and Customer

Using a Queue to Compute the capital Gain in a Sale of Stock• The Classes StockLedger and StockPurchase

Specifications of the ADT DequeUsing a Deque to Compute the Capital Gain in a Sale of StockSpecifications of the ADT Priority QueueUsing a Priority Queue to Compute the 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• Contrast to stack which is last-in, first-out

All additions are at the back of the queue

Front of queue has items 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 objectspublic interface queueInterface{ /** Task: Adds a new entry to the back of the queue.

* @param newEntry an object to be added */public void enqueue(Object newEntry);

/** Task: Removes and returns the front of the queue.* @return either the object at the front of the queue or null if the queue was empty */public Object dequeue();

/** Task: Retrieves the front of the queue.* @return either the object at the front of the queue or null if the queue is empty */public Object getFront();

/** Task: Determines whether the queue is empty.* @return true if the queue is empty */public boolean isEmpty();

/** Task: Removes all entries from the queue. */public void clear();

} // end queueInterface

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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 same order they were purchased• 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();public 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